Scientists Question the Existence of Planet Gliese 581g

The unexpected discovery of extrasolar planet Gliese 581g in September 2010 set off wild speculation about what appeared to be the most Earth-like planet found orbiting another star yet. The man who headed up the planet-hunting team which found it, Steven Vogt, had a moment of unsupported and possibly ill-advised optimism and even claimed there was a “100% chance” that life in some form was present on that planet. Australian SETI researcher Ragbir Bhathal even says he picked up a single anomalous signal from Gliese 581g two years ago.

Not everyone is celebrating, however. In fact, Swiss scientist Francesco Pepe isn’t even sure that Gliese 581g exists at all. Pepe works at the Geneva Observatory and shared his skepticism with New Scientist. Pepe’s team took data from the 3.6-metre HARPS telescope, owned by the European Southern Observatory in Chile. HARPS is specially designed to search for planets by detecting the telltale wobbles in their host stars caused by the gravitational influence of orbiting planets. Pepe says they just couldn’t find the wobble that should be occurring, if Gliese 581g is present.

For the moment, both Vogt and Pepe seem to be acting cordially (at least in public) about their debate. Pepe says he is “not trying to prove the nonexistence of a planet,” just to point out that more data is necessary before reaching a firm conclusion on the matter. Vogt says he’s confident in his research, and that the data about such a distant star is simply difficult to interpret correctly. Matters are complicated by the fact that Gliese 581 already has at least four and possibly five other planets other than Gliese 581g, and we are not entirely sure how elliptical their orbits are – meaning that the same data about star wobbles might potentially fit more than one theoretical scenario about the number of planets. It will take time to determine whether Vogt or Pepe can convince the bulk of their fellow astronomers.

If Gliese 581g does exist, it would be the sixth planet in that star system, and less than twice the size of Earth. It orbits its star at a distance just one-sixth the distance from the Earth to the Sun – which would be lethal in our system, but not so around the relatively cool red dwarf Gliese 581. However, it would be close enough to be tidally locked, meaning that it always presents the same face to the star. The same process affects our own Moon, which is why we only see one side of the Moon. This means that nearly half of Gliese 581g would be extremely hot, nearly half would be extremely cold, and a thin band along the border between the two would be comparatively warm and welcoming. Still, on the plus side, Gliese 581 is only 20 light-years away from Earth, making it far closer than most other candidate stars.