Whether or not Gliese 581 c, that intriguing world that may or may not offer temperatures conducive to life, will make a transit of its star is not yet known. But the principle that radial-velocity searches can identify a planet that is subsequently studied via transit received further validation today with the detected transit of a Neptune-class world around GJ 436. This is the smallest and least massive planet ever examined through transit methods, and it bodes well for future such studies of M-class stars.
The new transit comes courtesy of the Swiss team that includes Michel Mayor and Didier Queloz, recently in the news due to their work on Gliese 581 c — do these guys ever get any sleep? As Andy noted in a comment this morning, this ‘hot Neptune’ orbits closer to its star than the innermost planet of Gliese 581. GJ 436 is an M-class red dwarf, a type of star whose small radius makes the detection of such worlds by transit methods easier than would be the case for solar-type stars. And it gives credence to the idea that the Canadian MOST instrument might be able to detect a transit of Gliese 581 c if the planet is properly aligned.
Transits are chancy, and even here, the planetary passage is said to be ‘almost grazing,’ showing a duration half as long as would be expected for a central transit in front of the star. But it’s enough to yield helpful information indeed. GJ 436 b turns out to have a radius comparable to that of Uranus or Neptune. Here is what we know of its composition, from the discovery paper:
The mass and radius that we measure for GJ 436 b indicate that it is mainly composed of water ice. It is an “ice giant” planet like Uranus and Neptune rather than a small-mass gas giant or a very heavy “super-Earth”. It must have formed at a larger orbital distance, beyond the “snow line” where the proto-planetary disc is cool enough for water to condensate, and subsequently migrated inwards to its present orbit.
This planet, which may hold a small hydrogen/helium envelope, will not light up any media switchboards with questions about habitability. Its atmosphere is assumed to be hot, with temperatures ranging from 520 K to 620 K depending upon the albedo value, and the authors note that greenhouse effects may heat it to still higher temperatures.
So while the discovery team believes there are scenarios in which it could be considered an ‘ocean planet,’ GJ 436 b could hardly sport the kind of oceans we’re familiar with here on Earth:
Because of the high surface temperature, this would imply a steam atmosphere above supercritical water rather than an Earth-like situation. As methane and ammonia have very low condensation temperatures, this scenario would imply migration from a wide orbit.
Supercritical water is formed at high temperatures and pressures, showing no real distinction between its liquid and gaseous state. No gently lapping ocean waves in this scenario — in fact, supercritical water is used on Earth as a medium for destroying toxic substances without releasing emissions into the atmosphere. GJ 436 b would be one hostile place indeed, but what promise it holds for future transit studies. For that matter, are there other planets in this very system? Stay tuned.
The paper is Gillon et al., “Detection of transits of the nearby hot Neptune GJ 436 b,” accepted for publication in Astronomy & Astrophysics Letters, with abstract available.