Scientists Discover Exoplanet Less Than Twice the Mass of Earth

Snowblindeye writes with this excerpt from the European Southern Observatory: "Well-known exoplanet researcher Michel Mayor today announced the discovery of the lightest exoplanet found so far. The planet, 'e,' in the famous system Gliese 581, is only about twice the mass of Earth. The team also refined the orbit of the planet Gliese 581 d, first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist. Planet Gliese 581 e orbits its host star — located only 20.5 light-years away in the constellation Libra ('the Scales') — in just 3.15 days. 'With only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet,' says co-author Xavier Bonfils from Grenoble Observatory. Being so close to its host star, the planet is not in the habitable zone. But another planet in this system appears to be. ... The planet furthest out, Gliese 581 d, orbits its host star in 66.8 days. 'Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star,' says team member Stephane Udry. The new observations have revealed that this planet is in the habitable zone, where liquid water could exist. '"d" could even be covered by a large and deep ocean — it is the first serious "water world" candidate,' continued Udry."

Comments

Re:but what about Earth 2...

[Chris Burke]

This is very interesting but no where near as exciting as finding another Earth like planet. I suppose we will have to wait for the next generation of telescopes before we find it though.

Well the 'e' planet is somewhat earth-like in mass and possibly earth-like in composition. It's not in the habitable zone for the star, but the closer a planet is to the star the easier it is to detect, and this exoplanet is at the very edge of our ability to detect (thus why this is news -- smallest exoplanet ever found). So you're right, we'll have to wait for technology to advance to find earth-sized rocky planets in the habitable zone (especially of non-dwarf stars).

What is a little surprising though is how many planetary systems we have found that are very different to our own. I can't believe ours is unique but perhaps it's quite rare.

I'm not sure anything we've found suggests that our type of solar system is rare. The limitations of our detection method by and large assures we'd find systems different from our own first. Astrophysicists might not have expected to find gas giants very close in to stars, but if they exist, we were going to find those first. The two main things that seem to have changed to me are that 1) we've gone from having nothing but our own solar system as an example and thus assuming ours was the model for all of them, to have many more examples showing different types and 2) we've learned that solar systems seem to be pretty common.

If we get to the point where detecting a solar system like ours would be simple, and despite finding thousands of others we don't find any like ours, then maybe that points to rarity. Right now though I doubt we're anywhere near being able to say that.