NASA's Kepler planet-hunting probe has identified two potentially habitable planets only a little bigger than Earth, circling a star that's 1,200 light-years away. The planets could conceivably be covered by a global ocean, and they may well lead the growing list of alien worlds that can host life as we know it.

"These two planets are our best candidates for planets that might be habitable," said Bill Borucki, a space scientist at NASA's Ames Research Center who is the principal investigator for the $600 million Kepler mission.

The two habitable-zone planets, Kepler-62e and Kepler-62f, are part of a five-planet system that lies in the constellation Lyra, within a patch of sky that's been monitored by the Kepler space telescope over the past four years. The Kepler-62 parent star is about two-thirds the size of our own sun and about a fifth as bright.  Three of the star's confirmed planets circle the star in orbits so close that they'd be too hot for life. But the e and f planets are considered to lie in a zone where liquid water could exist, a ring of space that's defined as the habitable zone.

Water worlds?
Two members of the Kepler science team say their modeling suggests the two planets could be "water worlds" — with no land in sight.

"These planets are unlike anything in our solar system. They have endless oceans," Lisa Kaltenegger, an astronomer at the Max Planck Institute for Astronomy and the Harvard-Smithsonian Center for Astrophysics, said in a news release. "There may be life there, but could it be technology-based like ours?"

The report on the Kepler-62 system was published online on Thursday by the journal Science, and was the focus of a NASA news conference timed to coincide with publication. The water-world analysis, authored by Kaltenegger and Harvard's Dimitar Sasselov and Sarah Rugheimer, is contained in a separate paper that has been accepted for publication in The Astrophysical Journal.

The characterization of the two planets' habitability is based on an analysis of their size, plus what's known about the parent star. The Kepler data can show how wide a planet is, and how quickly it makes its orbit, by analyzing the telltale dips in light as the planet passes over its parent star. But Kepler can't make direct observations of a planet's mass. So, in Kepler-62's case, scientists had to make educated guesses about the planets' mass, composition and whether they had atmospheres.

Kepler-62e is 1.6 times as wide as Earth and orbits its star every 122.4 Earth days. Kepler-62f is 1.4 times Earth's width, with an orbital period of 267.3 Earth days. "It's highly likely they're rocky planets," Borucki told NBC News. "They might be water worlds, but they are so different, we just don't know."

David A. Aguilar / CfA

This artist's conception shows Kepler-62f as an ice-covered world, and Kepler-62e as an Earthlike planet with dense clouds. Other planets follow closer-in orbits and are not considered habitable.

NASA Ames / JPL-Caltech

The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring two-thirds the size of the sun and one-fifth as bright.

What would life be like?
Astrobiologists say the fact that the planets are bigger than Earth wouldn't be an obstacle for life. In fact, some experts argue that a super-Earth is more likely to have life than an Earth-sized planet. "If you and I walked on it, our weight would double," Borucki said. "But my weight has doubled since I was a teenager ... so we could do it."

If the planets had atmospheres like Earth's, Kepler-62e's surface temperature would be 86 degrees Fahrenheit (30 degrees Celsius), while Kepler-62f's temperature would be 19 degrees below zero F (-28 degrees C), Borucki said. "You'd see the sun being substantially larger than our sun, because it's so much closer," he said. "But it'd be darker, like walking around on a cloudy day."

In their research paper, Kaltenegger and Sasselov assume that Kepler-62e has a slightly cloudier atmosphere than Earth's, and that Kepler-62f has a thick carbon-dioxide atmosphere with a strong greenhouse effect. Without a thick atmosphere, Kepler-62f could get chillier than Mars. It might even look more like a Europa-style iceball than a Kevin Costner-style water world.

"Kepler-62e probably has a very cloudy sky, and is warm and humid all the way to the polar regions," Sasselov said. "Kepler-62f would be cooler, but still potentially life-friendly. The good news is, the two would exhibit distinctly different colors and make our search for signatures of life easier on such planets in the near future."

Habitable worlds ahead
Kepler-62e and Kepler-62f aren't the first habitable-zone planets to be identified by the Kepler team, and they won't be the last. A year and a half ago, Kepler-22b came to light as the mission's first potentially habitable planet. It's 2.4 times wider than Earth, which puts it halfway between our planet and Neptune on the size scale. Kepler-47c, unveiled last year, is also a habitable-zone planet — but it's 4.6 times wider than Earth, which makes it Neptune-sized.

This January, the science team discussed the habitability of another candidate planet, then known as KOI 172.02. The existence of that world has now been confirmed under the name Kepler-69c, with a size that's 1.7 times Earth's width. "Today we can announce that this is a bona fide planet," Thomas Barclay, an astronomer at Ames Research Center, said during Thursday's news conference. 

Three months ago, Kepler-69c was hailed as potentially the most Earthlike world detected beyond our solar system, but now researchers say Kepler-62e and Kepler-62f could be stronger contenders.

There will be more contenders ahead: Borucki said about four dozen of the more than 2,700 candidate planets being tracked by Kepler lie within their stars' habitable zones, and it takes about a year to confirm each candidate's existence through detailed analysis. "We really wish we were faster," he told NBC News. "I really wish we could knock off one a week."

Boruckin and his colleagues are poring through the oceans of observations coming in from the Kepler telescope, and although the spacecraft has had its problems, he's hoping that the flood of data will continue for years to come.

"When you're born a scientist, they leave out the gene for saying, 'We have enough data,'" Borucki joked.

More about the planet hunt:

• Alien Earths in our backyard?
• 17 billion hot Earths in our galaxy
• NBC News on planets | Cosmic Log on Kepler

Borucki, Kaltenegger, Sasselov and Barclay are among 64 authors of the Science paper, "Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone." Barclay and Borucki are among 31 authors of "A Super-Earth-Sized Planet Orbiting in or near the Habitable Zone around Sun-like Star," published in The Astrophysical Journal. Kaltenegger, Sasselov and Rugheimer are the authors of "Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the Case of Kepler-62e and -62f."

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA / JPL-Caltech / MSSS

This graphic shows the relative sizes of Earth, Mars and Europa, an icy moon of Jupiter.

A British astrobiology conference has revived a years-old debate over the best place to look for life elsewhere in the solar system: Mars, or the moons of Jupiter and Saturn?

"For reasons I don't really understand, the wider solar system and the potential for life there has not been high priority," The Telegraph quoted Robert Pappalardo, a senior research scientist at NASA's Jet Propulsion Laboratory, as saying on BBC Radio 4.

Pappalardo's remarks were occasioned by this week's astrobiology conference at the UK Center for Astrobiology in Edinburgh, Scotland. The center recently established the International Subsurface Astrobiology Laboratory, or ISAL, half a mile (1 kilometer) beneath the surface in Yorkshire's Boulby mine. Biologists will use that facility to see how organisms hold up in extreme environments, learn about life's chemical signatures, and test instruments that could look for those signatures on other worlds.

Someday, one of the worlds may well be Europa, an icy moon of Jupiter. With a diameter of 1,945 miles (3,130 kilometers), Europa is just slightly smaller than Earth's moon, and yet it is thought to contain more water than Earth's oceans beneath a miles-deep layer of ice. Researchers recently suggested that hydrogen peroxide in the ice could serve as an energy supply for simple forms of life in the ocean hidden below.

Europa is the focus of Pappalardo's research, and for months he has been urging NASA to support a $2 billion mission to study Europa at close range. However, proposals for NASA missions to Europa have been losing out, in part because of the cost of missions to Mars. Last week's federal budget proposal for the next fiscal year provides no funding for a Europa mission, but it does fund Mars missions such as Maven (launching this year), InSight (launching in 2016) and a new science rover (launching in 2020).

Kevin Hand (JPL-Caltech) / Jack Cook (WHOI) / Howard Perlman (USGS)

If Europa's ocean is 100 kilometers (62 miles) deep, and all that water were gathered into a ball, it would have a radius of 877 kilometers (545 miles). This graphic compares that hypothetical ball of Europan water to the size of the Jovian moon itself, as well as all the water on planet Earth. Europa is thought to have two to three times the volume of water in Earth's oceans.

At February's annual meeting of the American Association for the Advancement of Science, Pappalardo worried that NASA's study of the outer solar system would go "radio-dark" in 2017, when the Cassini mission to Saturn and the Juno mission to Jupiter are both due to end. He continued that theme in this week's BBC interview.

"I worry that if Europa exploration is delayed, but then finally it happens some day, we might look back and say 'Why didn't we do that sooner?' Imagine 50 years from now, we get a lander there and find signs of life. All this time we'll have been looking in the wrong place," he was quoted as saying.

Europa isn't the only moon that intrigues astrobiologists: In the Jovian system, Callisto and Ganymede also have icy shells and may hold hidden oceans. Meanwhile, Cassini has repeatedly observed geysers of water ice rising from the surface of the Saturnian moon Enceladus — suggesting that liquid water and perhaps life may lie beneath the surface. Saturn's largest moon, Titan, has a thick atmosphere and seas of hydrocarbon that some scientists think could harbor a totally alien kind of life.

As for Mars, astrobiologists say hints of life could well lurk beneath the surface. To some extent, the Red Planet has been winning out over Europa and Enceladus because it's easier to get to. Moreover, NASA's vision calls for sending astronauts to Mars and its moons in the 2030s. NASA's robotic missions serve as precursors for those human voyages, as well as steps in a long-term program to learn about life in the universe.

Europa's fans can take heart in the fact that the European Space Agency is planning its own mission to Jupiter's moons: The Jupiter Icy Moons Explorer, or JUICE, is due for launch in 2022 and arrival at the Jovian system in 2030. There's also talk of a sample return mission that would target Enceladus' geysers, and a proposal to drop a boat onto Titan's seas.

So what if all of these worlds — Mars and Europa, Callisto and Ganymede, Titan and Enceladus — turn out to be lifeless? Charles Cockell, who heads the UK Center for Astrobiology, addressed that scenario in an interview with the BBC.

"A lot of people think astrobiology is some sort of hunt for life, and if we don't find life, it will be a big disappointment," Cockell said. "But in fact, that's not the case. The discovery of many lifeless planets across the universe, the discovery that the Earth might be unique as a place for life, would be an astonishing discovery in itself. It would be a very lonely discovery, but it would be an astonishing discovery."

More about the search for life:

• Which alien worlds are most livable?
• Maybe we are alone, after all 
• Cosmic Log archive on astrobiology

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

WISSARD Project via Antarctic Sun

A laptop screen shows a video view of the borehole drilled through Antarctica's ice down to Lake Whillans.

The first signs of potentially exotic life have been spotted in a sample of water drawn from Antarctica's hidden Lake Whillans, a half-mile beneath the surface, according to reports from the scene.

The telltale green glow of cells stained with a DNA-sensitive dye could be seen when water from the lake was put under the microscope on Monday, Discover Magazine's Crux blog reported. "It was the first evidence of life in an Antarctic subglacial lake," science journalist Douglas Fox reported for The Crux. Fox is an embedded journalist reporting from Lake Whillans under the auspices of a National Science Foundation program.

The U.S. scientists in charge of the project to drill into Lake Whillans — known as the Whillans Ice Stream Subglacial Access Research Drilling, or WISSARD — will be more circumspect: They'll have to demonstrate that the green-glowing cells are truly alive and capable of growing in culture. They'll also conduct tests to make sure that the microbes are indigenous to the lake, rather than the result of contamination from the drilling operation.

Last year, Russian scientists analyzed water from Lake Vostok, an even deeper and bigger subglacial lake beneath Antarctica's Vostok Station, but the only microbes they found in the sample were surface-dwelling species that may have come from contaminated drilling chemicals rather than the lake itself.

During the current Antarctic research season, the Russians resumed their drilling at Vostok. They said earlier this month that they had reached transparent lake ice at a depth of 3.4 kilometers (2.1 miles). Since then, they've reported retrieving "fresh frozen" ice cores from slightly deeper levels.

The Russian and U.S. teams are drilling into the lakes in hopes of finding evidence of life forms that could have been living in the dark for thousands of years, or even millions of years. Theoretically, such organisms could live off the minerals in deep-buried rock, plus oxygen dissolved in the lake water.

The Whillans Ice Stream is a glacial river that pushes ice from the West Antarctic Ice Sheet into the Ross Ice Shelf. Lake Whillans lies about 800 meters (0.5 miles) beneath the ice, less than 400 miles (640 kilometers) from the South Pole. Just this past weekend, the WISSARD team reported that their borehole connected with the lake after several days of drilling. 

Fox quoted scientists as saying that Lake Whillans is just 5 to 6 feet (1.5 to 2 meters) deep, as opposed to the 20 to 30 feet (6 to 9 meters) that was expected. The first water samples that were brought up contained the ancient fossils of dead diatoms — tiny marine creatures that are thought to have been pushed down into the lake from West Antarctica.

The study of Lake Whillans and other subglacial lakes should shed light on Antarctica's climate history, as well as the long-term interaction between the continent's ice and the water and rocks that lie beneath. The discovery of novel life forms could open up an entirely new frontier for biologists. And even if the organisms found in the lakes aren't all that unusual, the drilling operations could set the stage for future missions to the ice-covered moons of Jupiter and Saturn, where similarly challenging conditions for subsurface life are thought to exist.

More about the mysteries beneath the ice:

• Saturnian moon Enceladus eyed for sample return mission
• Underground ocean goes deep on Jovian moon Europa
• Mission to drill into Antarctica's Lake Ellsworth suspended

For more about the WISSARD project at Lake Whillans, check out this report from The Antarctic Sun.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

AP file

Russian researchers at the Vostok station in Antarctica pose for a picture after reaching Lake Vostok in February 2012. Scientists hold a sign reading "05.02.12, Vostok station, boreshaft 5gr, lake at depth 3769.3 metres." The researchers now report that they have brought up fresh samples from the borehole.

Russian researchers say they have brought up fresh samples of clear ice from Antarctica's Lake Vostok, a huge reservoir of freshwater more than 2 miles (3.2 kilometers) beneath the surface.

Lake Vostok could contain water and perhaps living organisms that have been sitting undisturbed in the deep dark for up to 20 million years. The drilling operation also could set a precedent for far more ambitious efforts to find life beneath the ice of the Jovian moon Europa or the Saturnian moon Enceladus.

Because of the potential for contamination, scientists have been taking extreme care at Lake Vostok, which is situated 800 miles (1,300 kilometers) from the South Pole in East Antarctica. A year ago, the Russian drilling team reached the lake and brought up water samples. Some of the water was even served to Vladimir Putin, who was then Russia's prime minister and is now the country's president. But it wasn't clear whether those samples were actually from the lake or from the glacier above the lake, the Russian news service RIA Novosti reported.

This year's drilling operation is aimed at bringing up samples that can be linked more definitively to the lake itself.

"The first core of transparent lake ice, 2 meters long, was obtained on Jan. 10 at a depth of 3,406 meters," Russia's Arctic and Antarctic Research Institute said in a statement. "Inside it was a vertical channel filled with white bubble-rich ice."

The institute said that drilling operations would be extended another 24 meters with the existing cables, and that new cables were being delivered to the Vostok research station. The core samples were to be subjected to chemical and biological analysis.

Lake Vostok is about 160 miles (250 kilometers) long and 30 miles (50 kilometers) wide, making it the largest of Antarctica's nearly 400 subglacial lakes. Last year's drilling operation drew up samples from a depth of 12,366 feet (2.34 miles, or 3,769 meters). In October, Russian team members reported finding no native life within those samples. They said the only microbes they detected were traced to contaminants from the drilling oil.

The lake could serve as a laboratory for studying what Antarctica's climate and ecosystem was like millions of years ago. It may contain creatures unlike any that exist today. And as ambitious as all that sounds, the Vostok operation is seen as a mere warmup for future sampling missions to Europa, Enceladus and perhaps other icy moons in the solar system.

Planetary scientists see ample evidence that liquid water exists on those worlds, miles beneath the icy surface, and astrobiologists have theorized that internal heat may provide enough energy for organisms living within those hidden oceans.

Correction for 1:30 p.m. ET Jan. 14: I initially described Vostok station as 800 miles east of the South Pole, but that's not quite right: All directions from the South Pole are north, as commenters have pointed out. There is an "east" and "west" to the continent, and Vostok happens to be in East Antarctica. I've changed the reference to the location accordingly.

More about the mysteries beneath the ice:

• Saturn moon eyed for sample return mission
• Satellite shows Russia's 'moon shot' ice station
• Mission to drill into Antarctica's Lake Ellsworth suspended

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

SpaceX

An artist's conception shows a SpaceX Dragon capsule firing its retros to land on Mars. Scientists hope to use an advanced Dragon and a Falcon Heavy to deliver a life-detection experiment to Mars.

NASA officials emphasize that the Curiosity rover is not designed to look for signs of life on Mars, but other researchers say it could come across those signs in spite of itself — and still others are planning experiments to take on the "life on Mars" question directly for the first time in decades.

Every time the question comes up, it stirs a controversy: It was that way in 1976, when some of the researchers working on the Mars Viking mission contended that they saw evidence of biological activity on the Red Planet. It was that way in 1996, when researchers at Johnson Space Center said they spotted nanofossils inside a meteorite from Mars. And it was that way in 2004, when other researchers suggested that whiffs of methane detected on Mars hinted at the presence of life.

Perhaps because of those controversies, NASA tends to downplay the question during the current Curiosity campaign. "Whether life has existed on Mars is an open question that this mission, by itself, is not designed to answer," the mission's press kit declares. But Gil Levin, who still argues that Viking discovered life on Mars, believes Curiosity could confirm the discovery.

He says one of Curiosity's chemistry analyzers, known as Sample Analysis at Mars, or SAM, could detect organic compounds that would fill in the pieces that were missing during the Viking mission. And he's hoping that Curiosity's high-resolution color cameras will pick up the spectral signature of lichen-type growth on Martian rocks. Such organism might well have hitchhiked to Mars on meteorites that were blown into space from Earth.

"Preserved, frozen, they could survive the entry to Mars and grow under Martian conditions," he told me a few months ago.

Follow the methane
Robert Zubrin, a rocket scientist who's the president of the Mars Society, has a different perspective but arrives at the same bottom line when it comes to Curiosity. "It has the ability, in my view, to detect life on Mars," he told a Mars Society gathering on Saturday night.

The SAM analyzer not only has the capability to detect methane in the Martian atmosphere, but can also break down the distribution of different carbon isotopes to determine whether the gas was created through biological or purely geochemical processes, Zubrin said. "It could catch the scent of life on Mars," he said.

Even John Grunsfeld, NASA's associate administrator for science, acknowledged today that the analysis of Martian methane was one of the promising avenues of research available to Curiosity. Because methane tends to be broken down relatively quickly, the supply in the Martian atmosphere has to be replenished regularly. Emanations from volcanoes or gas hydrates could do it, but so far there's little evidence of that type of activity on Mars.

On Earth, much of the atmosphere's methane comes from biological sources, such as the digestive tracts of cows. Grunsfeld joked that if there were cows on Mars, the resolution of the camera on NASA's Mars Reconnaissance Orbiter is so good that "we would have seen them." But microbes? If Curiosity even hints that biology may be responsible for Martian methane, that could well become the prime focus of future Red Planet missions.

Drilling for life
Some NASA scientists and engineers are already thinking about those future missions. For example, Carol Stoker, an astrobiologist at NASA's Ames Research Center, is part of a team that's drawing up a proposal for a life-detection mission known as Icebreaker. During this week's Mars Society conference, Stoker said it was "quite possible that there is modern life on Mars," based on the chemical analyses conducted by NASA's Phoenix Mars Lander in 2008.

She'd like to see Icebreaker return to the north polar region visited by Phoenix, carrying a drill as well as a device known as a Signs of Life Detector, or SOLID for short. SOLID uses a microarray with hundreds of different antibodies to test for the presence of a wide variety of microbes, including the sorts of extremophiles that some experts think could survive beneath the surface on present-day Mars. 

"We believe we can answer the question, 'Is there Earthlike life at that site,' with a Discovery-class mission" costing no more than $425 million, Stoker said. She said a proposal could be prepared for NASA's next solicitation for Discovery-class or New Horizons missions, whenever that happens.

To hit that price tag, the experiment would be launched inside a next-generation SpaceX Dragon capsule, atop a next-generation Falcon Heavy rocket. The plans for a "Red Dragon" trip to Mars made a splash last year, but a lot of the technologies to back up the concept have yet to be developed. The Falcon Heavy is still under development, for example, and the proposed landing scenario for the Dragon relies on a system called supersonic retro propulsion.

"That's at a relatively low technology level," Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters, told me today. But some smart people are studying how to make the mission concept work — including Adam Steltzner, the head of the group at NASA's Jet Propulsion Laboratory that developed the unorthodox sky-crane system for putting Curiosity on the ground.

Could it work? And is it plausible to think biochemical tests at a single site could turn up evidence of life on Mars? Or is NASA's deliberate, step-by-step approach the way to go? Feel free to weigh in with your comments below.

More about Mars:

• Mars Curiosity comes in for a landing
• Last-minute guide to the Mars landing
• What will we see from Mars, and when will we see it?
• Why we're obsessed with Mars
• Mars probe provides radiation revelations

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Mark Wilson / Getty Images file

"Arsenic life" researcher Felisa Wolfe-Simon is flanked by Mary Voytek, director of NASA's Astrobiology Program, as well as chemist Steven Benner and astrobiologist Pamela Conrad during a NASA news conference on Dec. 2, 2010. Many of the claims made during that briefing have now been refuted in peer-reviewed research.

Nineteen months ago, NASA's experts on astrobiology hailed the initial report about arsenic-eating microbes as a "huge deal," but with the publication of two peer-reviewed papers that have refuted that report, the space agency now says the picture is "as yet incomplete."

The statement from Michael H. New, astrobiology discipline scientist at NASA Headquarters' Planetary Science Division, runs counter to the instant reaction that the "arsenic-life" controversy is finished. Since Sunday's online release of the two papers by the journal Science, a lot of folks have been talking about FAILs and nails (as in last nails in the coffin).

New took a different tack:

"NASA supports robust and continuous peer review of any scientific finding, especially discoveries with wide-ranging implications. It was expected that the 2010 Wolfe-Simon et al. Science paper would not be exempt from such standard scientific practices, and in fact, was anticipated to generate significant scientific attention given the surprising results in that paper. The two new papers published in Science on the microorganism GFAJ-1 exemplify this process and provide important new insights. Though these new papers challenge some of the conclusions of the original paper, neither paper invalidates the 2010 observations of a remarkable microorganism that can survive in a highly phosphate-poor and arsenic-rich environment toxic to many other microorganisms. What has emerged from these three papers is an as yet incomplete picture of GFAJ-1 that clearly calls for additional research."

University of British Columbia microbiologist Rosie Redfield, one of the authors of one of the newly published papers, said in a blog posting that NASA's response was "cowardly."

"I'm at a loss for words," she wrote.

It's easy to find commentaries on the Web indicting NASA as well as the authors of the original paper, scientific reviewers, the journal Science and journalists for their part in the arsenic-life controversy. Just as some folks scrambled to trumpet the news that evidence of life had been discovered on Titan, now there's a scramble to assign blame. But scientific sagas don't move as quickly as a Twitter stream, and it's a good bet that this particular saga isn't over quite yet.

Here's a sampling of the reaction:

• Washington Post: Journal retreats, authors stand ground
• Guest opinion on Retraction Watch: Science should issue retraction
• Phylogenomics on Storify: Twitter stream for #ArsenicLife
• Q&A on USA Today: Arsenic life studies released

Got more reaction? Feel free to pass along links or voice your own thoughts in a comment below.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

A year and a half after one team of researchers claimed they had bred a type of bacteria that could live on arsenic, suggesting that life is weirder than we imagine, two other teams have found that the microbe really doesn't do anything with the arsenic after all.

These two teams say that the microbe, known as GFAJ-1, is somewhat weird, due to the fact that it can survive amid ultra-high concentrations of arsenic. But they confirm the widely held view among microbiologists that GFAJ-1 did not rewrite the existing rules of life — an extraordinary claim that was implied by the initial study, which made a huge splash in December 2010.

"The new research clearly shows that the bacterium, GFAJ-1, cannot substitute arsenic for phosphorus," the journal Science, which published the initial findings as well as today's follow-up studies, said in an editorial statement.

Case closed?
One of the authors of the new research, University of British Columbia microbiologist Rosie Redfield, was among the most outspoken critics of the original study — and she said that as far as she was concerned, today's publication closes the case. "This isn't an area I have any special interest in, or any funding for," she told me in an email.

Over the past 19 months, Redfield has focused on the analysis of GFAJ-1's DNA more as a case study in open science — a perspective that focuses on freely sharing the results of the research process as they come to light. The study that she and her colleagues authored has been available for months on the ArXiv pre-print website. (The other study, conducted by researchers at ETH Zurich in Switzerland, became public just today.)

Science's editors decided to time today's online publication of the two studies to coincide with a talk that Redfield was due to give at a conference in Ottawa on evolutionary biology. Redfield said last week on her blog that she'd be discussing the results of her group's research, including the Science paper, during her talk. When I contacted her on Friday about the impending publication, she expressed surprise that the journal accelerated its publishing schedule.

"What? No!" she wrote in an initial email. "Must be because of my Evolpalooza talk that night."

The print version of the papers released tonight are to appear in Science later this month.

In the past, the lead researcher for the original study of GFAJ-1, Felisa Wolfe-Simon, has declined to comment in detail about the follow-up experiments that have raised questions about her group's work. She has said such comments would have to wait until those experiments were described in peer-reviewed research articles. But due to the publication in Science, Wolfe-Simon responded to my emailed inquiries at greater length.

She acknowledged that the follow-up experiments failed to find evidence that compounds containing arsenic, known as arsenates, were being taken up into the molecular machinery of GFAJ-1's life processes, such as DNA. However, she said those experiments were apparently conducted under conditions that differed from those surrounding the original experiment.

"We do not know the history of the cells in these new papers," she wrote. "In general, it requires more evidence to publish something unexpected — e.g., that cells can thrive in arsenic and that arsenate is found inside the cells, than something that everyone expects — e.g. that arsenate is not found inside cells or DNA.

"Our original work and data was in fact given high scrutiny, as standards are almost always higher for evidence for things that are unexpected. We are actively following the arsenic in our cells and will know more in the next few months." (The full email exchange is laid out in a comment below.)

Sensationalism and skepticism
The original point of the arsenic-life experiment was to see whether organisms on Earth could be coaxed to use arsenic, which generally acts as a poison, in place of phosphorus, which is generally seen as one of the essential chemical building blocks of life. The structure of those two elements on the atomic level is similar, which is a big reason why substituting one for the other is so lethal.

If some types of organisms, even bacteria, could live on arsenic, that would upset the mainstream view of how life works. Such a finding, if confirmed, would potentially lead to a wider search for "weird life" — not only on Earth, but also in extraterrestrial environments such as the Martian subsurface or the hydrocarbon lakes of Titan.

Wolfe-Simon and her colleagues conducted their search for arsenic-eating life by taking samples from the arsenic-rich sediments of California's Mono Lake, then turning up the dial on the arsenic and turning down the dial on the phosphorus in their laboratory's cell cultures. They isolated a strain of bacteria that grew in a setting with ultra-high concentrations of arsenic and seemingly negligible amounts of phosphorus. (The strain's name, GFAJ-1, stands for "Give Felisa a Job.")

Analysis of the cells led them to conclude that arsenic was being used in place of phosphorus, even in GFAJ-1's DNA molecules. The findings created a sensation when they were announced. "We're talking about an organism that we think ... is replacing phosphorus with arsenic," Mary Voytek, the head of NASA's astrobiology program, said at the time. "This is a huge deal."

The case sparked a huge backlash as well. Many scientists questioned the results — not only in comments to journalists, but also in blog postings and Twitter updates. Redfield suspected that the detection of arsenic was due to sample contamination rather than an uptake into DNA molecules. The experiment in which she was involved, conducted with Princeton's Marshall Louis Reaves as lead researcher, reported finding "only trace amounts of free arsenate" and no chemically bound arsenic compounds in the DNA samples they extracted from GFAJ-1.

In their Science paper, the researchers say the reason for the dramatically different results "is not clear," but they also note that "differences in DNA purity can readily explain" the discrepancies.

How GFAJ-1 works
The other study published today, with ETH Zurich's Tobias Erb as lead author, takes a wide-angle view of GFAJ-1, using mass spectrometry and other tools to trace the bacteria's chemical processes on the molecular level. They found that the microbes could grow with even less phosphorus than the tiny amount that was provided in the experiments by Wolfe-Simon and her colleagues. But when the phosphorus concentration was reduced to nearly nothing (less than 0.3 micromolar), no growth was observed.

Some arsenic compounds formed in the culture, but at a level that was more likely associated with non-biological chemical processes, Erb and his colleagues said. They noted that such compounds are also found in garden-variety E. coli bacteria when they're grown in cultures containing arsenic. This suggests that the detection of arsenic-containing compounds "might not be of physiological relevance," they wrote.

The two groups of researchers acknowledged that there was something extraordinary about GFAJ-1, in that it could grow amid ridiculously high concentrations of arsenic — roughly an order of magnitude higher than previously seen for other organisms, the Swiss-based scientists said. "The molecular basis for arsenate resistance in GFAJ-1 might be the subject of further investigations," they wrote.

It's also noteworthy that GFAJ-1 could survive amid ridiculously low concentrations of phosphorus. Wolfe-Simon and her colleagues said that was because the bacteria switched to metabolizing arsenic. But Reaves, Redfield and their colleagues said it was more likely that GFAJ-1 used a metabolic mechanism to enrich the tiny amount of phosphorus it could grab onto.

More research ahead
In her emails, Wolfe-Simon said the data reported in the newly published research did not contradict the thrust of her own studies, which are continuing. She said it's possible that the arsenic compounds taken up by GFAJ-1 become less stable "once cells are broken open."

"We expect to have our own results ready for publication in the next few months," she wrote. "We are focused on the questions, 'Where exactly is the arsenate going?' and 'How does this microbe survive in high arsenate?' These results will speak to the flexibility of the periodic table for life, so [they] merit the most thorough and careful analysis we can achieve."

In their statement, Science's editors took a different perspective.

"The new research shows that GFAJ-1 does not break the long-held rules of life, contrary to how Wolfe-Simon had interpreted her group's data," they said. "The scientific process is a naturally self-correcting one, as scientists attempt to replicate published results. Science is pleased to publish additional information on GFAJ-1, an extraordinarily resistant organism that should be of interest for further study, particularly related to arsenic-tolerant mechanisms."

Redfield agreed that GFAJ-1 was worthy of further study, even if she's not going to be doing it. "I think all organisms turn out to have interesting tweaks," she told me in her email. "We certainly know very little about the biology of GFAJ-1, and there are complications I never sorted out."

So just how big of a deal did the "arsenic life" controversy turn out to be? To my mind, the case seems likely to take its place among the other great disputed claims in science, ranging from cold fusion to Martian nanofossils and the missing-link primate. It also feeds into the debate over the best ways to distribute and verify scientific findings. Lots of folks will be weighing in on these questions over the next day or two, and you can have the last word in the comment section below.

Previous chapters in the weird-life saga:

• DNA study counters arsenic-life claims
• One year later, 'arsenic life' debate still percolates
• Strange find on Titan sparks chatter about life
• Mars methane mystery: What's making the gas?
• What exactly is life, anyway?
• Cosmic Log archive on arsenic life

In addition to Reaves and Redfield, the authors of "Absence of Detectable Arsenate in DNA from Arsenate-Grown GFAJ-1 Cells" include Sunita Sinha, Joshua D. Rabinowitz and Leonid Kruglyak.

In addition to Erb, the authors of "GFAJ-1 Is an Arsenate-Resistant, Phosphate-Dependent Organism" include Patrick Kiefer, Bodon Hattendorf, Detlef Günther and Julia A. Vorholt.

Science said the two papers, along with an editorial statement, were being released at 8 p.m. ET July 8 "to coincide with a related conference." That was a reference to Redfield's talk at the Evolution Ottawa conference.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA file

The Viking 1 lander sent back America's first pictures from the Martian surface in 1976. This picture shows off the lander's U.S. flag and Bicentennial logo as well as the planet's landscape.

Thirty-six years after an experiment conducted by NASA's Mars Viking lander sparked controversial claims about the presence of life on the Red Planet, NASA's next Mars mission could conceivably hint that those claims were correct after all.

At least that's the hope held by the experiment's principal investigator, Gil Levin, who is keeping the Mars Viking flame alive even in retirement. He still thinks Viking was "the most remarkable unmanned mission ever," but he worries that its legacy will be lost amid the scientific shuffle.

"Twenty or thirty years from now, when the economy permits NASA to rise again, there will be missions to Mars, and they will find life, and they will take credit for it and not mention Viking at all," he told me.

It might not take 20 or 30 years to bring Viking back into the spotlight, however. NASA's $2.5 billion Mars Science Laboratory mission is due to deliver the car-sized Curiosity rover to the Red Planet in August — and although the space agency insists that Curiosity doesn't have the capability to detect life, Levin believes it could show that his experiment was on the right track when it detected the chemical traces of organic activity.

GilLevin.com

Gil Levin was principal investigator for the Mars Viking probe's Labeled Release experiment.

Hopes of confirming the presence of life on Mars were riding high when the twin Viking landers touched down on Mars in 1976. The scientific payload included the Labeled Release apparatus, designed by Levin and his colleagues, as well as three other life-detection experiments. The Labeled Release experiment, or LR, was set up to take a bit of Martian soil and add a drop of water containing nutrients tagged with radioactive markers. The air above the mix was then monitored to see if it gave off a radioactive gas such as carbon dioxide or methane. That could be read as an indication that organisms in the soil were metabolizing the nutrients.

If the experiment came up with a positive response, a duplicate soil sample — the control — was heated to a temperature that should have been high enough to destroy microbes, but not to destroy any strong chemicals that might have produced a similar response sans life. 

The good news for Levin and the other life-hunters was that the LR experiment came out positive, and the control experiment came out negative. The bad news was that two of the other experiments came out negative, but they were based on different assumptions about potential Martian life. The really bad news was that the fourth experiment, conducted by Viking's Gas Chromatograph - Mass Spectrometer device, or GCMS, didn't detect any organic molecules in the soil.

The failure to find any organics led most scientists to assume that there was nothing living in the soil. Most scientists assumed that the LR findings were just a fluke. But not Levin.

"If these results are precisely the same as the results from biological entities on Earth, that's hard to get around," he told me. Dozens of explanations have been put forward for the LR results — for example, that the Martian environment is so chemically reactive, due to ultraviolet radiation, that the nutrients were broken down without life playing a part. Levin, however, says those explanations don't match up with the results produced during the LR experiments and the control experiments.

Hoping for new evidence
This might have ended up as one of those cold cases where nobody totally convinces everybody. But Levin says Curiosity's impressive array of scientific equipment could provide some hot new evidence. It has a suite of instruments known as Sample Analysis at Mars, or SAM, which is capable of detecting organic molecules in Martian soil or atmosphere. Another instrument suite, called ChemCam, can fire a laser blast at a soil or rock sample up to 23 feet (7 meters) away and use a spectroscopic imager to analyze the chemical composition of the vaporized material.

"I predict that one or more of these instruments, possibly all of them, will indeed find organic matter that the Viking GCMS missed," Levin said.

Finding organic molecules is not the same as finding life. After all, organic compounds have been detected within the interstellar stuff of distant galaxies, and it wouldn't be earth-shattering to detect them on Mars as well. But it would answer the main objection raised about the LR results.

Even more telling evidence could come from Curiosity's high-resolution cameras. Some of the pictures taken during the Viking mission showed colored patches on Martian rocks that were a fair spectrographic match for the color of lichen on earthly rocks. "The spectra were identical, but of course the images were not sharp enough to be able to make a conclusion, and everybody pooh-poohed it," Levin said.

Curiosity's color cameras will have much better resolution, and Levin said they "could detect sufficient detail to establish whether these might be lichenlike organisms." It might even be possible to take multiple looks at the same rocks, and track whether their appearance goes through the kinds of changes one would expect from lichen.

Levin said lichen, which is one of the hardiest types of organisms on Earth's surface, could conceivably have hitchhiked from Earth to Mars on meteorites. "Preserved, frozen, they could survive the entry to Mars and grow under Martian conditions," he told me.

The long search for life
The scientists who are in charge of Curiosity and the Mars Science Laboratory say that they're aiming for the same goal that Levin has in mind, but they argue that the search for life on Mars has to follow a step-by-step process.

"What the world needs to understand is that this is really the very beginning of a very systematic and deliberate form of exploration," Caltech's John Grotzinger, principal investigator for Mars Science Laboratory, told me. "The era of 'Star Trek' exploration is not over, but ... one must be more deliberate about it, because that's the way we do it on Earth, and we know that works."

Levin, however, thinks the evidence to come will show that Viking was working correctly 36 years ago. "To suggest that we should go back and start at a lower level ... means we throw away a billion dollars, in 1976 dollars. That's about $5 billion or $6 billion today that we don't have," he said.

He'd like to see a future Mars mission duplicate the LR experiment with a few added technological twists, including a check to see whether the active agent that Viking detected in the soil shows a preference for lefthanded or righthanded versions of the same molecule. Levin says that characteristic, known as chiral preference, would be strong confirmation of life, "since chemistry cannot distinguish chirality and reactions occur equally with both 'mirror images.'"

Levin also thinks the findings from Viking should be given another good, hard look.

"Let's convene a panel of astrobiologists," Levin said. "Let's have Levin present his data. Let's have the antagonists present their data. Let's examine this trove of data which we've never examined fairly."

Will that happen in Levin's lifetime? The researcher is now 88 years old, and nobody lives forever. But he's hoping that when the next episode in the saga of the search for life on Mars plays out ... maybe in the next few months ... the Viking missions will get their share of the spotlight.

"The stories increasingly omit any mention of Viking," Levin said. "I think Viking should be lauded rather than ignored."

More about Viking and the Mars saga:

• How the hunt for Mars life evolved
• Study suggests Viking found organics on Mars
• Were life's building blocks picked up on Mars?
• Did Viking probes find Mars life ... or kill it off?
• Did life on Earth actually come from early Mars?

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

This image shows a type of bacteria called GFAJ-1 that was said to incorporate arsenic in its cellular machinery.

Researchers say they ran a more rigorous version of the experiment that sparked a yearlong debate over the prospects for arsenic-based bacteria — but found no trace of arsenic within the organisms' DNA.

The findings, submitted to the journal Science this week and distributed openly via the ArxiV.org website, serve as the most definitive refutation to date of the "weird life" claims that caused such a stir in December 2010. "They match with what basically all the scientists had concluded a year ago," University of British Columbia microbiologist Rosie Redfield, the paper's senior author, told me.

Redfield had criticized the original study from the start, suggesting that the arsenic detected in a strain of bacteria known as GFAJ-1 was not actually incorporated into the machinery of life but was merely the result of insufficient purification. "We were much more meticulous about purifying the DNA before we analyzed it," she said today.

She and her colleagues worked with the same bacteria used for the original research, which had astrobiologist Felisa Wolfe-Simon as lead author and was published in Science. The bacteria were bred to live in a high-arsenic environment, with virtually no phosphorus present. The aim was to see whether arsenic compounds known as arsenates, which are typically poisonous to life as we know it, could be substituted for chemically similar phosphorus compounds known as phosphates. If that turned out to be the case, that would suggest that alien life forms could operate using biochemical processes radically different from Earth's.

In their paper, Wolfe-Simon and her colleagues said they saw evidence that the bacteria could be bred to live in the arsenic-rich environment, and that arsenates were detected in "macromolecules that normally contain phosphate, most notably nucleic acids and proteins."

Redfield and her colleagues were able to grow the bacteria amid high arsenic levels, under special conditions, but they found that the arsenic wasn't necessary for the bacteria's survival — and that the highly purified DNA from the bacteria did not contain detectable levels of arsenate.

Redfield noted that some arsenate stuck to the DNA even after what she thought would be sufficient purification, but was removed during a second round of washing. "That shows that arsenate does persist through steps in the DNA purification, but in a form that will wash away," she told me.

The researchers acknowledged that arsenate might occasionally get into the bacteria's biological machinery.

"Given the chemical similarity of arsenate to phosphate, it is likely that GFAJ-1 may sometimes assimilate arsenate into some small molecules in place of phosphate, such as sugar phosphates or nucleotides. Our results do not rule out the possibility that such assimilation could be beneficial," they wrote. "When it comes to DNA synthesis, however, GFAJ-1 does not appear to productively assimilate any arsenate."

Open review for results
The scientists behind the original study have said they would refrain from commenting on follow-up research until the peer-review and publication process is completed. Wolfe-Simon did not immediately respond to an emailed request for comment, but Science News' Rachel Ehrenberg quoted her as saying she and her colleagues never actually claimed that arsenate was being incorporated in GFAJ-1’s DNA.

"As far as we know, all the data in our paper still stand,” Science News quoted Wolfe-Simon as saying in an email. “Yet, it may take some time to accurately establish where the [arsenic] ends up."

That response left Redfield figuratively scratching her head — and literally wondering "WTF??" in a Twitter update. She pointed to several references in the original Science paper referring to DNA, including a sentence saying that the measurements "specifically demonstrated that purified DNA extracted from +As/-P [high-arsenic, low-phosphorus] cells contained As [arsenic]."

The paper written by Redfield and her colleagues is open for review and comment even in advance of its consideration for journal publication. That's consistent with Redfield's advocacy of an "open science" approach to research, as reflected in the regular updates posted to her RRResearch blog. Thanks to the blog, avid followers of the #ArsenicLife issue have known for weeks that the original results couldn't be replicated.

Redfield said she has received assurances that freely distributing the draft paper won't hurt the prospects for publication in Science — which goes against the traditional grain for peer-reviewed publication.

"What's happening, and I'm really pleased by this, is that interested people are reading the manuscript, and they're putting comments on it," she observed.

Redfield said she and her colleagues appreciated the feedback being posted to her blog by experts — as well as by non-experts. "Their comments are going to let us polish the manuscript to make it more accessible to non-experts," she told me.

More about the arsenic-life debate:

• A year later, the debate still percolates
• Nature: Study challenges existence of arsenic-based life
• Chemical & Engineering News: The arsenic aftermath

In addition to Redfield, the authors of "Absence of Arsenate in DNA From Arsenate-Grown GFAJ-1 Cells" include M.L. Reaves, S. Sinha, J.D. Rabinowitz and L. Kruglyak.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

This artist's impression shows Earth alongside the super-Earth known as 55 Cancri C, which is thought to be a little more than twice as wide as our planet and 7.8 times as massive.

Two years ago, Harvard astronomer Dimitar Sasselov stunned the world when he claimed there might well be 100 million Earth-size planets in the Milky Way. To some, the number sounded shockingly high. But the torrents of data that have come in from planet-hunters since then suggest that, if anything, the estimate was almost laughably low.

Just this month, researchers reported that there are probably more planets than stars in our galaxy, which would bring the total count well past the 100 billion mark. What's more, astronomers say the planets toward the lower end of the scale — "super-Earths" that are up to 10 times as massive as our own planet — are likely to be more common than Jupiter-scale planets.

"Small planets are really much more abundant than big planets," Sasselov told me last week.

Planet-hunters have already identified more than two dozen super-Earths beyond our solar system, including a batch of 16 announced on a single day last September. A couple of weeks ago, scientists spread the news about three planets smaller than Earth, and last week the science team for NASA's Kepler space telescope mission added still more super-Earths to the list.

That kind of planetary plenitude has even had an impact on the funny pages: "I don't know why this isn't the only thing people are talking about!" one character told another last week in the Arlo & Janis comic strip.

Basic Books

"The Life of Super-Earths" focuses on how the hunt for alien worlds and artificial cells will revolutionlize life on our planet.

It's the main thing that Sasselov is talking about, for more than one reason. He's a co-investigator for the $600 million Kepler mission, the director of the Harvard Origins of Life Initiative, and the author of a new book titled "The Life of Super-Earths." In the book, he makes the case that super-Earths could be as hospitable to life as our own planet, and perhaps even more so. Super-Earths that lie in the habitable zones around their parent stars — that is, the zones where water can exist in liquid form — would be prime candidates in the search for signs of extraterrestrial life.

"Life is not rare, it seems," the Bulgarian-born astronomer says.

Sasselov talked about the Kepler mission, the plenitude of planets and its implications for the search for alien life during our wide-ranging interview. Here's an edited transcript of last week's Q&A:

Cosmic Log: Do you look back at your estimate from two years ago and just shake your head at the idea that you were guessing so low? Were people making a fuss over something that now seems obvious?

Dimitar Sasselov:I feel that I was on the right track. Basically, yes, we have on one hand an even larger number of planetary candidates than I anticipated two years ago. The numbers went up. However, there is also a result which cancels those large numbers. There is a fly in the ointment. The caveat is that as it happens, most of our planetary candidates and confirmed planets are in relatively short orbits.

That means two things. First of all, they don’t directly tell you what the exact prediction about planets in the habitable zone should be.

Second, a lot of our small-planet candidates are in compact, multi-planet systems. Planets are closely packed next to each other, and these planets usually are within the orbit of Mercury around a star which is not that different from the sun. So there must be something extraordinary about the way they formed. It's quite possible that the formation and evolution required to create such architectures in planetary orbits is different in some fundamental way from planetary formation and early evolution in our solar system.

Jon Chase / Harvard

Dimitar Sasselov is a professor of astronomy at Harvard University.

So it is still a question mark as to what these planets are telling us, and what they are made of.

For the Kepler-11 system, we have the mean density of the planets. Those little planets are very low-density planets. They’re nothing like a bigger version of Earth. They have envelopes of hydrogen, or probably hydrogen and helium. They're like mini-versions of Neptune and Uranus. There are no planets like that in our solar system, so we don't know much about them.

It’s a cautionary tale there. Yes, there may be plenty of planets that are just two to three times more massive than our own Earth. But their mean density may be very low, because they formed farther out and migrated inward, and ended up in the moderate temperature regions of their planetary systems.

What would happen if we have a very large number, maybe billions, of super-Earth-size planets in the habitable zones — but half of them, or even nine out of 10 of them, are these mini-Neptunes? Would I consider them Earthlike? Definitely not, because they don't have the same geochemistry.

So while on one hand, the numbers have gone beyond my expectations, the diversity has gone beyond my expectations, too. And that means we might have a lot of planets with something different from an Earthlike geochemistry. Looking at the physics and the geochemistry is the only way we can go to the next step — and that is the search for signatures of life.

Q: What is the next step? How do you go from Kepler and planet detection to getting at the more fundamental questions?

A: To me, the next big step is to go from discovery and detection of planets like our Earth, to understanding their geochemistry. We have to do that to be effective in searching for biosignatures. The way we would do the first step — that is, understanding geochemistry — is by finding enough planets that are close to us. Kepler's candidates are a little bit too far for a good follow-up on characterization. So in terms of a practical approach, we should be gearing up for surveys of the nearby population of stars, and discovering those nearby planets.

There, the news from Kepler is good, because the statistics are high. If the statistics were low, then it would take more of an effort. Once we make that survey, and we can practically accomplish that in the next 10 years, we can jump onto those planetary candidates, and do atmospheric analysis, and try to understand the diversity of their atmospheres. This is a necessary step to talk about the signatures of life. Otherwise, we'd be looking blindly.

Q: Some people might say, well, let's just look for oxygen or methane, or something we associate with life on Earth. 

A: That wouldn't be prudent at all. If we just look at biosignatures as we understand them on our own Earth today, they correspond to a particular moment in time in which the microbial communities on this planet have managed to change the atmosphere in a particular way. For about half of the history of life on Earth, the atmosphere wasn't anything like what it is today. It would be foolish to just assume that all life shares the same biochemistry and the same history.

Theoretically speaking, we should not assume that all planets that otherwise resemble Earth have the same geochemical cycle. There are alternatives.

Q: What sort of mission would work for this next step?

A: There are two approaches that need to be taken. The first one, when it comes to discovery, is a combination of space- and ground-based surveys. The space surveys would use smaller arrays of telescopes in orbit, and would scan the entire sky by observing the brightest stars, nearest to us, in a selective manner. But as opposed to concentrating in one direction, which was necessary due to the design of Kepler, we can select the nearest stars over the entire sky.

This can also be done from the ground for a particular subset of stars, which are the M stars. These stars are so much smaller than a sunlike star that the transits for Earth-size planets are much more prominent. You can see them using ground-based telescopes. You don't need to go to space. The trick is to do the whole sky and catch all those M dwarfs, and catch the transits.

Q: One of themes in your book is that we shouldn't limit the planet search to Earth-size planets, because the planets that are bigger than Earth — the super-Earths — might be more conducive to life than even our own planet. How can that be?

A: What we're finding out about super-Earths places them front and center as the most suitable places for life to emerge. These are planets that are only slightly bigger than Earth. In terms of size, we're talking about an average of 50 percent larger. In terms of mass, we're talking about two, three, five times as massive — maybe 10 in some cases, but overall, made of the same stuff.

Then you just compare the whole range of planets, from Mars to Earth to the largest super-Earths. In all different levels of comparison, the super-Earths end up being equal or slightly better when compared with Earth.

For example, one of the problems a planet could encounter is the ability to keep water liquid on the surface, and to have the good chemical exchange between the interior and the surface. That’s very difficult to do if you don’t have an atmosphere. An atmosphere in the habitable zone is difficult to keep, because it evaporates over the course of billions of years. If you have a small planet, made of rock but still low mass, like Mars is, eventually you lose more of your atmosphere than if you have a bigger planet. There is no negative factor, it is just more of a good thing.

Here's another example. A lot of people would say we have it good here on Earth because the moon keeps the axis of Earth's rotation more stable than it otherwise would be. It's the kind of momentum effect you get when you're on a bicycle — you can let the handlebars go and you still go straight. In a similar way, the existence of the moon out there cancels out the additional push and pull from the other planets, which could from time to time turn the axis of Earth dramatically and change the climate. This is what we think happened a few times on Mars. The more massive a planet is, the less vulnerable it would be to these effects.

Q: Is it always "the bigger, the better," until you get into a Neptune-class ice giant?

A: It's always the bigger the better. There's either no difference, or it's better. I didn’t find anything which was actually detrimental about having a big planet. Larger g-force, having more gravity on the surface, has a small effect when it comes to building biological structures, such as the membranes of cells. The list goes on and on. Everything gets better when you're slightly bigger.

Q: How long do you expect this book to stand up? I suppose that's an occupational hazard when you're writing about planet-hunting.

A: I would say it should stand up until we discover life out there on another planet, or in the lab when we manage to put it together as an artificial minimal cell. Then, of course, we'll open a whole new chapter in the history of science — and it will be so exciting that I wouldn't care. If a new book needs to be written, I will be happy to do so.

More about the planet search:

• NASA mission piles on the planets
• 160 billion planets in the Milky Way?!
• Three newfound worlds are smaller than Earth
• Flash interactive: How other worlds are found
• SETI researchers check signals in exoplanet study
• Millions of Earths? Talk causes a stir
• Cosmic Log archive on planets

Dimitar Sasselov will talk about the planet search during a book tour that takes him to Boston on Thursday and on Feb. 17, to New York on Feb. 6, San Francisco on Feb. 8 and Seattle on Feb. 10.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

Planetary Habitability Lab / UPR

This "periodic table" of exoplanets, including confirmed planets as well as candidates from NASA's Kepler mission, places exoplanets into 18 categories based on mass and temperature. The numbers keep track of how many worlds are in which categories. Click on the image to see a larger, more readable version.

Researchers have set up an online "periodic table" for extrasolar planets ranging from Hot Mercurians to Cold Jovians, with Earthlike worlds right in the middle. 

The Habitable Exoplanets Catalog, drawn up by the University of Puerto Rico's Planetary Habitability Laboratory, is aimed at pigeonholing the hundreds of worlds that are being identified by NASA's Kepler space telescope and other planet-hunting projects. Eventually, the tally of exoplanets is expected to mount into the thousands, and that's where researchers hope the proposed catalog will come in handy.

"One important outcome of these rankings is the ability to compare exoplanets from best to worst candidates for life," Abel Mendez, the laboratory's director and principal investigator for the project, said today in a news release.

Also today, Kepler's scientists said they've confirmed the existence of their first exoplanet solidly within the habitable zone of its solar system, where water could exist in liquid form at a pleasant 72 degrees Fahrenheit (22 degrees Celsius). That certainly sounds livable, but Mendez told me that the planet, known as Kepler-22b, doesn't quite fit into the sweet spot for habitability because it's closer in size to Neptune than to Earth.

"I confirmed its radius, and Kepler-22b is a low-end Warm Neptunian, very close to a Superterran," Mendez said in a Twitter back-and-forth from NASA's Ames Research Center in California, where he was presenting his research at the Kepler Science Conference.

Neptunians are likely to have a gaseous rather than a rocky composition, which might make it tough for life as we know it on Kepler-22b. However, the situation might be more hospitable on a moon orbiting the planet, just as it is in the movie "Avatar" for the inhabitants of Pandora, a fictional moon orbiting the gas giant Prometheus.

How the catalog was created
The Habitable Exoplanets Catalog sets up a matrix of 18 pigeonholes based on temperature and mass: Planets in the Hot Zone would be too close to their parent suns for water to exist in liquid form. Water would exist only as ice in the Cold Zone, but could take liquid form in the Warm Zone. The catalog sets up six categories of planetary mass: Mercurians (think Mercury), Subterrans (Mars-size), Terrans (Earth-size), Superterrans (up to 10 times as massive as Earth), Neptunians (Neptune-size) and Jovians (Jupiter-size).

To figure out which planets fit which categories, the catalog draws upon a variety of resources, including the Kepler database of candidates, the Extrasolar Planets Encyclopaedia, the Exoplanet Data Explorer, the Earth Similarity Index, the Habitable Zones Distance metric and the Global Primary Habitability index.

The initial classification of more than 1,600 confirmed planets and yet-to-be-confirmed candidates puts only 16 potential worlds in the habitable categories — that is, Warm Subterrans, Warm Terrans and Warm Superterrans. But that list will grow: The Kepler team announced today that its tally of candidates has risen to 2,326, based on the first 16 months of the space telescope's mission. Forty-eight of those candidates are said to lie in their stars' habitable zones.

"The tremendous growth in the number of Earth-size candidates tells us that we're honing in on the planets Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable," Natalie Batalha, Kepler's deputy science team lead at San Jose State University, said in a NASA news release. "The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods."

Mendez and his colleagues are working on software to keep the Habitable Exoplanets Catalog updated. "The computers are doing the job," he told me. "I am trying to automate everything, but it takes time."

Right now, the world in the database that's judged most similar to Earth is a candidate known as KOI 736.01, which is 1,750 light-years away and is estimated to have a surface temperature of 55 degrees F (286 Kelvin). But the top prospect for surface habitability is KOI 255.01, a Warm Superterran that's 1,169 light-years away with a surface temperature of 86 degrees F (303 K). Some researchers believe super-Earths can be even more conducive to life than Earth.

Gliese 581d, a world that orbits a red dwarf just 20 light-years from Earth, shows up among the Sweet 16 on both lists.

The search revs up
So what's next? "I hope this database will help increase interest in building a big space-based telescope to observe exoplanets directly and look for possible signatures of life," Jim Kasting, a planetary scientist from Penn State, said in the Planetary Habitability Laboratory's news release.

A habitability index could help scientists set the priorities for future observations, but they don't necessarily need to wait until a new super-space telescope is launched. During the Kepler conference, the California-based SETI Institute announced that it was once again searching planetary systems for radio signals that could serve as evidence of extraterrestrial intelligence. Some of Kepler's planetary candidates are among its first targets.

"For the first time, we can point our telescopes at stars and know that those stars actually host planetary systems — including at least one that begins to approximate an Earth analog in the habitable zone around its host star," Jill Tarter, director of the institute's Center for SETI Research, said in a news release. "That's the type of world that might be home to a civilization capable of building radio transmitters."

Tarter and her colleagues makes use of the Allen Telescope Array, a network of radio antennas in northern California that had to be put into hibernation due to money troubles. The SETI Institute was able to restart work at the array thanks to contributions made by the public through the SETIStars.org website, as well as funding from the U.S. Air Force to assess the array's utility for space situational awareness (that is, monitoring the skies for hazardous asteroids and space debris).

Tarter said the highest priority would be given to Kepler planets that are located within their stars' habitable zones. But the search for extraterrestrial intelligence won't stop there.

"In SETI, as with all research, preconceived notions such as habitable zones could be barriers to discovery," she said. "So, with sufficient future funding from our donores, it's our intention to examile all of the planetary systems found by Kepler."

More about the planet quest:

• Which alien worlds are most livable?
• City lights could point to E.T.'s home
• Super-Earth on the 'edge of habitability'
• Interactive: How scientists search for planets
• Astronomers find 18 alien planets, and they're huge

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

ESO

One of the several planets within the Gliese 581 star system, called Gliese 581d, ranks among the most potentially habitable alien worlds on a new scale.

Astronomers have come up with a livability index for alien planets and moons, and the winners are ... Titan in our own solar system, and the Gliese 581 planets in the extrasolar league.

Rating systems for Earthlike and habitable planets may not make much difference now, but the developers of the Earth Similarity Index and the Planet Habitability Index say they could be crucial in the years ahead.

"With a new generation of telescopes and missions on the way, the discovery of many more exoplanets can be expected," they write in a paper to be published in the December issue of the journal Astrobiology. "That, in turn, will drive the need for a classification scheme for assigning astrobiological potential for exoplanets based on estimates derived from quantitative data of their probability for supporting life."

If such a scheme could truly reflect whether or not a given planetary environment is habitable, that could drive the priorities for exploration in our own solar system, as well as high-resolution observations of extrasolar systems.

Habitability indexes have been in the works for at least the past couple of years. Traditionally, astrobiologists have focused on three conditions that appear essential for life on Earth: organic compounds, the presence of liquid water, and an energy source such as the sun or undersea volcanoes. But in the search for alien Earths, those conditions aren't easily determined, and they may even be irrelevant.

The newly proposed indexes take a two-track approach to the classification challenge.

"The first question is whether Earthlike conditions can be found on other worlds, since we know empirically that those conditions could harbor life," Dirk Schulze-Makuch, an astrobiologist at Washington State University who is one of the study authors, said in a news release. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."

The Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as the size and temperature of its parent star, and compares those parameters with Earth's. Earth has the maximum global ESI of 1. Mars has a 0.70 rating, and Mercury is the next on the list with 0.60. For what it's worth, the dwarf planet Pluto and Neptune's moon Triton register a measly 0.075 and 0.074, respectively. And Enceladus, the icy Saturnian moon that is thought to harbor a subsurface ocean and perhaps life, is right down there with them at 0.094.

Looking beyond the solar system, the researchers worked up ESI values for a variety of extrasolar planets. The top finishers were Gliese 581g (whose existence is in dispute) with 0.89, and Gliese 581d with 0.74.

But that's just the first part of the job: The researchers' Planet Habitability Index looks at a different set of factors: Does the planet have a rocky or frozen surface? Is there an atmosphere, and how thick is it? How about a magnetic field? How much energy is available, either through tidal flexing or from the parent star? Could there be organics present, and is a liquid solvent available for chemical interactions?

By those measures, Earth has a relative PHI of 0.96, which is nearly as close as you can get to the maximum of 1. Based on what's known about the rest of the solar system, the runner-up is not Mars, as you might expect, but the Saturnian moon Titan (0.64 vs. 0.59 for Mars). The Jovian moon Europa is next on the list (0.47), but Enceladus (0.35) ranks lower than Venus, Jupiter and Saturn (0.37).

The authors stress that expectations based on earthly life may not apply to extraterrestrial environments.

"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” they write. "For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life."

So how does the Gliese 581 system's PHI look? Gliese 581g's value was estimated at 0.45, 581d registered 0.43, and 581c came in at 0.41. By that scale, the chances of finding life in a red-dwarf system 20.5 light-years away (or sustaining life if we ever get there) are about as good as they are for Europa. OK, but not great.

It's important to keep a couple of things in mind about this research: First of all, there's a fair amount of speculation about the various factors and their relative value for habitability. Further observations may shift the values for those factors, as well as the mathematical formula into which they're fed.

Perhaps more importantly, the numbers game can't take the place of actual observation and exploration. The ESI and PHI may well turn out to be thought experiments like the Drake Equation, which takes your assumptions about a variety of cosmic factors (How many planets like Earth come into existence every year? How likely is it that intelligent civilizations arise on alien Earths? How long do they last?) and turns them into a number. At least that's the message from David Morrison, director of the Carl Sagan Center for the Study of Life in the Universe, headquartered at the SETI Institute in Mountain View, Calif.

Here's what Morrison told me in an email:

"Very interesting. Discussing such conceptual indexes is a good way to organize our thinking about worlds that may be suitable for life. But it doesn’t actually add value, in my opinion. For the Earth Similarity Index, we already have thought that liquid water, and a solid surface, and enough gravity to hold on to a substantial atmosphere, are important indications of habitability. Hence the interest in Earth-size planets within the habitable zone (meaning surface liquid water is possible). To go further, as by considering the composition of the atmosphere, we are quickly into the effort to identify life by its chemical signatures, not just habitability. The broader habitability index in also interesting, but we just don’t know how to define habitability. And if Titan is an example, we may never have the data on exoplanets that could distinguish the hydrocarbon liquid lakes that we see on Titan.

"Bottom line: This (like the Drake Equation) is a good teaching tool. It helps is to organize our thoughts. But I doubt it will be very useful as a research tool, because we know so little about what properties truly define habitability. Without a much better idea of what alien life is like, we don't know how to define habitability. And probably nature is much more creative than we can imagine."

What do you think? Where would you target the search for extraterrestrial life, and what criteria would you use to prioritize the targets? Feel free to weigh in with your comments below.

More about the search for alien life:

• A new equation for life
• Case builds for habitable alien planet
• Super-Earth on the edge of habitability
• More about astrobiology from Cosmic Log

In addition to Schulze-Makuch, the authors of "A Two-Tiered Approach to Assessing the Habitability of Exoplanets" include Abel Mendez, Alberto G. Fairen, Philip von Paris, Carol Turse, Grayson Boyer, Alfonso F. Davila, Marina Resendes de Sousa Antonio, David Catling and Louis N. Irwin.

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or following the Cosmic Log Google+ page. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.