Rethinking Gliese 581g

A huge stir zipped through the scientific community and beyond recently with the announcement by astronomers of the discovery of a world with only three or four times the mass of Earth that orbited in the habitable zone of the red dwarf star Gliese 581, only 20 light years away. The mass told us the planet was a rocky world like Earth, and the location of the orbit told us that liquid water could exist on its surface.

Well, maybe not.

Other astronomers are now questioning whether the world, dubbed Gliese 581g, even exists. The discovery of this exoplanet-- as well as of most of the hundreds of other such planets so far discovered-- was done by monitoring tiny movements by the parent star that could be caused by the gravity of planets tugging on it. Obviously, more massive planets close to the star are easier to detect than less massive ones. A few astronomers who have looked at the Gliese 581g discovery data say they see no obvious pattern in it. The Gliese 581 system already has four confirmed exoplanets, and the announcement of "g" was coupled with the announcement of another, even smaller world. Any pattern, therefore, would be extremely complex and subtle, possibly difficult to distinguish from the random "noise" always present in such endeavors.

So, does a slightly larger version of Earth orbit Gliese 581, or not? Answering that question is within our capability, and will be even more within it as technology develops. So, we will know what actually exists in the system of this particular red dwarf, but it could take a while.