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Your guide to exoplanets

Planetary Society
An online "trading card"
highlights the Gliese
876 planetary system.

The planet-hunting game is shifting into high gear with the launch of NASA's $591 million Kepler mission, but how can you possibly keep track of all those alien worlds without a program? Fortunately, the Planetary Society has just the thing: a free online catalog of exoplanets that will keep up with a tally expected to escalate into the thousands.

The catalog is designed to cater to all types, ranging from avid fans who gobble up the vital stats for every newfound world to the casual spectator who may wonder what the heck an exoplanet is, said Bruce Betts, the nonprofit group's director of projects.

"We've tried to make it accessible," he told me today. And that means including an animated "trading card" for each of the 300-plus planets detected so far.

The whole database is automated, which will make it easy to update the catalog with fresh finds from Kepler as well as from its European counterpart, the Corot space telescope, and a phalanx of ground-based telescopes monitoring faraway planetary systems.

How do scientists find faraway planets? A variety of methods come into play: Some research groups check for a characteristic gravitational wobble in a star's motion. Others watch for the way an unseen planet might distort the light coming from a more distant source. Kepler and Corot are designed to watch for the faint dimming of light when a planet passes over its parent star's disk (or when the planet passes behind the star). Our interactive on "Other Worlds" explains all those methods.

The technique used by Kepler, known as transit photometry, has even been used to analyze the atmosphere of an alien planet. In some cases, methane and water vapor have been detected - which are hopeful signs for the future identification of alien life.

Betts said the Planetary Society is committed to keeping the catalog current - which is something we haven't been able to do with our "Other Worlds" interactive. The society is getting advice from some of the top troops in the planet-hunting business, including Caltech graduate student Darin Ragozzine, a member of Mike Brown's dwarf-planet search team; and Berkeley Professor Geoffrey Marcy, a leader of one of the longest-running exoplanet search efforts.

"We live in an exciting era of discovery with exoplanets, with new worlds found every month, sometimes every week," Marcy said in the Planetary Society's news release. "The Planetary Society's online Catalog of Exoplanets is the perfect resource for the public to keep pace with new discoveries."

The Planetary Society's catalog isn't the only one out there: For years, the Extrasolar Planets Encyclopaedia has been the authoritative source for vital statistics about all the alien worlds detected so far. Another Web site, SolStation.com, provides a digest of discoveries for a wide assortment of stars, many of which have planetary systems associated with them.

Betts said the Planetary Society's catalog doesn't include some of the planetary systems reported over the years - including the pulsar planets, which were the first-ever worlds ever found orbiting an alien star (this star happened to be a radio-emitting neutron star), and free-floaters that could be considered oversized planets or undersized brown dwarfs.

Speaking of dwarfs, let's not leave out the dwarf planets in our own solar system: So far, five worlds fit the IAU's disputed definition: Eris, Pluto, Haumea, Makemake and Ceres. Uruguayan astronomers currently list more than a dozen additional candidates. Mike Brown thinks there should be around 50 on the list already, with the potential for thousands.

Is there something wrong with having thousands of dwarf planets in our solar system? Not in my book. I'd favor counting them as a subcategory of the full planetary list - just as I'd favor including the pulsar planets, the free-floaters and other weird worlds beyond our solar system. We'll have plenty of planets to choose from, once Kepler, Corot and other planet-hunting telescopes get deep into their surveys.

Or do you think we should be more discriminating with the planet label? Feel free to give vent your views as a comment below. And for more about the planet search, check out these archived posts: