Earth-like planet found?
I'm just reading a European Southern Observatory press release about a potential planet that is 1.5 times the diameter of the Earth and five times the mass. There are some things about it that are niggling me so I'll write more when I've actually read the paper that the discoverers are planning to submit as a Letter to Astronomy & Astrophysics. In the mean time, the Bad Astronomer and News in Space (MP3: 32.6 MB) have their takes on the news.
Update (11:50 BST): I've now read the paper which hasn't been accepted yet (that doesn't mean it won't be!). One of the things in the press release that concerned me was the statement that the radius of the planet was 1.5 that of the Earth. The reason for my concern is that this planet (and the others around the same red dwarf star) were found using the radial velocity method whereby the changes in velocity of the parent star are monitored. This method can only tell you the periods of planets around the star. However, using the period, the size of the change in velocity, and Kepler's laws you can work out the mass and size of the orbit. This still doesn't give you the physical size of the planet. To do that you either need the planet to pass infront of its parent star or to know the composition (rocky, gas etc) and make a guesstimate at the size from mass and density. In this case, the authors have relied on a paper by Valencia et al (2006) which makes various assumptions to get a relationship between the mass of 'super-Earths' and their sizes. So the 1.5 Earth-diameter figure relies on this model being valid for a planet forming near a red dwarf star. It may be fine but I'd like to wait to see.
The other thing I'm slightly confused over are the final masses (why no uncertainties on these?). The quoted masses seem to be the minimum masses that the planets could have and the true mass depends on the inclination of the orbital planes. So, they could be larger. Having said all this, it is good to see that the authors have considered other possibilities to explain their data. For instance, they considered large sunspots on the star. In the end they dismiss these because other measurements show the star to be quite inactive.
Comments: Earth-like planet found?
Freebetv sent me this to my phone this morning:
European astronomers have spotted what they say is the most Earth like planet yet outside our solar system, with balmy temperatures that could support water and, potentially, life.They have not directly seen the planet, orbiting a red dwarf star called Gliese 581. But measurements of the star suggest that a planet not much larger than the Earth is pulling on it, the researchers say in a letter to the editor of the journal Astronomy and Astrophysics.
Posted by pete on Wednesday 25th Apr 2007 (11:04 UTC)
Doesn't the fact that Gliese 581 is a red dwarf also suggest it's planets would form in a metal-poor enviroment?
Posted by Rob on Wednesday 25th Apr 2007 (11:38 UTC)
Aren't a lot of red dwarfs, flare stars? That would not seem to translate to 'balmy weather' to me.
Posted by agesilaus on Wednesday 25th Apr 2007 (22:42 UTC)
Rob: Yes, Gliese 581 is metal poor, they authors thing this explains why all the planets are relatively small.
agesilaus: While many dawrfs are flare stars, this one is quite quiet.
Stuart: I noted the mass issue as well, but as I read the paper, the orbital solutions constrain the masses quite strongly.
I have made a celestia ssc file for the Gliese 581 system if anybody wants to do a virtual fly through.
Posted by Ian Musgrave on Friday 27th Apr 2007 (12:01 UTC)
Ian, the pre-print still quotes the masses as m.sin(i) so the masses will all scale with a change in inclination angle of the orbits to the line of sight.
I also notice that the latest version of the pre-print (27 April) claims that it has been accepted by A&A.
Posted by Stuart on Friday 27th Apr 2007 (17:53 UTC)
It's the diameter, and therefore surface gravity and density that seems the weakest. It's radial velocity data - not transit data. So the diameter is a guess. It may be a good guess, but it could be completely off the mark.
The Hubble Space Telescope (HST) Fine Guidance Sensors (FGS) have been used to measure side to side wobble (proper motion) of a similar star. FGS is good to something like 17th magnitude, and 581 is something like 10th. Any idea if radial wobble combined with proper motion wobble would constrain the masses of the planets better? I can't seem to wrap my head around it to tell for sure. Last time it was done with 20 hours of HST time. Perhaps two days of HST time, 6 days apart could do it for this 13 day orbit. Then again, with three (or so?) planets, maybe it'll take more effort.
Posted by Stephen on Wednesday 09th May 2007 (14:15 UTC)
I thought 581 was slightly metal poor compared with the Sun. The implication is that it is nearly as metal rich as the Sun.
disclaimer: I don't know anything, except what my parrot tells me.
Posted by Stephen on Wednesday 09th May 2007 (14:20 UTC)
Stephen, I'm not sure if the HST would see a wobble. The star is pretty close (~20 light years) but the planets are not very hefty so wouldn't induce much of a positional wobble.
Posted by Stuart on Wednesday 09th May 2007 (14:46 UTC)