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The warmth of a sun has long been thought to be a key ingredient to life.

But astronomers say 'rogue', sun-less planets that wander the stars could still harbour extra-terrestrials.

While it sounds like science fiction, these planets may offer scientists a new avenue in their search for alien life.

The warmth of a sun has long been thought to be a key ingredient to life. But astronomers say 'rogue', sun-less planets that wander the stars could still harbour extra-terrestrials

This is according to Sean Raymond, an astrophysicist with the Laboratoire d'Astrophysique de Bordeaux in France, who has taken a look at how life can form on rogue planets.

Writing in Aeon, he says: 'To have any chance of life – at least life like our own – a free-floating Earth would need liquid water.'

A planets needs to keep warm for liquid water to form, and without the sun, this heat would have to come from its interior.

'A layer of ice on a planet's surface can act as a strong insulator, locking in a planet's heat,' claims Raymond.

Globulettes - regions of dust and gas that can form planets - could make rogue more common than first thought. Rogue planets are of particular interest to astronomers because they represent objects that have likely failed to form into a star

THE LONELIEST PLANET 

PSO J318.5-22 was detected 80 light years away from Earth and it is estimated to have a mass six times that of Jupiter

Several years ago, an international team of astronomers discovered a large planet floating freely through space with no star to orbit.

Planets traditionally travel in a uniform, singular direction, around a star. However, the free-floating planet, named PSO J318.5-22, was found without a host.

Its movement isn't structured, and scientists do not understand how it formed and they are baffled by what - if anything - controls it.

PSO J318.5-22 was detected 80 light years away from Earth and it is estimated to have a mass six times that of Jupiter.

Astronomers believe it formed 12 million years ago, and is considered a newborn in planetary terms - Earth is thought to be around 4.5 billion years old.

Dr Michael Liu of the Institute for Astronomy at the University of Hawaii identified the rogue planet from its 'faint and unique heat signature'.

We have never before seen an object free-floating in space that that looks like this.

'It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,' explained Liu.

'If the ice layer is thick enough, then a planet can maintain an ocean of liquid water beneath the ice.'

He calculates that the ice layer needs to be at least 10 km (6 miles) thick to prevent it from freezing for billions of years. 

Raymonds comments echo a similar conclusion reached by scientists in Chicago several years ago.

They calculated that life can cling on to a planet for billions of years without a star to provide a direct source of warmth.

Research by astrophysicists Dorian Abbot and Eric Switzer from the city's university discovered that heat would come from the breakdown of radioactive elements inside the planet's core.

The scientists named their discovery a 'Steppenwolf' planet because they claimed any life found there would 'exist like a lone wolf wandering the galactic steppe'.

However the pair refused to speculate on what life forms would be discovered, but agreed that they would me microscopic in size.

As well as internal heat, Raymond says another way a planet can keep warm is using a thick atmosphere.

'A free-floating Earth with a thick hydrogen atmosphere could keep its surface temperature above the freezing point of water,' writes Raymond.

'The planet could have lakes and oceans (and possibly life) on its surface.'

Rogue planets are hard to find because, without orbiting a star, they reflect very little light.

But 50 have now been found in the past 15 years, leading some to suggest they may be abundant in the universe.

Rogue planets are of particular interest to astronomers because they represent objects that have likely failed to form into a star.

Brown dwarfs, for example, are substellar objects that are not massive enough to sustain fusion, but are still more than 13 times the mass of Jupiter.

The exact boundary between a giant planet and a low-mass brown dwarf is still being debated, but finding more rogue planets could shed light on this mystery.

And this could indicate that there are more rogue planets than we know of in the cosmos.