The “500” -Alien Planets Common in Milky Way & Beyond

M100 Less than 20 years after confirming the first planet beyond our own solar system, astronomers have sighted exoplanet No. 500, all of which are less massive than Jupiter -ranging between 15 and 50 percent of Jupiter's mass- and the planets' distances from Earth range from 58 light-years to 196 light-years.

The 500th find comes less than two months after another watershed moment — the discovery of the first potentially habitable extrasolar planet. And astronomers are sure to announce other big milestones soon, as data rolls in from instruments like NASA's planet-hunting Kepler space observatory.

The milestone was reached Friday (Nov. 19), according to the Extrasolar Planets Encyclopedia, a database compiled by astrobiologist Jean Schneider of the Paris-Meudon Observatory. The new exo planet count now stands at 502 alien worlds.


The 500th alien planet currently appears to be one of four newly sighted extrasolar worlds, based on Schneider's list. They appear on the list just after another extrasolar planet, HIP 13044 b, which astronomers announced last week to be from an alien galaxy.

"In some sense, 500 is an artificial milestone," said Jon Jenkins of the SETI (Search for Extraterrestrial Intelligence) Institute and analysis lead for the Kepler mission, in an interview with space.com. Astronomers aren't just discovering scorching-hot gas giants anymore, he added. "They are finding smaller, rocky worlds, too — planets that could be a lot like Earth."

The lists of extrasolar planets is unofficially maintained by Schneider and NASA's Jet Propulsion Laboratory, which keeps a count called "PlanetQuest: New Worlds Atlas," which currently lists 497 planets.

Kepler has already identified more than 700 "candidates" — stars possibly harboring alien planets — that await further observation and confirmation. A high percentage of these will probably pan out, according to Jenkins.

"Our false positive rate continues to be rather low," he continued in his interview with space.com. "It appears to be below 20 percent. We're starting to see the family portraits of these extrasolar systems. We're getting a more complete picture of the distribution and frequency of planets beyond our solar system."

Some of these alien stars will likely host more than one planet, as multi-planet systems appear to be somewhat common. Reaching exoplanet number 1,000 is soon within reach. Multiply Kepler's infant-effort success rate times the number of known galaxies and the mind boggles at the number of potential "Twin Earths" in the universe. A typical spiral galaxy (M100 above in Coma Berenices) similar to the Milky Way could house thousands of potential candidates. If this were our own galaxy the solar system would be located near the outer edge of an arm.

Casey Kazan via space.com

Image credit: NASA/HST photo

"The Galaxy" in Your Inbox, Free, Daily