Two 'Habitable' Planets Never Actually Existed

Gliese 581 d and g, two planets that are just 20 light years from Earth and orbit within its star's habitable zone, are widely considered to be two prime candidates where we could find extraterrestrial life. Problem is, neither planet exists.

A new study published today suggests that previous reports of the planets' potential habitability, and, really, their mere existence, were greatly exaggerated. That's because Gliese 581 d and g were originally detected using the "Doppler method," which measures a planet's back-and-forth wobbles that are induced on it as it orbits a star. This isn't nearly as effective as the way the Kepler Space Telescope detects planets, using the shadows planets cast on their star as it orbits.

Paul Robertson, an astronomer at Penn State University, writes in Science that the existence of both 581d is an "artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g." That's because the existence of planet g was always based on the gravity exerted on it by planet d, which we now know doesn't exist. In fact, Robertson writes that the existence of planet g "was simply leftover noise created by stellar activity." Poor guy.

"Gliese 581 d and (the now less widely believed) 581 g were considered to be among the first exoplanets likely to host habitable environments if they were rocky," he wrote.

No longer.

So, what the heck happened here? The team says that the false readings that were originally believed to be planets were actually due to intense magnetic activity on the star itself—much like sunspots on the sun. This crazy intense activity created false positives for planet d and g. When Robertson studied the sodium and hydrogen emissions coming off the star, that much became obvious.

The discovery is a bit of good news and bad news. The good news, of course, is that now we know not to look to Gliese 581 d as our new home. We also know more about the system than we ever have before, which is good considering that back in 2008, a team of researchers sent a high-powered radio signal containing a "digital time capsule" the Gliese 581 system. Luckily, that signal, called "A Message from Earth," was sent to planet c, a planet that Robertson says almost definitely exists. The bad news, of course, is we just lost two other potential candidates for life.

"Our improved detection of the real planets in this system gives us confidence that we are now beginning to sufficiently eliminate Doppler signals from stellar activity to discover new, habitable exoplanets, even when they are hidden beneath stellar noise, said Robertson. "While it is unfortunate to find that two such promising planets do not exist, we feel that the results of this study will ultimately lead to more Earth-like planets."

So there's that. So long, Gliese 581 d and g. It was nice not knowing you.