NASA

An artist's conception shows NASA's Kepler space telescope observing a planetary transit.

NASA is getting plenty of advice — and sympathy — as it assesses whether its Kepler planet-hunting telescope can be revived after the failure of its reaction-control system. The reactions from scientists and engineers range from repair tips to an Audenesque elegy. Here's a sampling:

How to fix Kepler
The reason why the $600 million Kepler spacecraft can no longer search for planetary transits is that two of its four gyroscopic reaction wheels can no longer spin. Mission managers say Kepler needs at least three of those wheels in working order to hold its position still enough to stare at alien stars.

The most recent part to fail is known as reaction wheel 4. The mission's deputy project manager, Charlie Sobeck, told reporters that the Kepler team could try putting some reverse torque on that wheel in hopes of freeing it up.

Two other possibilities were raised by Scott Hubbard, who headed NASA's Ames Research Center during the development of the Kepler mission and is now a consulting professor of aeronautics and astronautics at Stanford University.

One option would be to try turning on reaction wheel 2, which failed last July. "It was putting metal on metal, and the friction was interfering with its operation, so you could see if the lubricant that is in there, having sat quietly, has redistributed itself, and maybe it will work," Hubbard said in a Stanford Q&A.

"The other scheme, and this has never been tried, involves using thrusters and the solar pressure exerted on the solar panels to try and act as a third reaction wheel and provide additional pointing stability," he said. The mission's principal investigator, Ames' Bill Borucki, said on Wednesday the thrusters couldn't hold the spacecraft stable enough for planet-hunting. Nevertheless, it might be one of the options under consideration.

For the time being, Kepler has been put into a holding pattern that should minimize its thruster fuel consumption and give the Kepler team several months to weigh all the options, the costs and the potential scientific benefits.

Going beyond Kepler
Even if the Kepler spacecraft can't be revived, Borucki says that only half of the data collected so far have been fully analyzed. He estimates it'll take another two years or so to complete the analysis.

Meanwhile, NASA has just given the go-ahead its next planet-hunting satellite: the Transiting Exoplanet Survey Satellite, or TESS. That $200 million project would put a telescope array in space in 2017 to perform an all-sky survey, looking for exoplanets in orbit around the nearest and brightest stars. That strategy is markedly different from the one used by Kepler, which stared at a relatively small patch of sky straddling the constellations Cygnus and Vega.

This October, the European Space Agency plans to launch a space probe called Gaia to conduct a census of more than a billion stars in the Milky Way. Gaia could detect thousands of distant planetary systems, and measure their orbits and masses using a technique known as astrometry.

ESA is working on another planet hunter called the Characterizing Exoplanets Satellite, or CHEOPS, which is due for launch in 2017. CHEOPS would conduct high-resolution transit observations of stars that have already been found to host planets. 

The $8.8 billion James Webb Space Telescope, which NASA bills as the successor to the Hubble Space Telescope, could conceivably analyze the atmospheres of alien planets. It's currently due for launch in 2018.

Paying tribute to Kepler
NASA's associate administrator for science, John Grunsfeld, said it's too early to consider Kepler "down and out." But many astronomers fear that Kepler's planet-hunting days are finished.

"I think 'The mission is not over' means 'the mission is over,'" Caltech's Mike Brown said in a Twitter update on Wednesday. "Might be other things it can do. But, kids, I think the mission is over."

Alan Boss, an astrophysicist at the Carnegie Institution for Science who's part of the Kepler team, was similarly downbeat. In an email sent to AAAS MemberCentral, he called this week's setback a "disaster":

"I am afraid that the loss of this second reaction wheel effectively means the partial loss of Kepler's main science goal: determining the frequency of Earth-sized planets orbiting their stars at distances such that liquid water could occur on the planets' surfaces. Kepler has taken an outstandingly impressive four years of data, but we still need another three or so years of outstandingly impressive data to be certain of the frequency of Earth-size planets. Right now we have enough data to make an intelligent extrapolation about what that number is, but that is not the same as actually determining that number. Kepler was planned to do that for us. There is no other mission in sight that can reproduce for us what Kepler was in the process of doing. The upcoming (2017) NASA TESS Mission will help to push the exoplanet field forward, but it is not designed to find Earthlike planets around sunlike stars, like Kepler was."

"This is one of the saddest days in my life. A crippled Kepler may be able to do other things, but it cannot do the one thing it was designed to do."

Another Kepler team member, Geoff Marcy of the University of California at Berkeley, told KQED that he felt dizzy and teary-eyed over the spacecraft's situation. "It’s a loss for our species," he said. "That sounds dramatic, but we pride ourselves as a species of exploration, seeking answers beyond the horizon, answers about our place in the universe. And Kepler was answering those questions."

Marcy went so far as to tweak W.H. Auden's poem "Funeral Blues" to pay tribute to Kepler. Here's the astronomer's elegy to a spacecraft:

Stop all the clocks, cut off the Internet,
Prevent the dog from barking with a juicy bone,
Silence the pianos and with muffled drum
Bring out the coffin, let the mourners come.

Let jet airplanes circle at night overhead
Sky-writing over Cygnus: Kepler is dead.
Put crepe bows round the white necks of doves,
Let the traffic officers wear black cotton gloves.

Kepler was my North, my South, my East and West,
My working week, no weekend rest,
My noon, my midnight, my talks, my song;
I thought Kepler would last forever: I was wrong.

The stars are still wanted now; let's honor every one,
Pack up the moon and dismantle the sun,
Pour away the ocean and sweep up the woods;
For nothing will ever be this good.

With thanks to W.H.Auden.

For a video rendition of "Funeral Blues," check out this clip from "Four Weddings and a Funeral."

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's 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

An artist's conception shows NASA's Kepler space telescope observing a planet making a transit across an alien star. (Star and planet not to scale.)

NASA's planet-hunting Kepler space telescope suffered a second failure in its reaction-wheel control system, forcing a suspension of its search for alien planets while the space agency determines whether the four-year mission is truly finished.

"It's certainly not good news," Charles Sobeck, deputy project manager for the $600 million mission at NASA's Ames Research Center, told reporters Wednesday.

But Sobeck and other mission managers emphasized that there was still a chance that the probe could be revived. "I wouldn't call Kepler down and out just yet," said John Grunsfeld, associate administrator for science at NASA Headquarters.

The problem has to do with the reaction wheels that are part of Kepler's fine-pointing system. The space telescope identifies worlds in far-off solar systems by watching for the telltale dips in starlight when the planet's disk passes over its parent sun. But in order to make those observations, Kepler has to hold itself in a precise position with the aid of four gyroscopic reaction wheels. One of the wheels failed last July, but Kepler could still do the job with the other three.

On Sunday, however, the spacecraft put itself into safe mode when it couldn't stay in its proper orbit around the sun, 40 million miles (64 million kilometers) from Earth. When the mission team did its regular check-up with Kepler on Tuesday, they found that a second reaction wheel wasn't working. In a mission update, NASA said the problem was probably caused by "a structural failure of the wheel bearing."

That forced an end to Kepler's planet quest. "We need three wheels in service to give us the pointing precision to make this work," the mission's principal investigator, William Borucki of NASA Ames, told NBC News.

Sobeck said the spacecraft itself could remain stable as long as it had fuel for its thrusters, but the thrusters aren't capable of providing the precise pointing that Borucki and his colleagues need. Over the next several months, members of the Kepler team will assess their technical options, and gauge what kind of science could be accomplished using those options, said Paul Hertz, astrophysics director at NASA Headquarters. 

There's still a chance that the reaction-wheel system could be restored — for example, by trying to spin the wheel backward and forward, just as you might spin the wheels of a car that's stuck in a snowdrift. Sobeck said it might even be possible to revive the wheel that was shut down last July. "When we turn it on, it just might start spinning, we don't know," he said.

But mission managers also acknowledged that Kepler's planet-hunting days may be over. Hertz pointed out that the spacecraft outlasted its 3.5-year primary mission, and was currently into an extended mission costing $20 million a year.

Even if the spacecraft's control system can't be revived, it will still take another couple of years to analyze the trillions of bits of data already collected, Borucki said. The mission has already identified 132 confirmed planets and 2,740 additional candidates yet to be confirmed. Some of those worlds are thought to lie within the habitable zones of their planetary systems.

"The prime reason for the existence of this mission is to determine whether Earths are common or rare in our galaxy," Borucki said. So far, the evidence suggests that there are billions of Earth-size planets in the Milky Way. Scientists have not yet identified an Earth-size planet in an Earthlike orbit around a sunlike star, but Borucki voiced confidence that the crucial evidence was tucked away somewhere in the readings that have already been beamed down from Kepler.

"I am really delighted, frankly, with what we've accomplished," he said.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's 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'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.

ESO

An artist's impression shows a planetary system around a red dwarf star.

An analysis of data from NASA's Kepler planet-hunting mission suggests that about 6 percent of all red dwarf stars should have habitable, Earth-sized planets — and because red dwarfs are the most common stars in our galaxy, the nearest Earthlike planet could be as close as 13 light-years.

"We thought we would have to search vast distances to find an Earthlike planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted," Harvard astronomer Courtney Dressing, the lead author of the data-crunching study, said in a news release.

That doesn't mean we can just hop out the back door and head for a red dwarf: Although 13 light-years is relatively close in astronomical terms, it would still take more than 50,000 years to cover that distance using current propulsion technology. But the finding could lead astronomers to cast a wider net in the search for the conditions conducive to extraterrestrial life.

Dressing presented her team's findings on Tuesday during a news briefing at the Harvard-Smithsonian Center for Astrophysics, or CfA. The research paper is expected to be published in The Astrophysical Journal.

Red dwarfs are thought to account for about 75 percent of the stars in the Milky Way: They're smaller, cooler and fainter than our sun — so faint, in fact, that no red dwarf is visible to the naked eye. But the fact that they're cooler means that closer-in planets are more likely to be habitable.

That's good news for planet-hunters: The Kepler space telescope, which was launched in 2009, detects alien worlds by looking for the subtle dimming of starlight as a planet passes in front of its alien sun. Closer-in planets would cover more of the star's disk, making them easier to detect.

Dressing and her colleagues sifted through the 158,000 stars targeted by the Kepler probe to identify all the red dwarfs. She said the stars' sizes and temperatures were calculated using computer models "that are more appropriate for these small red stars." Previously, those stellar characteristics were derived using a less precise, one-size-fits-all type of computer model, she said.

The fresh analysis showed that almost all of the stars were smaller and cooler than previously thought. That means the worlds detected around those planets would be proportionately smaller as well, bringing more of them into the Earth-sized category.

The astronomers identified 95 Kepler candidate planets that are circling red dwarfs. When they ran those candidates through their fine-tuned computer model, they found that three of them were roughly Earth-sized, with the right temperature to sustain liquid water and life. And when they factored in their estimates for the proportion of planets that would have gone undetected, due to the limitations of the Kepler mission's observing method, they concluded that 6 percent of all red dwarfs should have an Earth-sized, habitable planet.

"That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought," the CfA's David Charbonneau, a co-author of the study, said in Wednesday's news release.

Because our sun is surrounded by a swarm of red dwarfs, the statistics suggest that the most probable distance for such a habitable planet would be 13 light-years, if all the surrounding stars could be examined with a suitable telescope. Kepler isn't designed for such a survey — but a new type of space telescope, or a big enough network of ground-based telescopes, could take on the job. For example, NASA's James Webb Space Telescope, currently due for launch in 2018, could study the thermal characteristics and perhaps even the atmospheres of nearby red-dwarf planets, Charbonneau said.

The researchers said that a habitable planet circling a red dwarf would be markedly different from Earth: It would probably be locked into an orbit that kept one side of the planet perpetually facing its alien sun. Charbonneau said the heat could conceivably be transported around the globe via a thick atmosphere or ocean.

Also, red dwarfs are known to be quite variable in their emissions, with occasional strong flares of ultraviolet light. "If that were to happen on Earth, it would cause havoc," Charbonneau told journalists.

But Dressing said alien life could conceivably adapt to such stresses. "You don't need an Earth clone to have life," she said.

Red-dwarf planets might have at least one edge over Earth in the habitability department: Astrobiologists have estimated that our planet could be rendered inhospitable to life in the next couple of billion years, due to a long-term increase in solar radiation. Red dwarfs are different in that regard. "They are incredibly long-lived," Charbonneau told journalists. "They never show their age."

He said it's conceivable that some of the planets circling red dwarfs could remain habitable for 10 billion years or more.

Caltech astronomer John Johnson said the newly reported research marks one more step toward taking the search for alien Earths out of the realm of science fiction and putting it squarely in the realm of science fact. "Now I think the conversation is starting in earnest," he said. 

More about the planet search:

• Estimate suggests our galaxy has 17 billion hot Earths
• How to take a trip to Alpha Centauri ... and why
• Kepler space telescope recovers from glitch
• Cosmic Log archive on planets

The three habitable-zone planetary candidates identified in this study are Kepler Object of Interest (KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI 2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI 854.01, 1.7 times the size of Earth in a 56-day orbit. All three are located about 300 to 600 light-years away and orbit stars with temperatures between 5,700 and 5,900 degrees Fahrenheit (3,150 to 3,260 degrees Celsius). For comparison, our sun’s surface is 10,000 degrees F, or 5,500 degrees C.

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.

First published 11 a.m. ET Feb. 6. Last updated 12:53 p.m. ET Feb. 6.

NASA / JPL-Caltech / T. Pyle

An artist's rendering shows a typical close-in Earth-size planet, Kepler-20e, which is about 0.87 times as wide as our planet but orbits its parent star more closely than Mercury orbits our sun.

A simulation based on data from NASA's planet-hunting Kepler mission has determined that about one out of every six stars has an Earth-sized planet, which would translate to at least 17 billion such worlds in our Milky Way galaxy. And that's not even counting the alien Earths we'd want to live on.

These 17 billion planets would be circling their parent stars more closely than Mercury orbits our own sun — which means that, in many cases, the planets would be too hot for liquid water to exist. A few such worlds already have been found, including a "lava planet" known as Alpha Centauri Bb that's just 4.3 light-years away from us.

Someday, the type of simulation that astronomers used to estimate the number of hot Earths can be used to estimate how many habitable Earths could provide a home for life as we know it in the Milky Way. But not just yet.

"For an estimate of Earth-sized planets in the habitable zone, it's simply too early to call," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, or CfA.

Fressin and his colleagues lay out their estimates for Earth-sized planets, as well as bigger worlds, in a paper that's been accepted for publication in the Astrophysical Journal. Their research is being discussed today at the American Astronomical Society's winter meeting in Long Beach, Calif.

The estimates are based on a list of 2,400 planet candidates that have been detected by the Kepler probe since its launch in 2009. Kepler looks for planets in a patch of sky overlapping the constellations Cygnus and Lyra, by checking for the faint dimming of a star as an alien world passes across its disk. One of the challenges is to make sure the dimming is really caused by a planet, rather than some other phenomenon such as an eclipsing binary star. Another challenge is that Kepler is sure to miss some planets, because those planets are not in a position to block the light of its parent star, as seen from Earth.

Now that the Kepler mission has been churning out detections for more than three years, there's enough of a database to arrive at some statistical conclusions about the total number of planets in the Milky Way — at least 100 billion. There's also enough data to determine what the breakdown of detections should be, and even how many of those detections will be wrong.

"We have a knowledge of false positives that's good enough that we can do a study from scratch," Fressin said.

The simulation suggests that the false-positive rate should vary depending on the size of the planet candidates, from a low of 6.7 percent for small Neptune-scale planets to a high of 17.7 percent for Jupiter-type giants. The false-positive rate for close-in planets between 0.8 and 1.25 times as wide as Earth is 12.3 percent. When all these factors were added to the calculations, the astronomers arrived at a breakdown for five types of planets currently detectable by Kepler:

• 17 percent for Earths with orbital periods up to 85 days.
• 26 percent for super-Earths (1.25 to 2 times as wide as Earth) with orbits up to 145 days.
• 26 percent for small Neptunes (2 to 4 times Earth's width) with orbits up to 245 days.
• 3 percent for large Neptunes (4 to 6 times Earth's width) with orbits up to 418 days.
• 5 percent for giants (6 to 22 times Earth's width) with orbits up to 418 days.

The results indicate that for every size of planet except for gas giants, the type of star doesn't matter. Earth-sized planets should be just as likely to form around red dwarfs as around sunlike stars. That runs counter to what was previously thought.

"Earths and super-Earths aren't picky. We're finding them in all kinds of neighborhoods," the CfA's Guillermo Torres, a co-author of the study, said in a news release.

The researchers emphasized that these are just minimum estimates — and that as Kepler provides more planet candidates at smaller scales and wider orbits, the numbers could increase. Eventually, such simulations could spit out a long-sought number: the tally of Earth-sized planets in the Milky Way expected to have conditions capable of supporting life.

"This result is a significant step towards the determination of eta-earth, the occurrence of Earthlike planets in the habitable zone of their parent stars," they wrote in their research paper.

More about the planet search:

• Kepler mission adds 461 potential planets to list
• 'Exocomets' are common across the Milky Way
• 2013 might be the year for first 'alien Earth'
• Alien planets face danger from binary stars
• Cosmic Log archive on the planet search

In addition to Fressin and Torres, the authors of "The False Positive Rate of Kepler nd the Occurrence of Planets" include David Charbonneau, Stephen Bryson, Jessie Christiansen, Courtney Dressing, Jon Jenkins, Lucianne Walkowicz and Natalie Batalha.

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.

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.

The science team for NASA's Kepler planet-hunting mission nearly doubled their list of confirmed planets beyond our solar system in one fell swoop today, announcing the discovery of 26 planets spread among 11 star systems. Their sizes range from just a little bit bigger than Earth to super-Jupiter-size, but they're all closer to their parent stars than Venus is to our own sun.

The accelerating pace of discovery is matched by the diversity seen in the worlds discovered so far, one of the Kepler mission's co-investigators, Harvard astronomer Dimitar Sasselov, told me today.

"There is more diversity out there than our limited imaginations could come up with, which is good," he said.

The $600 million Kepler mission, launched in 2009, now has a list of 61 confirmed planets, and another 2,326 planetary prospects that have yet to be confirmed. At this rate, Kepler's worlds could soon account for the majority of the exoplanets detected beyond our solar system — a tally that now stands at more than 700.

"Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky," Doug Hudgins, Kepler program scientist at NASA Headquarters, said in a news release. "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."

The Kepler space telescope searches for other worlds by staring at more than 150,000 stars in that fist-sized patch of sky, straddling the constellations Cygnus and Lyra. Kepler's instruments can detect the faint dips in starlight that occur on a regular basis as a planet passes over the disk of its parent star, as seen from Earth. By analyzing the patterns of those passes, also known as transits, Kepler's scientists can figure out the orbit and the size of a potential planet — but not its mass.

An alternative method has to be used to confirm that Kepler is actually looking at a planet rather than something else, such as mutually eclipsing binary stars. The mission's early discoveries were confirmed by looking at the candidates' stars with ground-based telescopes and checking for the telltale gravitational wobbles that are caused by big, close-in planets.

Transit timing variations
Most of the planets added to the list today were confirmed using a different backup method. The Kepler mission's astronomers analyzed subtle changes in the intervals between the transits, caused when multiple orbiting planets exert gravitational pull on each other. The resulting data on acceleration and deceleration can be used to confirm the planets' existence and calculate their masses.

"By precisely timing when each planet transits its star, Kepler detected the gravitational tug of the planets on each other, clinching the case for 10 of the newly announced planetary systems," said Dan Fabrycky, an astronomer at the University of California at Santa Cruz and the lead author for a paper confirming four of the planetary systems, known as Kepler-29, 30, 31 and 32.

Other newly confirmed planetary systems include Kepler-25, 26, 27 and 28, described in a paper with Fermilab's Jason Steffen as lead author; and Kepler-23 and 24, which was the focus of research led by the University of Florida's Eric Ford. In today's release, Ford said the transit timing variation method "dramatically accelerated" the pace of planetary discovery.

Yet another study, led by Jack Lissauer, a planetary scientist at NASA's Ames Research Center, detected five planets around Kepler-33, a star that is older and more massive than the sun. Those five planets range in size from 1.5 to five times the width of Earth, and they're all closer to their parent star than Mercury is to our own sun.

"The approach that was used to verify the Kepler-33 planets shows that the overall reliability of Kepler's candidate multiple transiting systems is quite high," Lissauer said in today's release. "This is a validation by multiplicity."

Five of the newly confirmed planetary systems (Kepler-25, 27, 30, 31 and 33) contain a pair of worlds that are bound together in a 1:2 resonance. That means the inner planet makes two circuit for every one circuit made by the outer planet. Four other systems (Kepler-23, 24, 28 and 32) have two planets that are linked in a 2:3 resonance, like Pluto and Neptune in our own solar system.

"These configurations help to amplify the gravitational interactions between the planets, similar to how my sons kick their legs on a swing at the right time to go higher," Steffen said.

Fifteen of the 26 planets announced today are Neptune-size or smaller, and the orbital periods range from six to 143 days. The planets' distances from Earth range from 623 light-years (for Kepler-25) to 4,064 light-years (for Kepler-29).

Great expectations
Sasselov, who has just come out with a book about the Kepler quest titled "The Life of Super-Earths," marveled that so many of the newfound worlds are in multiple-planet systems. He recalled that when the Kepler mission was proposed to NASA, more than a decade ago, "there was one little sentence that said maybe two or three of the systems will have multiple transiting planets."

None of the planets announced today would be conducive to life as we know it, because their orbits are so close to their parent stars. It's likely to be just a matter of time before Kepler achieves its main goal — confirming the existence of Earth-size planets in Earthlike orbits around sunlike stars. Unfortunately, it may be a matter of more time than initially expected.

Funding for the Kepler mission is due to run out in November, but the mission's scientists "don't have enough to statistically complete the core goal of the mission," Sasselov said. It turns out that the data collected by the telescope is "noisier" than expected. That means more observations will be required to confirm the mission's trickiest planetary finds.

The Kepler team has applied for a four-year extension, and is currently waiting for a decision from NASA executives.

More about the planet search:

• 160 billion planets in the Milky Way?!
• Three newfound worlds are smaller than Earth
• Flash interactive: How other worlds are found
• Two new 'Tatooine' planets with two suns
• SETI researchers check signals in exoplanet study
• Cosmic Log archive on planets

The planetary confirmations are described in four research papers:

• "Transit Timing Observations from Kepler: II. Confirmation of Two Multiplanet Systems via a Non-parametric Correlation Analysis'' by E.B. Ford et al., The Astrophysical Journal.
• "Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations," by J.H. Steffen et al., Monthly Notices of the Royal Astronomical Society.
• "Transit Timing Observations from Kepler: IV. Confirmation of 4 Multiple Planet Systems by Simple Physical Models," by D.C. Fabrycky et al., The Astrophysical Journal.
• "Almost All of Kepler's Multiple Planet Candidates are Planets, and Kepler-33 5-planet System," by J. Lissauer et al.

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.

Planet-hunters say they've detected the first world that's absolutely known to circle two stars, like Luke Skywalker's home planet Tatooine in the fictional "Star Wars" saga.

"Once again, what used to be science fiction has turned into reality," said Carnegie Institution astronomer Alan Boss, a member of the team for NASA's Kepler mission and a co-author of a paper on the discovery in the journal Science.

To mark the occasion, NASA invited John Knoll of Industrial Light and Magic, the special-effects company behind the "Star Wars" movies, to sit in on today's announcement. "When I was a kid, I didn't think it was going to be possible to make discoveries like this," Knoll told journalists.

Tatooine serves as the setting for the first movie in the series, released in 1977 and now subtitled "A New Hope." The saga's main character, Luke Skywalker, could watch a double-sunset as he toiled in the desert on his uncle's moisture farm, aided by his trusty robots C-3PO and R2-D2.

Luke probably couldn't stand on the surface of Kepler-16b, which orbits a red and an orange star in the constellation Cygnus, 200 light-years from Earth. It certainly wouldn't be a desert. The planet is most like Saturn in our own solar system — too cold for life as we know it, most likely with a thick, gassy atmosphere. "This one's just outside the habitable zone," the paper's lead author, SETI Institute astronomer Laurance Doyle, told me.

But if Han Solo were to park the Millennium Falcon on one of Kepler-16b's hypothetical moons, there'd be plenty of double-sunsets. In fact, because the two suns orbit each other, each sunset would bring a different configuration, with the small red sun occasionally crossing over the larger orange one. "You might get two eclipses every 41 days," Doyle said.

© Lucasfilm Ltd. & TM. All Rights Reserved

Luke Skywalker surveys a double sunset on the planet Tatooine in "Star Wars: A New Hope."

How the Tatooine planet was found
It's the complex crossings of the suns and the planet that tipped off Doyle and his colleagues to Kepler-16b's existence. NASA's Kepler space telescope, launched in 2009, stares at a 105-square-degree patch of sky in the constellations Cygnus and Lyra, looking for the telltale signs of something dark moving across a star. Kepler watches for periodic dips in the light coming from 155,000 stars. When those dips are detected, scientists use sophisticated software to figure out if the pattern could be caused by a planet.

One of the big challenges is that such dips can also be caused by one of the companions in a double-star system crossing over the other one. This is what's known as an eclipsing binary. The Kepler team has found hundreds of eclipsing binaries, including Kepler 16 — but scientists saw something extra in Kepler 16's pattern of dimming and brightening. "We saw extra dips in the light curve," Doyle recalled.

In the Science paper and 12 pages of supporting material, the Kepler scientists describe the painstaking process used to figure out what was behind those extra dips. They analyzed the pattern of the dips, as well as the varying lengths of time it took for objects to cross over each other (a method known as transit timing variation, or TTV). That resulted in a gravitational model demonstrating that the pattern could only be caused by a planet and two suns passing across each other repeatedly, as seen from Earth's point of view.

R. Hurt (SSC) / JPL-Caltech / NASA

It's theoretically possible for the Kepler-16 system's two suns to line up directly behind the planet Kepler-16b, as shown here.

The team found that Kepler-16b is almost exactly a third as massive as Jupiter, and three-quarters as wide — which makes it comparable to Saturn. It's somewhat denser than Saturn, but not quite as dense as water — which suggests it's half-gassy (with a helium-hydrogen atmosphere) and half-heavy (with an icy-rocky core).

Both of the two suns are smaller and dimmer than our own sun, and they orbit each other once every 41 days. The Kepler-16b planet is in a nearly circular orbit around both stars. It takes 229 days to make one circuit at a distance of 65 million miles — which is similar to the parameters for Venus' 225-day orbit. Because the twin suns are dimmer, Kepler-16b is colder than Venus, with an estimated surface (or cloud-top) temperature of -100 to -150 degrees Fahrenheit (170 to 200 Kelvin).

"You better have your long underwear," Boss joked.

Doyle said it was lucky that Kepler happened to be watching now. The orbital characteristics are such that the planet-sun transits won't be visible from Earth starting in the 2014 time frame. "In 2018, the primary transits will stop for 24 years. And in 2014, the secondary transits will stop for 45 years. Delay Kepler, and a lot wouldn't have happened," he said. 

Looking back and looking ahead
The Kepler team says Kepler-16b is the first confirmed, unambiguous example of a planet orbiting two stars. Several years ago, astronomers wondered whether binary-star systems, which make up more than half of our Milky Way's stellar population, would be too unstable to harbor planets for long. Since then, theoretical models have shown that double-sunset planets could be far more common than previously thought.

There have been a number ofl tentative reports of double-star planets. Last year, astronomers reported detecting a "Tatooine planet" that orbited one of the stars in a binary-star system. That research team used a different analysis method known as astrometry.

Boss said the case for Kepler-16b was more solid, not only because it orbited two stars in a close-in binary system, but also because Kepler's transit observations were "rock-hard solid."

"With astrometric observations, you're always a bit uncertain if it's real," Boss said.

Beyond the "Star Wars" angle, Kepler-16b is significant because it shows once again that a wide variety of star systems can foster planets, and perhaps habitable planets at that. "This is an example of another planetary system, a completely different type that no one's ever seen before," Doyle said. "That's why people are making a big deal out of this."

William Borucki, an astronomer at NASA's Ames Research Center who serves as the Kepler mission's principal investigator, said the research "confirms a new class of planetary systems that could harbor life."

"Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars," Borucki said in a NASA news release. "This milestone discovery confirms a theory that scientists have had for decades but could not prove until now."

Doyle said Kepler-16b almost certainly will not be the last double-sunset planet discovered by the $600 million Kepler mission. When the numbers all added up, "I didn't feel like it's the end of 20 years of searching ... it felt like the beginning of something" he said. "I predict that in the next couple of months, we're going to have some more."

But time's running out for Kepler. Boss noted that the current mission plan calls for the telescope to be "out of business one year from now." That would be a shame, Boss said, because it looks as if it will take longer than expected for Kepler to get the data to identify Earthlike planets in Earthlike orbits around sunlike stars — which is the mission's prime objective. The reason for that is that the readings from alien suns are unusually noisy. "It turns out that most stars are not as quiet as the sun," Boss said.

Kepler's scientists are already talking about seeking an extension of the mission. That could be a challenge in this era of tightening budgets, but Boss argues that it could be a long time before NASA gets another opportunity to launch a planet-hunting mission.

"Kepler has become, in essence, our only Terrestrial Planet Finder," Boss said. "This is it, for the foreseeable future."

Extra credit: Doyle says that anyone with a good telescope (8-inch mirror or larger) and a CCD camera could record a Kepler 16 planetary transit next June 28 from China and other parts of northeastern Asia. The light from the star system would be seen to dip by about 1.7 percent, if observers train their telescopes on the stars at just the right time. "They'll be able to measure the next transit since the discovery of the planet," Doyle said.

More about weird planets:

• Q&A with planet hunter Laurance Doyle
• 'Super-Earth' just might support life
• Planetary six-pack poses a puzzle
• Probe finds planetary 'missing link'
• Planets spotted in changing orbits
• 'Invisible' planet spotted with new technique
• Looking for alien Earths? Here they come
• Cosmic Log archive on the Kepler mission
• Search for Kepler planets on msnbc.com

This report was last updated at 5:20 p.m. ET.

In addition to Doyle and Boss, the authors of "Kepler-16: A Transiting Circumbinary Planet" include Joshua A. Carter, Daniel C. Fabrycky, Robert W. Slawson, Steve B. Howell, Joshua N. Winn, Jerome A. Orosz, Andrej Prsa, William F. Welsh, Samuel N. Quinn, David Latham, Guillermo Torres, Lars A. Buchhave, Geoffrey W. Marcy, Jonathan J. Fortney, Avi Shporer, Eric B. Ford, Jack J. Lissauer, Darin Ragozzine, Michael Rucker, Natalie Batalha, Jon M. Jenkins, William J. Borucki, David Koch, Christopher K. Middour, Jennifer R. Hall, Sean McCauliff, Michael N. Fanelli, Elisa V. Quintana, Matthew J. Holman, Douglas A. Caldwell, Martin Still, Robert P. Stefanik, Warren R. Brown, Gilbert A. Esquerdo, Sumin Tang, Gabor Furesz, John C. Geary, Perry Berlind, Michael L. Calkins, Donald R. Short, Jason H. Steffen, Dimitar Sasselov, Edward W. Dunham, William D. Cochran, Michael R. Haas, Derek Buzasi and Debra Fischer.

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

Gemini Observatory / AURA

The Gemini Observatory's image of Kronberger 61 shows the shell of ionized gas surrounding a dying star.

Professional and amateur astronomers are teaming up to study a cosmic "soccer ball" with a tricky goal in mind: understanding how the death throes of a star are affected by the company it keeps.

The focus of this game is Kronberger 61, a planetary nebula discovered several months ago by Austrian amateur astronomer (and professional physicist) Matthias Kronberger. He belongs to a group called the "Deep Sky Hunters," which combs through imagery from the Digital Sky Survey and other sources looking for celestial objects worthy of further study. The hunters have found about 100 faint planetary nebulae, shells of glowing ionized gas that are thrown off by sunlike stars in the waning years of their lives.

Kronberger 61 is worth noting for aesthetic reasons alone: The image above, captured by the Gemini North telescope in Hawaii, highlights the nebula's emissions from twice-ionized oxygen. The dying star can be seen as a point of bluish light close to the center of the ball-shaped nebula.

But this soccer ball, also known as Kn 61, is also notable because of its location. It happens to be within the Kepler planet-hunting probe's field of view, an 105-square-degree area that takes up about as much of the sky as your hand held at arm's length. There's a chance that Kepler could determine whether there are planets or faint companion stars circling Kn 61's main star.

"Kn 61 is among a rather small collection of planetary nebulae that are strategically placed within Kepler's gaze," Orsola De Marco of Australia's Macquarie University said in the Gemini Observatory's news release about the find. "Explaining the puffs left behind when medium-sized stars like our sun expel their last breaths is a source of heated debate among astronomers, especially the part that companions might play. It literally keeps us up at night!"

The Kepler science team has now added Kn 61 to its target list of more than 150,000 stars, and within months, astronomers might be able to determine whether the star has companions, said George Jacoby of the Giant Magellan Telescope Organization and the Carnegie Observatories (Pasadena). "This was not an object that was known by Kepler to be valuable early on," Jacoby told me.

Jacoby serves as principal investigator for the program to get follow-up observations of Kn 61 with Kepler, and also acts as the liaison with the Deep Sky Hunters.

"Without this close collaboration with amateurs, this discovery would probably not have been made before the end of the Kepler mission," Jacoby said in today's news release. "Professionals, using precious telescope time, aren't as flexible as amateurs who did this using existing data and in their spare time. This was a fantastic pro-am collaboration of discovery."

The Deep Sky Hunters have identified yet another planetary nebula in the Kepler field, and possibly a third prospect. Jacoby said astronomers would be playing an "odds game," hoping that one of the nebulae will reveal something interesting about the effects of companion objects on a dying star's gaseous shell. If the gamble pays off, the scientific payoff could be significant.

De Marco said that planetary nebula present a "profound mystery."

"Some recent theories suggest that planetary nebulae form only in close binary or even planetary systems — on the other hand, the conventional textbook explanation is that most stars, even solo stars like our sun, will meet this fate," she said. "That might just be too simple."

Will this pro-am team hit the goal, or will luck be against them? The project has already produced a beautiful image of a ghostly planetary nebula, and it's sparked some intriguing scientific questions. So the way I see it, they've already scored.

Update for 3 p.m. ET: Jacoby sent along further information about Kronberger 61: The star is located in the constellation Lyra, very close to the western edge of Cygnus. Determining its distance "is a very difficult question, because these kinds of objects (planetary nebulae) have been very resistant to having their distances measured accurately." Jacoby's rough estimate is 13,000 light-years, "but it could be half that or twice that." He says Kronberger discovered the nebula in January, using data from the Digital Sky Survey.

"The star is very likely to have a mass about 60 percent that of the sun," Jacoby wrote in his email. "The age of the star is much harder to estimate, but it is likely between 2 billion and 8 billion years old. The nebula around the star was probably blown off about 15,000 to 30,000 years ago (after accounting for the time delay due to the distance of 13,000 light years, or 28,000 to 43,000 years ago if you include that light travel time)."

More about planetary nebulae:

• New clues to amazing space shapes
• Student 'hoots' for Owl Nebula in contest
• Dying star belches up a toxic brew
• Slideshow: Hubble's greatest hits

The discovery and the new Gemini images were presented today at "Planetary Nebulae: An Eye to the Future," an International Astronomical Union symposium in Puerto de la Cruz in the Canary Islands.

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

NASA Kepler via Jason Rowe on Flickr

This graphic shows the 1,235 candidate planets reported so far by NASA's Kepler mission, in proportion to their parent stars. For comparison's sake, our own sun is shown as well. It's just below the top row on the right, with Jupiter and Earth silhouetted against the disk. Click on the image to see larger versions on Flickr.

Two years after its launch, NASA's Kepler space telescope has detected more than 1,200 potential planets circling distant stars by tracking the slight dimming of starlight. This graphic, drawn up by Kepler science team member Jason Rowe, shows all 1,235 worlds in proper proportion to their respective stars. Some of the stars are thought to have multiple planets. To spot them all, you'll have to check Rowe's Flickr page for higher-resolution views.

The stars' sizes range from 6.1 times larger than our sun to just a third as wide. For comparison's sake, Rowe has included our own sun, off by itself beneath the top row. Jupiter appears as a speck in silhouette, and Earth is an even more minuscule speck. The colors of each star reflect how it would look if we could see it up close, outside Earth's atmosphere.

Nearly all of these candidates have yet to be confirmed as planets rather than binary-star companions or computer glitches. Kepler's scientists expect that 80 to 90 percent of them will turn out to be honest-to-goodness exoplanets. They also expect to find plenty more candidates as the mission continues, including some specks as tiny as Earth.

Check out these marvels from Kepler's planetary menagerie:

• Planet probe spots hot prospects
• Oops! Hopes for alien Earth go poof
• A tourist's guide to the Kepler-11 planetary system
• Planetary six-pack poses a puzzle
• Probe finds planetary missing link
• Planets spotted in changing orbits
• Planet-hunter finds five lightweight worlds
• Kepler snaps its first images

So what about our own planetary neck of the woods? A few years ago, Kokogiak blogger (and former MSNBC colleague) Alan Taylor drew up a similar graphic showing the relative size of all known solar-system bodies that are wider than 200 miles. It's a pretty wide array. Click on the thumbnail graphic below to see the full-size display, and explore our "New Solar System" interactive to learn more about the solar system's lineup.

Alan Taylor / Kokogiak

This graphic shows all known solar-system bodies wider than 200 miles. Click on the image to see larger versions.

Tip o' the Log to Discovery News' Ian O'Neill and Astronomy Picture of the Day.

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto." 

NASA / JPL

An artist's conception shows an alien Earthlike planet.

I should have known it was too good to be true: Last month, it looked as if a world known as KOI 326.01 was the best hope among the Kepler mission's 1,235 candidates to be a second Earth. It was thought to be a bit smaller than Earth, and even better, it was located in a "habitable zone." That's the area of space surrounding a star where water could plausibly exist in liquid form. Those two characteristics — smaller than Earth, and in the habitable zone — put KOI 326.01 in a class by itself.

No more, unfortunately. A fact-checker at Discover magazine, Mara Grunbaum, called up the Kepler team for more information about the planet, presumably because it was going to be featured in a future issue. Members of the team, including San Jose State University's Natalie Batalha, double-checked their figures and determined that the planet candidate is actually somewhat warmer and much larger than originally estimated.

"The details of the planet need to be hammered out, but this certainly means that this is not an Earth-size planet in the habitable zone," Batalha is quoted as saying in a posting to the 80 Beats blog, part of Discover magazine's corner of the blogosphere. You can get the whole story behind KOI 326.01's demotion there.

That's the way it goes with these Kepler candidates: As more observations are gathered, checked and double-checked, the basic statistics for any particular candidate may need to be revised. And in some cases, the "candidate" may not exist at all. Instead, the supposed planetary detection may turn out to be merely the effect of two stars passing in front of each other. It will take months or years more to turn those candidates into confirmed planetary detections. But if you're hoping to hear about alien Earths, don't give up hope yet. Kepler's hunt continues.

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