Or rather: the exoplanet we thought was there actually didn’t exist in the first place.

I wrote about the exoplanet euphoniously known as Gliese 581g when it was first announced back in 2010. It was potentially a huge deal: the first planet with small enough mass to be rocky orbiting within its star’s habitable zone. (“Rocky” means it has a solid surface, as opposed to Jupiter and the like, which are gaseous.) The habitable zone, for those keeping track at home, is the range of distances from the host star where a planet’s surface could potentially have liquid water. “Habitability” isn’t the whole story, of course: Venus is in the habitable zone for the Sun, but it’s far less habitable than Mars, which lies outside it.

But Gliese 581g has more troubles than just habitability: it probably doesn’t exist at all. What looked like a planet was actually something else:

Now the verdict is in: Gliese 581g is an ex-planet, not an exoplanet. A new study, published in the journal Science, showed that what seemed to be the sign of a planet was more likely to be from the star’s “weather”—the same sort of magnetic fluctuations that cause prominences and sunspots on the Sun. Not only that, but a second planet in the same system, Gliese 581d, probably doesn’t exist either, for the same reasons. [Read more…]

Admittedly, the case for Gliese 581g was never that strong from the start. However, it’s one thing to say “we didn’t detect it” and another to say “we’ve figured out why some astronomers see it and others don’t”, and this new study is the latter.