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.

Posted in astro blog by Stuart on Wednesday 25th Apr 2007 (10:42 BST) | Permalink
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