A home from home: Five planets that could host life

       Gliese 582g
~    Similarity to Earth (ESI): 0.92
~    Minimum mass: 2.2 times that of Earth
~    Distance: 20.3 light-years

By Paul Rincon Science editor, BBC News website

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?

Earth "twin"

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.   READ MORE