Exoplanet Gliese 581c is referred to as the "Goldilocks Planet" for extraterrestrial life. It appears to be small enough to be rocky planet, not a gas giant like almost all the other planet discoveries, and it's at the right distance from its star so that the surface temperature might be between water's freezing and boiling point, if greenhouse gases in its atmosphere doesn't make it too hot.
Typical reporting about the size of the planet, from CNN
: "new planet is about five times heavier than Earth...." From MSNBC
: "it has a radius just 50 percent larger than Earth's, a mass five times greater...." Even from the European astronomers who discovered it
: "Astronomers have discovered the most Earth-like planet outside our Solar System to date, an exoplanet with a radius only 50% larger than the Earth and capable of having liquid water. Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered a super-Earth about 5 times the mass of the Earth that orbits a red dwarf....
The problem with all these reports is that the planet is very likely to have a significantly higher mass than five earths. The planet was discovered with the radial velocity method, which only determines the minimum mass. The radial velocity mass determination is based on the equation msini, where m, the actual mass, is multiplied by the trignometric sine of i, the inclination of the planet's orbit with respect to earth. The only situation with current technology where we know what i is, though, is if the planet transits directly in front of the star with respect to earth, partially occulting its light, and that doesn't appear to be the case for Gliese.
To be fair to the European astronomers, they do add a footnote saying, "Using the radial velocity method, astronomers can only obtain a minimum mass (as it is multiplied by the sine of the inclination of the orbital plane to the line of sight, which is unknown). From a statistical point of view, this is however often close to the real mass of the system." I don't know the basis for that second sentence, though, unless they define "close" very loosely.*
Gliese 581c has five earth masses only if the inclination is edge on, i=90, and sini=1. There is no reason why the inclination shouldn't be random, so 45 degrees is the 50th percentile likelihood, where the real inclination and therefore real mass is equally likely to be greater or lesser than that figure. The sin of 45
is .707, and five divided by .707 is seven earth masses. The planet has a 50% chance of massing between five and seven earth masses, and 50% chance of being more than that. If the planet has 10-12 earth masses, then it's a Uranus-to-Neptune size gas giant (sometimes called ice giants), and nothing like the earth. At an inclination of 30 degrees or less (33% chance), the mass will be at least five divided by .5, or 10 earth masses.
So unless I've messed up, the Goldilocks planet has a 33% chance of being a gas giant. And even before that level, there's a problems with temperature. The larger the planet, the more likely it will trap enough greenhouse gases to experience a runaway greenhouse effect
that in some distant future will make the Earth resemble Venus.
So it's certainly possible that the planet's mass is close to five earth masses, and maybe the smaller mass levels might allow it to escape the runaway greenhouse, but I wouldn't count on it being habitable.**
*I see even wikipedia says
"One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass. Usually the true mass will be within 20% of this minimum value, but if the planet's orbit is almost perpendicular to the line of sight, then the true mass will be much higher." Again, I don't know the basis for the 20% statement - am I missing something?
**Some good comments at Bad Astronomy, especially this one
: "Definitely I am aware that this is a minimum mass, however I think the small scale of the system suggests the masses cannot be too much greater, or planets b and c would very likely be unstable." That argues against my inclination argument, but raises other reasons for the planet not being habitable.
UPDATE: Lab Lemming has some good ideas in the comments. I still think even with LL's ideas, it should be considered substantially more than 5 masses.