I have two labs on Tuesdays, which is even more exhausting than it sounds, so I went to be early last night. And woke up to find a whole new planet. You guys need to give me some warning about these things...
Anyway, the planet in question orbits a red dwarf star a mere 20 light years from Earth, has a mass of roughly five times that of Earth and an estimated radius half again Earth's (which would mean a bit more than double the surface gravity, for those planning to set SF stories there). It orbits its star in about 84 days, which seems pretty short, but is much slower then the previously discovered Neptune-sized planet in an orbit with a 5-day period, and also puts in in the region where water ought to be liquid. Depending on what sort of atmosphere it has, they estimate the surface temperature to be between 0 and 40 C (32-104 F).
There is, of course, a news story in the Times, and also a ton of blog commentary. I particularly want to note the post at systemic, mostly because I wasn't previously aware that there is a whole blog dedicated to extrasolar planets. Welcome to the future.
(Link via James, who else?)
Comments
Where did you get the 84 day orbit period from? According the the post at that you link to (and the Times article), it has an orbital period of 12.9 days.
Posted by: Matthew | April 25, 2007 8:29 AM
Matthew-
The announcement (here) was for two new planets around Gl 581 (a first planet was already known, so now we know three), and the second of these new planets has an orbital period of about 84 days.
Posted by: Rick | April 25, 2007 9:06 AM
Where did you get the 84 day orbit period from? According the the post at that you link to (and the Times article), it has an orbital period of 12.9 days.
From skimming the press release rather quickly, and grabbing the period of the wrong planet. I should really stop trying to blog at 6:30 in the morning.
Posted by: Chad Orzel | April 25, 2007 9:16 AM
Thanks Rick, I hadn't noticed that in my quick glances through the articles. From my reading of the article, I think the planet that everyone is excited about (5x mass of Earth, could have water) has a 13 day orbit, and the other possible planet (8x mass of Earth) has the 84 day orbit. Is this correct?
Posted by: Matthew | April 25, 2007 9:18 AM
Matthew-
That's my understanding; the planet that everyone is excited about has a mass of about 5x mass of Earth, distance from Gl 581 of about .07 AU, and a period of about 13 days. The preprint (pdf) describes this planet as being "within the habitable zone around the star", and "probably the most Earth-like of all known exoplanets".
Posted by: Rick | April 25, 2007 10:16 AM
I have a question about the estimated size: How do they get that figure? I understand the estimated mass figure (which is, I believe, a lower bound), but the size seems strange.
They're claiming a specific gravity of 8, but it's a metal-poor system.
Posted by: Craig Helfgott | April 25, 2007 12:14 PM
Craig-
It seems to be the result of a model. In the preprint, for the value of the radius of 1.5 * radius of Earth, they reference a paper (Valencia et al) which presents a model for large terrestrial planets (with M = 1 to 10); the model has the mass and radius being related by R ~ M^0.27. By this relation, when M = 5, R = 1.5.
Posted by: Rick | April 25, 2007 1:27 PM
Craig, the high density is due to the gravitational self-compression of a 5 earth-mass rocky/metal body. For example, iron at the Earth's surface has a density of about 8 g/cc, but in the inner core it is around 13 g/cc. In the core of a 5 earth mass planet, it would be closer to 18-20 g/cc. The rocky mantle experiences similar densification.
Posted by: Lab Lemming | April 30, 2007 9:27 PM