Artist rendering of an Earth-like planet in the Gliese system.
Image courtesy of ESO.
The new planet goes by the romantic name Gliese 581g and is part of a system of six known planets orbiting the same red dwarf star about 20 lightyears away from Earth. Two of these planets, 581c and 581d, orbit on either side of 581g - on the warmer and cooler edges, respectively, of the habitable zone (HZ). Meanwhile, planet g lies smack dab in the middle of the HZ, with a presumed equilibrium surface temperature of around 228 K (-45 C). That's pretty cold. Keep in mind, however, that the equilibrium temperature of the Earth is actually about 255 K (-18 C). We can thank Earth's atmosphere and the resulting greenhouse effect for keeping our planet nice and toasty. The same goes for Gliese 581g. A greenhouse effect would support liquid water, one of the cosmic benchmarks for potential development of life. There is an important difference, however: planet g is tidally locked to its star, much like the way our moon is tidally locked to the Earth. This means that one side of the planet always faces its star, while the opposite side always faces away. According to Steven Vogt, an astronomer from the UC - Santa Cruz team, a temperate and liveable climate would probably only exist at the boundary between the blazing hot sunny side and the frigid, perpetually dark side. This might be a card in life's favor, however. To quote Vogt, "Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude."
Habitable zone of the Gliese system as compared with that of our solar system. Gliese 581g has been discovered in the HZ between planets c and d.
Image courtesy of ESO.
Altogether, there are many factors that distinguish Earth from planetary newcomer Gliese 581g. For instance, the planet is at least three times as massive as Earth, and thus orbits its parent star far closer than we do the sun. Its surface gravity is likely stronger than ours, and it may not necessarily have the same rocky composition or protective atmosphere as we do. But it's a start, and it is certainly conceivable that life could have developed there. Unfortunately, since planet g never transits its star relative to our line of sight on Earth, researchers currently do not have the technology necessary to analyze its composition or exact mass. That being said, the discovery of planet g is the first of its kind, and it gives astronomers and astrobiologists hope of finding many more planets like it. As Vogt put it, "If these are rare, we shouldn't have found one so quickly and so nearby. The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy." His brazen comments are currently catching a lot of flack on the airwaves, but in theory, he's right. We'd be silly not to acknowledge the likelihood of life elsewhere in the universe, and this discovery is as good a start as any.