A LONELY planet, wandering through space without a star or companions, has caught the eye of astronomers searching for alien life.
The gaseous exoplanet, dubbed PSO J318.5-22, is just 80 light years from Earth and has a mass only six times that of Jupiter.
Having formed 12 million years ago, the planet is considered a newborn among its peers.
"We have never before seen an object free-floating in space that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone," said research team leader Michael Liu of the Institute for Astronomy at the University of Hawaii at Manoa.
"I had often wondered if such solitary objects exist, and now we know they do."
The researchers, whose study was published in the Astrophysical Journal Letters, identified the planet from its faint and unique heat signature using the Pan-STARRS 1 wide-field survey telescope on the Haleakala volcano of Hawaii's Maui island.
This image shows the free-floating planet PSO J318.5-22.Picture: N. Metcalfe / Pan-STARRS 1 Science Consortium.
The discovery comes as the same time that a planetary "graveyard" some 150 light years away reveals it once had the right conditions for life - and planets like Earth may have orbited the star known as GD 61, British astronomers reported in the journal Science.
But it is the wanderer that has caught the public's imagination.
The Astronomers suggest the newly found planet may have the lowest mass of all known freely floating objects.
Other telescopes in Hawaii showed that the planet has similar properties to those of gas giants orbiting around young stars, but PSO J318.5-22 is nowhere near a host star.
During the past decade, researchers have found about a thousand extrasolar planets using indirect methods, including planet-induced wobbling or dimming of their host stars.
An artists impression shows a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. Picture: AFP/Mark A. Garlick, space-art.co.uk, The University of Warwick and The University of Cambridge
But only a handful of these planets have been observed directly since most are orbiting around young stars less than 200 million years old and thus very bright.
PSO J318.5-22 "is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth," said co-author Niall Deacon of the Max Planck Institute for Astronomy in Germany.
Meanwhile, the discovery of the "planetary graveyard" raises hopes of observing a system with life-supporting conditions.
"This planetary graveyard swirling around the embers of its parent star is a rich source of information about its former life," said co-author Boris Gansicke of the Department of Physics at the University of Warwick.
Around 200 million years ago, GD 61 lost its power and began sucking in the nearest planets with its extreme gravitational pull, devouring them to pieces.
Now that Sun is what is known as a white dwarf, or a dying star that is circled by planetary debris.
Astronomers have typically studied living stars and the planets that circle them in the search for other worlds that are a reasonable distance from their stars and therefore not too cold or too hot for life.
But a handful of promising far-away discoveries by the NASA Kepler mission have been limited to the size and density of planets in this so-called Goldilocks zone.
Their compositions, whether rocky like Earth or gassy like Jupiter, have remained a mystery because astronomers couldn't get close enough to peer at their surfaces, or inside them.
The latest analysis focused on a dead planet that has been broken to bits, allowing scientists to analyse the fragments and actually see inside the contents.
Previous research has examined 12 destroyed exoplanets orbiting white dwarves in this way, but never before has the signature of water been found.
Using ultraviolet spectroscopy data, scientists have shown that the fragments contain about 26 per cent water by mass, far greater than the Earth's 0.02 per cent.
The team detected magnesium, silicon, iron and oxygen in the white dwarf's atmosphere, making up the main components of rocks.
The high proportion of oxygen indicates the presence of water.
"Those two ingredients - a rocky surface and water - are key in the hunt for habitable planets outside our solar system so it's very exciting to find them together for the first time outside our solar system," said Mr Gansicke.
The findings are based on data from the Hubble Space Telescope and the W.M. Keck Observatory on Mauna Kea on Hawaii on planets far beyond the solar system.
"The finding of water in a large asteroid means the building blocks of habitable planets existed - and maybe still exist - in the GD 61 system, and likely also around substantial number of similar parent stars," said lead author Jay Farihi, from Cambridge's Institute of Astronomy.
"A system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces," Mr Farihi said.
"Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system."
While the findings offer fresh hope of someday locating other planets where life exists, they also provides a sobering reminder of what lies ahead for Earth, perhaps six billion years from now, when alien astronomers may be studying the fragments of our solar system.
