You may or may not have heard about this one. The news broke just recently, and it's getting a ton of buzz on the scientific side of the web. Reports of over 200, Jupiter-like (large, gaseous) exoplanets have been discovered in the past 10-or-so years, so what's the big deal deal with 581 c?
It might have water.
It might be habitable.
With that in mind, it's not difficult to see why nerds, geeks, astronomers, astrobiologists, and whoever else might fit the bill are positively salivating.
A few stats on 581 c:
1) Located at an extremely close 20.5 light years away, it orbits the star Gliese 581 in the constellation of Libra. For context, the nearest star system to Earth is Alpha/Beta/Proxima Centauri at 4 light years away.
2) Gliese 581 is a red drawf star, relatively small with respect to our own Sun. Red dwarf stars are some of the most common in our neighbourhood; of the 100 closest stars to Earth, 80 are red dwarves. These stars have a life span of up to 100 times longer than our own Sun's estimated 10 billion years (5 of which have passed).
3) 581 c is estimated to be about one and a half times larger than Earth, with a mass of about 5 times greater. The density is reported at 8.1 g/cm^3 - Earth's is 5.5 (Mercury and Venus are 5.4 and 5.2 respectively. Mars is 3.9 g/cm^3). Those numbers may not mean much, but it suggests that 581 c follows the typical terrestrial model of a thin crust (< 2) and dense, diffrentiated core (10-12 g/cm^3) which is clearly much larger than the Earth's. Gravity is also stronger as a result of the increased mass, and 581 c probably has a stronger magnetic field (important for blocking star radiation).
(Of course, I'm thinking the flip-side of this coin is that if the core is so large, the mantle likely is too, and there's probably a lot of interior heat. If we look at the Moon, the patchy areas we can see with the naked eye and called 'Maria' (seas) are large basaltic lava flows that appear on the Earth-facing side as a result of the Earth's gravity. If we have a similar situation here with 581 c's synchronous orbit, it could mean the day side is (or at one time was) a volcanic mess like Venus.)
4) The planet is much closer to its star than is the Earth to the Sun, but as Gliese 581 is a red drawf, 581 c lies right in the habitable zone. It's in a synchronous orbit, meaning it rotates at the same rate it revolves, always presenting the same face to its star (much like the Moon always faces the Earth as a result of synchronous orbit). Its orbital period is 13 days.
5) Speculation about water is off the charts. As the surface temp should be within a 0-40C range, this certainly means that water could exist in liquid form. Some are even postulating 581 c contains vast oceans of water. I think they might be overly optimistic, but hey, it's possible, right?