My local paper is running a story on page 11A entitled “Astronomers Report Earth-like Planet.” It’s a tantalizing headline, but obviously one that bears further investigation. For what’s being reported here is background information on one of the 45 planets — I should say ‘candidate’ planets — recently discussed at the Boston meeting of the IAU. These have been extracted from the HARPS planet survey, but we’ll probably have to wait until mid-June for further confirmation, which may well occur at the upcoming Extrasolar Super-Earths workshop in Nantes.
This would be an interesting world if things do play out, a rocky ‘super Earth’ just over four times as massive as Earth, and hence the smallest world yet in our attempt to find planets not so different from our own. If the press continues to generate a buzz about this, we should look at the contrast with the Gliese 581 story. There we wound up with two planets of astrobiological interest, one apparently on the inner edge of the habitable zone and probably across it, too hot for life, with another on the outer edge. The jury is out on both in terms of habitability, but the odds went down considerably as various teams ran the numbers.
But while Gliese 581 is an M-class dwarf, the HARPS survey has been looking at F, G and K-class stars, the latter two classes not much different from our Sun. If we were to find a rocky world in the habitable zone of one of these, we would be a step closer to an ‘Earth-like’ world than a hot, tidally locked planet in tight orbit around a red dwarf. No wonder the press is interested. But again, we’ll have to await confirmation and the inevitable follow-up studies to the Geneva team’s work, nor have I seen any verification of the McClatchy news story’s further claim that the potential new world orbits in the habitable zone of its star (see below).
Somewhat misleading headlines aside, what really came out of the Boston IAU session was the growing understanding of how frequently rocky worlds occur. Based on the recent findings, they could outnumber Jupiter-class planets by three to one. Sara Seager (MIT) is being widely quoted on this, including this from the McClatchy story:
“The mass of the planets and the sheer number of them represents a huge step toward finding planets of the Earth’s mass and ones that might be suitable for life as we know it. What amazes me is that these planets may be very, very common.”
No wonder Seager sees the HARPS windfall as “…the beginning of the detailed exploration of super-Earths.” Excitingly, we’re also looking at the growing possibility of finding such a world in transit. To my knowledge, the 45 HARPS planet candidates all orbit in less than fifty days (making the habitability question seemingly moot around F, G and K-class stars), with some in orbits as short as ten days. Bagging a transit to follow up the HARPS radial velocity studies becomes easier when orbits are close and frequent, and such a transit would provide information about the planet’s diameter, density and composition, not to mention allowing potential studies of its atmosphere. But unless HARPS has other planets up its sleeve, ‘habitability’ may not be a factor in the next headline.