The planet Gliese 581c, located just 20 light-years from the solar system (in the constellation Libra) and for a time one of the smallest known extrasolar planets, has captured attention in recent years as one of the most likely candidates for life-bearing planets outside of our own solar system. As a planet just several times the mass of Earth and orbiting within the habitable zone of its star, the red dwarf Gliese 581, Gliese 581c certainly does hold out considerable appeal for planet-hunters hoping to find a planet like our own, capable (at least in theory) of supporting Earth-like life. However, we now know that while the conditions on Gliese 581c may be favourable enough to support life, it would be quite unlike life currently found on Earth, with the possible exception of certain so-called extremophile species, living in the harshest and least hospitable conditions on our own planet.
– About Gliese 581c –
Gliese 581c was discovered in 2007 by a Swiss team at the University of Geneva, using the HARPS instrument in Chile and the Canadian MOST space telescope. It is believed to have a mass of about 5-10 Earths, a radius somewhat larger than our own planet’s (a necessity, given the extra mass, though precisely how large depends on whether the higher mass is due to more water or more rock), and substantially stronger gravity. This puts it in a category of extrasolar planets known as “super-Earths”: probably rocky or Earth-like planets, but several times the mass of our own planet.
Gliese 581, the planet’s star, is a red dwarf star, only about one-third the mass of our own Sun. This means that the habitable zone is actually much closer in, to take advantage of the much smaller amount of energy being radiated outwards. Gliese 581’s orbit is less than seven miles from its star, a fraction of the distance from the Sun to our innermost planet Mercury. At that distance, its “year” lasts less than two weeks here on Earth.
– Prospects for Life –
Although there has been considerable excitement about the prospects for life on Gliese 581c, given the above limitations, it must be immediately understood that what life might exist there would be dramatically different than what exists on Earth. A red dwarf star emits far less light than our Sun; even at close range, the equivalent of our plants would need to be much more efficient in converting starlight into energy.
Moreover, given its proximity, Gliese 581c would have to be tidally locked, a process in which a small body orbiting in close proximity to a larger one develops a day equivalent to one orbital period. The best example close to Earth is our own Moon, on which one day is equivalent in length to one orbit around the Earth. In both cases, the consequence is that the same hemisphere is always facing toward the larger object. In other words, half of Gliese 581c experiences the harshness of permanent day, and the other half the bleak cold of permanent night. If life were to evolve, it would almost certainly have to cling to a perilous existence along the thin band in between these two extremes. Such life would live a marginal and rugged existence, probably similar to extremophile bacteria here on Earth – in short, Earth-like life, but only the most isolated and hardy of Earth’s species.
For the moment, there simply is not enough known about Gliese 581c to say whether there is some other mechanism that alters this balance and makes more Earth-like life possible. It is possible that the atmosphere helps with heat circulation, allowing a cooler climate around much of the planet. On the other hand, it is also possible that the atmosphere facilitates the sort of runaway greenhouse effect seen on Venus, where the entire surface is so hot that no liquid water can exist and human space probes can function only for a few minutes or hours before their systems fail in the overwhelming heat.
In short, while Gliese 581c does seem to be within the habitable zone of its star, there are many reasons to believe that Earth-like life simply cannot exist on its surface. However, this does not mean that there is no life on Gliese 581c at all: it may simply be very different from that on Earth. Moreover, Gliese 581c is only among the first of what is already a growing catalogue of Earth-like planets. Most astronomers accept that, given sufficient technological improvements, finding another Earth-like planet in the galaxy is only a matter of time and effort.
– Technological Limitations and Other Candidate Planets –
It also must be admitted that, given our present technology, we will have considerable difficulty finding planets capable of supporting Earth-like life. The best candidate, of course, would be a planet roughly the size of Earth, orbiting a yellow star at an orbit roughly somewhere between that of Venus and Mars. However, finding such a planet would be extremely difficult: it would be small enough that only a few very finely-tuned instruments, namely the Kepler space telescope, could actually see it, and, moreover, even they would only detect it at a very specific point in its orbit. Observing a planet identical to Earth, after all, would require training a telescope on its star for the equivalent of years. So far, the planets we find are those that are easiest to find: large planets, orbiting extremely close to their star so that their “years” last mere weeks on Earth. Our telescope technology is just too limited for us to be able to find true Earth-like planets, and therefore Earth-like life, very easily.
Advances in planet-hunting telescope technology are finding more Earth-like candidates every year. Indeed, another planet orbiting Gliese 581, named Gliese 581d, also orbits within the star’s habitable zone and it is believed may have a surface made up wholly or entirely of water, making it a strong candidate for life. Moreover, NASA’s Kepler space telescope is far more capable of finding extrasolar planets than any other technology yet built. Over the next several years, it is expected that Kepler will locate a substantial number of small, rocky planets, potentially finding some with much more Earth-like conditions than those found on Gliese 581c.