I’ll try not to be prematurely excited here, but there are rumors running around that a team of astronomers have found 2 more planets around Gliese 581 (we already know of one planet there), which might be located in the habitable zone, meaning that the distance to the star is just right so that the temperature is in the range where liquid water can exist. And liquid water is the one thing that makes a planet habitable by human standards.

The team hasn’t yet released any data, so we have to wait for that before drawing any conclusions, but if this is true, it is exciting news. Also, the method used to discover the planets make it hard to tell exactly what size and composition they have. If you’ve followed Reduced Mass for a while, and remember our articles on exoplanets, you’ll remember how most planets have been found using the method of radial velocity. This method measures a characteristic wobble of the star caused by orbiting planets. Until now this method has been cursed by not being able to find small (rocky) planets in large enough distances from the planets for them to be habitable. But now the technology has improved to the point where this is actually possible.

Thats all good, but there are still some problems with this method. For one it can only give a lower limit on the mass, meaning that it could be more massive than the proposed 5 Earth masses, depending on the how the solar system is tilted compared to our line of sight. Secondly it can only deduce the (err, lower limit of the) mass of the planet, and not the size, which is very important to know weather it is made of rock or gas. Rock is good, gas is bad, from a habitable point of view of course. So even if the data holds, and this is a 5 Earth mass planet in the habitable zone, we won’t know if it’s actually habitable before we are able to either analyze the light from the planet itself and determine the chemical composition of the atmosphere (if any), or somehow measure the size of it (meaning its radius).