Free-floating blue planet
Artist’s impression of the free-floating planet.

Sydney: What looks like a young, lone planet roaming through space with no star to orbit could become a benchmark for uncovering the nature of massive planets outside our Solar System, astronomers report.

It is estimated to be about 100 light-years away from our Solar System, could be either a 'failed' small star or a planet expelled from its system, according to the study published in Astronomy & Astrophysics.

"To me, the best thing is that this object is a really easy-to-study prototype of the 'normal' giant planets we hope to discover and study with the upcoming generation of direct imaging instruments," said lead author Philippe Delorme from Institut de Planétologie et d'Astrophysique de Grenoble, France.

Brown dwarf or ejected planet

A number of free-floating space objects with potentially planetary masses have been discovered previously. There is currently no measurable way for astronomers to ascertain how such objects have formed and, therefore, exactly what they are.

One possibility is that they formed at the centre of an accretion disk - the flattened, spinning cloud of gas and dust that funnels material onto a forming star - but without gathering enough mass to become self-sustaining, nuclear fuel burners. These 'failed stars' are known as brown dwarfs.

A second scenario could be that they formed as orbiting planets inside an accretion disk and were subsequently ejected from the developing planetary system.

While, according to Delorme, "we cannot say which hypothesis is more probable" for the newly identified free-floating object, its unique position and closeness to our Solar System means astronomers can study it in much greater detail than what has been possible for the more distant objects found previously.

"Firefly in a car headlight"

"Looking for planets around their stars is akin to studying a firefly sitting one centimetre away from a powerful car headlight. This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up," Delorme said in a media statement.

The researchers used two very powerful telescopes to record images and spectra of the object.

"This is the first time we can get a good quality spectra of a planetary-mass object and therefore measure its temperature, gravity and what molecules its atmosphere contains," Delorme said.

His team estimated that the bright 'space rover' has a temperature of around 430˚C, a mass of four to seven times that of Jupiter and is between 20 and 200 million years old - a youngster in galactic history. They established with 87% probability that it is a member of a stream of young stars known as the AB Doradus moving group, which is the closest such group to our Solar System.

Delorme and colleagues believe their discovery will help to better understand how star formation processes may produce planetary-mass objects, or how planets may be ejected from planetary systems.

"If this object is an ejected planet, and not a planetary-mass brown dwarf, it hints that ejections are common during planetary formation and that there are many ejected exo-Neptunes and exo-Earths wandering alone, lost in space, as some other studies have hinted," Delorme said.

Benchmark for stargazing

"The fact is we have absolutely no idea whether a given object was formed as a planet or a brown dwarf. There is at present no way to distinguish between the two, based on the spectrum emerging from the object," said Chris Tinney, an astrobiologist at the University of New South Wales, Australia, who was not involved in the research.

"The interesting thing about this object is that you can use it to get a model for what the planets we detect indirectly around other stars might actually look like."

Delorme's team will now focus on getting more accurate and direct measurements of the object's motion and distance from our Solar System (currently estimated at about 100 light-years), so they can confirm whether it belongs to the AB Doradus moving group and hence is young and has a planetary mass.

"It will be a benchmark to understand the physics and chemistry of massive exoplanets," Delorme said.