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Worlds Apart

The chances of planets with similar conditions as our Earth existing were thought to be slim at best. However, in recent years the number of Earth-like planets discovered has steadily increased. The most recent, Gliese 581 g, is particularly exciting as it is the nearest Earth-like planet found to date, lying a mere 20.5 light years away from our own solar system. To put this into perspective, the nearest star to Earth, Proxima Centauri, is 4.2 light years away.

Our own planet, Earth, sits comfortably about 93 million miles from the sun. There's plenty of water and an atmosphere of nitrogen, oxygen and a little bit of carbon dioxide. Conveniently for us, the Earth spins on an axis, which prevents one side of our rocky blue planet developing extremes of temperature. Earth, like the newly found Gliese 581 g, lies in a narrow band known as the 'habitable zone' of a solar system, a place neither too far nor too close to its star, where temperatures and other conditions are best for sustaining life. This fact is often referred to as the ‘Goldilocks effect’–not too hot, not too cold, but just right.

The discovery of Gliese 581 g was made in September 2010 by teams led by Stephen Vogt from the University of California, Santa Cruz and Paul Butler from the Carnegie Institute in Washington, DC. They made their observations at the Keck Observatory in Hawaii. Whilst anyone can go out on a clear night and observe stars light years away, observing the planets that orbit these stars is a tricky business; they are far too small to be seen by any telescope we have today. The planet Gliese 581 g was detected using a technique known as Doppler spectroscopy. The method works on the principle that the gravity of planets within a solar system exerts small amounts of attraction on the parent star, causing it to 'wobble' slightly about a central point. These ‘wobbles’ or small movements of the parent star, are detected using telescopes and interpreted by astronomers to predict the existence of planets. Previously studied wobbles of the Gliese 581 star revealed four other planets in the system. Astronomers recently found two new signals, representing Gliese 581 f, a planet on the edge of this system, and Gliese 581 g, the newly discovered Earth-like planet.

Gliese 581 g, like our own Earth, lies in the middle of the habitable zone of its own star, which is a small red dwarf that emits infra-red light. Remarkably, many of its physical characteristics are not too dissimilar to our own planet: its mass is estimated to be 3.1 to 4.3 times that of Earth's, its radius 1.3 to 1.5 times wider and gravity about 1.6 times as strong. Gliese 581 g is thought to be a rocky planet, based on the estimations of its mass. Simulations suggest that the lower the mass of a planet, the more likely it will be rocky in composition. The similarities, although important, appear to end here. Gliese 581 g does not undergo a day or night cycle as one side of the planet continuously faces its sun. This, you might imagine, will create a planet with extremes of temperature on the dark and sunny sides, separated by a narrow 'twilight zone' where conditions may be more favourable; stand here and you will be seeing an eternal sunset. Owing to the slightly stronger gravity on Gliese 581 g than Earth, the atmosphere is thought to be sufficiently dense to maintain enough air circulation to create a habitable area on the surface. At this point little is known about the actual atmospheric and solid composition of the planet as no methods exist that allow us to see past the intense glare emitted from the Gliese 581 star. Any predictions on whether there will be water, a key substance for the life to arise and answer the question, “Is there anybody out there?” are not yet possible.

The discovery of Gliese 581 g, quite early on in the search for extrasolar planets, suggests that habitable planets are much more common than previously thought. "If you take the number of stars in our galaxy – a few hundred billion – and multiply them by 10 or 20 per cent, you end up with 20 or 40 billion potentially habitable planets out there," says Stephen Vogt, "It's a very large number." Whether humans will ever be able to visit these far away worlds, is, however, another story altogether.

Declan Valters is an undergraduate student in Geosciences

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