Those following the Gliese 581 story have been awaiting the results of the MOST observations with great interest. The Canadian mission put the red dwarf under study for six weeks after the recent flurry of speculation regarding a possible habitable planet, Gliese 581 c, in the system. If the planet made a transit, moving across the face of its star as seen from Earth, then we could learn more about its size and makeup.
The results are now in, and no transit occurred. But a second issue is a bit more satisfactory. During the observation period, Gliese 581 showed little change in brightness, indicating a level of stability that would prove beneficial to the growth of life, whether on Gliese 581 c or the more distant (and massive) Gl 581 d, which may orbit on the outer edge of the star’s habitable zone. Here’s Jaymie Matthews (University of British Columbia and a MOST mission scientist) on the matter:
“The climate there should not be a wild rollercoaster ride that would make it difficult for life to get a foothold. It also suggests the star is quite old, and settled in its ways, so that the planets around it have been around for billions of years. We know it took about three and a half billion years for life on Earth to reach the level of complexity that we call human, so it’s more encouraging for the prospects of complex life on any planet around Gliese 581 if it’s been around for at least as long.”
Note that phrase ‘any planet around Gliese 581.’ I assume it’s an oblique reference to the growing evidence that Gl 581 c is not the Earth-like world first suggested. In fact, recent work on the planet suggests it resembles Venus much more than Earth, a seething cauldron far too close to its primary to provide a reasonable foothold for life. But Gl 581 d may still be in the running, and it is encouraging in any case to know that a star like this can produce candidate planets near the needed limits. It won’t be long before we have another candidate, and a transiting one at that.
Bringing MOST to bear on this interesting system was useful in another regard. The observations rule out sunspots or other activity on Gliese 581’s surface that might have posed an alternative explanation for the 13-day cycle that has been interpreted as a planet. No such variations occur in the MOST findings, adding to our confidence that Gliese 581 c is indeed there. Whether it is habitable or not seems increasingly in doubt.