Gliese 581 c is a planet orbiting the main sequence red dwarf star Gliese 581. Its discovery was announced on April 24, 2007, and achieved through the tried-and-true radial velocity method. What makes Gliese 581 c unique is its size and location. It is located a mere 20.5 lightyears from Earth in the direction of the constellation Libra. Gliese 581 c is the smallest exoplanet discovered to date, at about five earth masses, and appears to be located in the habitable zone of its parent star. While these two pieces of information are the only things known about the planet, they invite speculation on a number of exciting possibilities.
With its higher mass, Gliese 581 c is almost certainly larger than Earth. Assuming it has a similar structure (rocky mantle, large iron core), Gliese 581 c would be about half again Earth's size. This assumption also leads to a surface gravity around 2.2 times that on Earth. On the other hand, a body of largely water and ice would be just under twice the size of Earth, with a surface gravity around 1.25g. Given the method of discovery it is impossible to directly determine the planet's radius, however it in all likelihood lies somewhere between these two extremes.
Due to its location in Gliese 581's habitable zone, Gliese 581 c may have liquid water on its surface. Indeed, this is the very definition of "habitable zone". Water in its liquid form is the key ingredient for life as we know it on Earth. In fact, everywhere on Earth that has liquid water also has life, be it microscopic or human. Depending on the planet's albedo, the mean surface temperature may be anything between -3°C and 40°C, mimicking most climates on Earth. The actual surface temperature may in fact be much higher than these estimates, depending on atmospheric composition and possible greenhouse effects.
The orbit of Gliese 581 c is only 11 million kilometers from its parent star. Compare this with Earth's orbit of 150 million kilometers. This makes a "year" on the planet a mere 13 earth days long. Because of this proximity, it is likely that Gliese 581 c is tidally locked, with one side always facing its parent star (similar to the moon's relationship with earth). This of course limits the prospects for life as we know it, but does not eliminate the possibility. The narrow ring of twilight between the night and day sides may have a suitable environment for life to develop. Alternatively, a sufficiently thick and active atmosphere could evenly distribute heat from the day side to the night side.
While these are all exciting ideas, the fact remains that very little of the planet is actually known. And, while 20.5 light years is a relatively short distance cosmologically, it would still take our best rockets thousands of years to get there. In the coming decades, as the next generation of space telescopes launches, we will doubtlessly learn much more about Gliese 581 c. We will flesh out the unknown bits of its size, composition, and orbital characteristics. At best, we will determine the composition of its atmosphere (assuming it has one), and perhaps detect subtle hints at the presence of life. But no one alive today will see this planet up close. It may be a lush world filled with exotic flora and fauna the likes of which no one could have possibly imagined... or a desolate rock, covered with a blistering, uninhabitable wasteland on one side, and a kilometers thick layer of frozen carbon dioxide on the other. It could be a hundred years before humanity knows for sure.