Possible terrestrial planet around Gliese 581

The European Southern Observatory has released a press release claiming to have found a possible terrestrial planet in the habitable zone around the red dwarf Gliese 581.

Rampant speculation has ensued.

I’ll give the facts as I see them, but since I’m not an astronomer I may need to be corrected.

The radial acceleration method measures the pull exerted on the host star by an orbiting planet. These measurements can be done extremely precisely. Since they rely on the law of gravitation, they can detect smaller masses if you have a small star and a close planet.

The wobble that is detected gives the orbital period (which, given the mass of the star, yields the orbital distance), and the amplitude, which is dependent on the mass of the planet and the geometric configuration.

The reported mass is a minimum mass- that is, it is the mass if, from the exoplanet’s POV, our sun lies in the plane of that exoplanet’s ecliptic. Otherwise, the actual mass is greater than this minimum mass by 1/cos X, where X is the angle between our sun and their ecliptic. Fancy spherical geometry* thus suggests that the actual mass is unlikely to be more than 30% higher than the minimum mass.

So this new exoplanet probably has an actual mass between 5.5 and 7 or so Earth masses.

The mass, and the orbital period are all we can actually determine at this point in time.

There have been reports of a planetary radius. As far as I can tell, these assume a terrestrial (or icy) composition, with a little bit of gravitational self-compression thrown in.

Obviously, such assumptions are speculative- the terrestrial planets in our own system don’t even have the same bulk composition as each other. Neither do the icy moons.

The presence or absence of water is also speculative. It is worth noting, however, that on Earth, most of the water is believed to have been added late in planetary formation. The late veneer hypothesis suggests that volatiles and highly siderophile elements were added to the Earth late in its formation from impacts of outer solar system objects. While our solar system has several gas giants to gravitationally deflect bodies inwards, no such giant planets exist in the Gliese 581 system outward of planet c (planet b is only 8 earth masses- compared to 318 earth masses for Jupiter).

Additionally, spectroscopic attempts to identify water in other exoplanet- the hot jupiters HD 209458 and HD 189733- failed to identify water in the atmosphere of either planet. This is despite theorists claiming that it had to be there.

Of course, an unusual result shouldn’t be surprising. Ever since we went to the moon, the rule of thumb for planetary exploration is that they have been stranger than anything we could hypothesize. So my guess is that Gliese 581c will adhere to this rule.

At the moment, all we can do is guess about this planet’s composition. But hopefully, the Terrestrial Planet Finder (recently bloged at green gabbro), if funded, will be able to image it, and determine the IR spectra. The key fact about this new discovery is that if the TPF gets flown, it now has one definite target to analyze. And since the signal for this planet was 3 times the LOD, hopefully more will follow.

Of course, if there is anyone living there, and they look out towards the galactic rim, this is what they will see:

Orion is to the left, the Pleadies are to the right, and somewhere in this picture is our sun. Can you find it?

* I've only estimated this, feel free to quantitatively set me straight.