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.