A study released recently revealed that one of the most promising exoplanets discovered inside a nearby stars habitable zone might be no more than data ghosts caused by star activity.
A couple of years ago, Gliese 581 was the subject of a lot of attention as it was pretty close to Earth at about 22 light years distance, and also apparently had quite a few exoplanets in orbit around it in, or very close to, its habitable zone. A lot of people got very excited very quickly as it seemed that the then current generation of exoplanet detection had struck it rich practically without effort – and that exoplanets were pretty common.
Unfortunately it appears that at least two exoplanets discovered around Gliese 581 – specifically Gliese 581d and 581g are little more than data ghosts caused by sunspots on the surface of Gliese 581 itself.
Gliese 581g has been suspected of being a phantom planet for a few years, being listed as “Unconfirmed” in an encyclopaedia of exoplanets and the subject of several papers casting doubt on its existence.
Last month, a study confirmed beyond reasonable doubt what had long been suspected – that 581g did not exist, and neither did 581d. The authors state that an unfortunate confusion of signals is responsible for the mistaken identification of the two exoplanets. What has apparently happened is that some incorrect assumptions had been made about the long term brightness of the star and how much that brightness was affected by the presence of magnetic anomalies – ie, Sunspots.
These sunspots are apparently a long term feature of Gliese 581 that stay in the same place on the stars surface, while the star itself rotates – about once every 130 days. The planets appeared to have orbital periods of almost exactly one half and one quarter of this period. Correcting for the stellar activity signal eradicated the signals for 581d & g, conclusively proving that the exoplanets were actually false positives caused by filtering problems.
On the postive side, it appears that the correction boosted the signal evidence for three other planets around Gliese 581 – b,c and e, so the study is not a complete bust!
The study, “Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581″ (Robertson, Mahadevan, Endl & Roy) was published in the journal “Science” July 3rd 2014. (Arxiv.org pre-publication edition)