It's one of the big questions: Are we alone on this blue marble or is there life elsewhere in the cosmos? To shed some light, astronomers are searching for habitable worlds circling far-off stars.
A team has now published updated evidence for a planet that could be the most Earth-like yet. According to the US Planetary Habitability Laboratory, it would be the fifth potentially habitable world known outside our Solar System.
So what do we know about these five Earth-like planets, and how likely is it that they could support life?
The discovery of Gliese 581g was announced in September 2010 by a US-led team. But as soon as they made the announcement, doubts began to surface. The team at the Geneva observatory which had discovered all four other planets around the star Gliese 581 failed to detect it in their own data. However, the original discoverers of 581g have now published an analysis using a greater amount of data to provide more promising evidence for its existence.
This would be significant because the Earth Similarity Index (ESI) , devised by a team including Dirk Schulze-Makuch from Washington State University and Abel Mendez from the University of Puerto Rico (UPR) in Arecibo, shows that Gliese 581g is the most Earth-like planet discovered to date. The ESI measures characteristics of exoplanets on a scale from zero to one, with one being identical to Earth. Accordingly, the online Habitable Exoplanets Catalog , based at UPR, has decided to include it in their list of the most promising worlds to support life.
Like the other worlds in the catalogue, Gliese 581g orbits in a "sweet spot" around its star - the habitable zone, or Goldilocks zone - that is neither too hot nor too cold to allow for liquid water. It is just over twice the mass of Earth and, although the planet is closer to its parent star than is Earth, it receives about the same light flux (a measure of the star's apparent brightness) as our planet because Gliese 581 is a red dwarf star and therefore dimmer than our own Sun. Steven Vogt, from University of California, Santa Cruz, US, one of original discoverers, said Gliese 581g orbits "squarely in the star's Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility". But it remains to be seen whether the new evidence will convince the doubters.
New kid on the block
Announced in February 2012 by a team from the US Carnegie Insitution and Germany's Goettingen University, Gliese 667Cc belongs to a class of planets known as Super Earths - a class of planet with a size in between that of rocky worlds like Earth and Mars and the gas giants like Jupiter and Saturn.
Roughly 4.5 times the mass of Earth, Gliese 667Cc takes 28 days to complete a single orbit of its star, which is located just 22 light-years away.
The exoplanet absorbs about as much energy from its star as Earth does from the Sun, which means surface temperatures could be similar to Earth's. This would allow for the presence on the planet's surface of liquid water - one of the pre-requisites for life.
However, astronomers say this cannot be confirmed without further information on the planet's atmosphere. Nevertheless, co-discoverer Guillem Anglada-Escude has said of 667Cc: "This planet is the new best candidate to support liquid water and, perhaps, life as we know it."
With a radius 2.1 times that of Earth, Kepler 22b is bigger than some others in the list of five. But its orbit of 290 days around a Sun-like star resembles that of our own planet.
Astronomers do not yet know for sure whether it has a predominantly gaseous, rocky or liquid composition. But to some, its size suggests a composition closer to the gaseous Neptune than to Earth. At the time of its discovery, exoplanet hunter Geoff Marcy, from the University of California, Berkeley, told the AP news agency that he would "bet my telescope that there is no hard, rocky surface to walk on".
But this is not necessarily bad for its prospects for hosting biology: mission scientist Natalie Batalha said that if Kepler 22b were mostly ocean with a small rocky core, "it's not beyond the realm of possibility that life could exist in such an ocean".
Another Super-Earth, with a mass 3.6 times that of our planet, HD85512b orbits on the inner margins of its habitable zone. This means it is closer to Venus than Earth in the amount of energy it absorbs from its star. However, that would not necessarily preclude the possibility of life.
How hospitable the planet is may depend on how much cloud cover it has , since clouds reflect solar radiation. Its discoverers calculate that if HD85512b were to have more than 50% cloud cover (on the same order as Earth), it could offset the planet's proximity to its star enough for water to stay liquid at the surface.
However, any life forms on HD85512b will have to be suited to very balmy conditions, "It's going to be really muggy, just think about the muggiest day you can think of," co-discoverer Lisa Kaltenegger, from the Max Planck Institute in Germany, said at the time of its discovery. "We're not saying it's habitable for you and me."
Discovered in 2007 by a team at the Geneva Observatory using the HARPS planet-finding instrument in Chile, Gliese 581d is probably too large to be formed entirely of rocky material. Instead, it is a serious candidate for an ocean planet, according to one of its discoverers, Stephane Udry.
One of a system of five planets, it is the immediate neighbour of Gliese 581g. However, Gliese 581d orbits at a greater distance from its parent star, on the colder, outer fringe of the habitable zone. As a result, sunlight on Gliese 581d has about 35% the intensity that is does on Mars.
But computer simulations of the exoplanet's atmosphere suggest that under some scenarios, greenhouse gases would allow surface temperatures to remain above 0C, allowing water to stay liquid. Indeed, says Dr Udry, the planet "could even be covered by a large and deep ocean".
Since it is located only 20.3 light-years away - in the Constellation Libra - astronomers should be able to resolve some of these questions in the future, with direct spectroscopic observations of the planet's atmosphere.