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.

European astronomers have announced the discovery of more than 50 new planets beyond our solar system, including 16 that are just a notch above our own planet in mass. They say their record-breaking findings suggest that more than half of the stars like our sun possess planets, and that many of those worlds are less massive than Saturn.

The pick of the litter is a planet that's already been in the spotlight: HD 85512 b, a world at least 3.6 times as massive as Earth that's located 36 light-years away in the constellation Vela. HD 85512 b is the only one of the 16 super-Earths on today's list that is located in its star system's habitable zone. That's the area around a star where scientists believe water could exist in liquid form, which would make a rocky planet potentially livable.

HD 85512 b's status came to light a couple of weeks ago in a paper submitted to the journal Astronomy and Astrophysics, but the team behind the discovery provided more details about that super-Earth and the dozens of other worlds in papers presented today at the Extreme Solar Systems II conference in Wyoming.

The findings came from the team behind the High Accuracy Radial Velocity Planet Searcher, or HARPS, which is installed at the European Southern Observatory's 11.8-foot (3.6-meter) La Silla Observatory in Chile.

"The detection of HD 85512 b is far from the limit of HARPS, and demonstrates the possibility of discovering other super-Earths in the habitable zones around stars similar to the sun," University of Geneva astronomer Michel Mayor said in today's news release from the ESO.

Super-Earths, which range from Earth's mass to worlds 10 times more massive, are of particular interest to planet-hunters because it's thought that they could be even more conducive to the development of life than our own planet. When the search for extrasolar planets began more than 15 years ago, the telescopes used for the task could only detect giant planets like our own solar system's Jupiter. Since then, the techniques and tools used for the search have become much more sensitive.

HARPS, for example, can detect the slight gravitational wobble caused by planets as small as Earth, if they have incredibly close-in orbits. HARPS' observations of 376 sunlike stars has led the team to conclude not only that more than half of such stars are surrounded by planets (maybe as many as 70 or 80 percent), but also that about 40 percent of sunlike stars have at least one planet less massive than Saturn.

One of the team members, Lisa Kaltenegger of the Max Planck Institute for Astronomy and the Harvard Smithsonian Center for Astrophysics, told journalists today that the latest round of findings marked a new age in the search for habitable planets.

"We are actually entering an incredibly interesting time in our history," she said.

Keeping track of the habitables
ESO's Markus Kissler-Patig said the discovery of HD 85512 b could be one of the first entries in "a good catalog of habitables" marked for further study. Kissler-Patig is the project scientist for the ESO's European Extremely Large Telescope, or E-ELT, which is slated to be built over the next decade at a cost of 1 billion euros ($1.4 billion).

HD 85512 b "is in the zone where we can directly image it," Kissler-Patig said, and that means astronomers could theoretically analyze its atmosphere for the signatures of life, such as the presence of oxygen, methane and water vapor.

The HARPS team members were able to figure out the minimum mass and orbital characteristics of HD 85512 b, but they couldn't determine its density, composition or the nature of its atmosphere — which means astronomers will have to wait for the completion of E-ELT or similar high-resolution observing instruments to confirm that the world is truly habitable.

Francesco Pepe, a colleague of Mayor's at the University of Geneva, said that the HARPS team's discoveries include 10 worlds described in papers submitted to Astronomy and Astrophysics, including HD 85512 b, and 49 planets reported today at the Wyoming conference. Eight of the new planets were detected as part of the Swiss-led CORALIE search effort in Chile, he said. The ESO says this is the largest number of extrasolar planets reported at one time.

Pepe said the findings pointed up a fresh mystery for planet-hunters to ponder: the existence of a "planet desert" between low-mass worlds and gas giants. Relatively few planets have been found at a level around 30 times the mass of Earth. "It may point towards different formation mechanisms" for planets like Earth and Neptune vs. planets like Jupiter and Saturn.

HARPS isn't the only instrument engaged in the search for extrasolar planets: Two space telescopes, NASA's Kepler and the European Space Agency's Corot, are detecting planets by looking for the telltale dimming of their parent stars. Kepler and Corot can determine how big a planet is, but they can't tell how massive it is. In contrast, HARPS can determine the mass but not the size.

Unfortunately, Kepler can't be used to confirm HARPS' discoveries, nor can HARPS confirm Kepler's. The good news is that the William Herschel Telescope in the Canary Islands is being outfitted for a HARPS North instrument that will begin operation next year and facilitate the follow-up of Kepler detections. 

Today's revelations bring the official tally of extrasolar planets to 645.

Other findings from the Extreme Solar Systems II conference:
• Over at the "Dynamics of Cats" blog, Steinn Sigurdsson quotes Kepler team members as saying they have identified 1,781 candidate planets, with up to 27 of those confirmed. Among the reported candidates are 123 potential worlds that are less than 1.25 times as wide as Earth, and 121 that are in the nominal habitable zones of their parent stars.

Jon Lomberg

An artist's conception shows storms on a brown dwarf.

• Astronomers say they have observed brightness changes on a failed star, also known as a brown dwarf, that may indicate a storm grander than any seen yet on a planet. The stormy brown dwarf is known as 2MASS 2139.

"We found that our target's brightness changed by a whopping 30 per cent in just under eight hours," the University of Toronto's Jacqueline Radigan said in a news release. "The best explanation is that brighter and darker patches of its atmosphere are coming into our view as the brown dwarf spins on its axis."

Radigan is the lead author of a paper being presented this week at the Extreme Solar Systems II conference.

More about alien planets:

• Previously: Super-Earth on the 'edge of habitability'
• Interactive: How scientists search for other worlds
• 'Invisible' planet discovered using new technique
• Background: Looking for alien Earths? Here they come

Authors of  “The HARPS search for Earth-like planets in the habitable zone, I — Very low-mass planets around HD20794, HD85512, HD192310" include F. Pepe, C. Lovis, D.D. Ségransan, W. Benz, J. L. Bertaux , F. Bouchy, X. Dumusque, M. Mayor, D. Queloz, N.C. Santos and S. Udry.

Authors of "The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets" include M. Mayor, M. Marmier, C. Lovis, S. Udry, D.D. Ségransan, F. Pepe, W. Benz, J. L. Bertaux , F. Bouchy, X. Dumusque, G. Lo Curto, C. Mordasini, D. Queloz and N.C. Santos.

Authors of "High amplitude, periodic variability of a cool brown dwarf: Evidence for patchy, high-contrast cloud features" include Jacqueline Radigan, Ray Jayawardhana, David Lafreniere, Etienne Artigau, Mark Marley and Didier Saumon.

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.