Amid the breathless excitement of finding a potentially habitable exoplanet, there’s been a bit of a buzzkill cropping up in the news. At an exoplanet conference in Turin, a Geneva-based team announced that they have not found evidence of Gliese 581g, casting a shadow of doubt on its discovery in the minds of some. What does this really mean?
The Geneva team used data from HARPS, or the High Accuracy Radial Velocity for Planetary Searcher, a powerful spectrometer on a 3.6-meter telescope in Chile. HARPS has been enormously successful at detecting exoplanets using the radial velocity method, or measuring the gravitational tugs on stars by their planets by watching the stars’ spectral lines “wobble” back and forth due to the Doppler effect.
For the discovery work on Gliese 581g, the US-team, led by Steven Vogt and Paul Butler, used data from HARPS over a 4.3 year period and data from another instrument, HIRSE, over an 11 year period. HIRSE is the High Resolution Echelle Spectrometer on one of the 10-meter Keck Telescopes in Hawaii. Their analysis searches for planets in the Gliese 581 system using both sets of data, confirming the presence of planets b, c, d, and e while showing evidence for the two new planets, f and g. (That’s right, there were two, but Gliese 581g get’s all the fame for being in the habitable zone!)
Francesco Pepe of the Geneva-based group using HARPS announced that they have added to their data set, now spanning 6.5 years, and that their data does not show evidence of Gliese 581g. However, the precision is not enough to prove that the planet does not exist. This contrasts with the work of Vogt et al., which argues that the false alarm probability for these six planets is extremely low.
I noticed an important point when I took a closer read of the Vogt et al. preprint. In it, they explicitly state that both sets of data, from HARPS and from HIRSE, were needed to detect all 6 planets in the Gliese 581 system. So really, it is no surprise that the HARPS-only dataset cannot confirm the presence of the potentially habitable planet g. However, some sort of independent confirmation of this planet will likely be needed to firm up the detection.
You may be thinking, shouldn’t it be easy to tell whether a whole planet exists or not? Well, maybe if you are right up close to it, but astronomers have to work very hard to detect these faint signals of worlds around other stars. Though Gliese 581 is “only” 20 light-years away, next-door in Galactic terms, that is still over 100,000,000,000,000 miles away!
In fact, Vogt et al. state in their summary that, “Finally, it is important to keep in mind that, though all six plants presented here are well supported…, caution is warranted as most of the signals are small.”
So here we are, on the raggedy edge of science, where important discoveries are painstakingly wrestled from nature’s clutches and debates rage on in the peer-reviewed journals. I would certainly stay tuned.
Image credit: NASA