Up to 170 ‘Rogue Planets’ detected by astronomers

By Mark Waghorn via SWNS

Up to 170 'rogue planets', floating in space with no sun to orbit, have been detected by astronomers - the largest group to date.

They lie in a star-forming region close to our sun in the southern constellations of Upper Scorpius and Ophiuchus.

The free-floating worlds drift aimlessly through space. There could be billions in the Milky Way.

They have masses comparable to those of the planets in the solar system.

First author Dr Nuria Miret-Roig, of Bordeaux University in France, said: "We did not know how many to expect and are excited to have found so many."

They have been known about for decades. But their prevalence is an enigma. Our sun has at least eight - and most other stars have some.

An international team combined observations from space and ground-based telescopes.

The mysterious worlds lurk far away from any star illuminating them - and would normally be impossible to detect.

But in the few million years after their formation, they are still hot enough to glow.

Sensitive cameras on the VLT (Very Large Telescope) and VISTA (Visible and Infrared Survey Telescope for Astronomy) identified them directly.

They have masses similar to Jupiter's - and would be too cold to harbour life.

To spot so many the researchers used data spanning about 20 years from a number of telescopes.

They included the European Space Agency’s Gaia satellite. It's a big step in the exploration and understanding of our universe.

Dr Miret-Roig said: "We measured the tiny motions, the colours and luminosities of tens of millions of sources in a large area of the sky.

"These measurements allowed us to securely identify the faintest objects in this region, the rogue planets."

Rogue planets may even outnumber the stars in the galaxy. Experts believe they hold the key to its evolution.

Project leader Dr Herve Bouy, also from Bordeaux, said: "The vast majority of our data come from ESO observatories, which were absolutely critical for this study.

"Their wide field of view and unique sensitivity were keys to our success.

"We used tens of thousands of wide-field images from ESO facilities, corresponding to hundreds of hours of observations, and literally tens of terabytes of data."

The study in Nature Astronomy suggests there could be many more of these elusive, starless planets that we have yet to discover.

Dr Bouy said: "There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star."

Analysing the newly found ones could reveal clues to how they form. One theory is they are collapsed gas clouds too small to make a star.

Another is they were previously bound to a host star - before hurtling into interstellar space.

It's hoped further advances in technology will solve the riddle of nomadic planets.

The researchers plan to study them in greater detail with ESO's forthcoming ELT (Extremely Large Telescope).

It's currently under construction in the Atacama Desert in Chili, and is due to be working before the decade is out.

Dr Bouy said: "These objects are extremely faint and little can be done to study them with current facilities.

"The ELT will be absolutely crucial to gathering more information about most of the rogue planets we have found."

The exact number of rogue planets found by the team is hard to pin down as they can't measure the masses of the probed objects.

Those about 13 times bigger than Jupiter are most likely not planets - so cannot be included.

So the researchers analysed the planets' brightness to provide an upper limit to the number spotted.

The brightness is, in turn, related to age as the older a planet, the dimmer it is as it cools down.

If the studied region is old, then the brightest objects in the sample are likely above 13 Jupiter masses, and below if it's on the younger side.

Given the uncertainty in the age of the region, this method provided a rogue planet count of between 70 and 170.

A previous study by US scientists suggested the Milky Way could harbour more than 100 billion rogue planets.

The Nancy Grace Roman Space Telescope - due for launch in 2027 - is to build the first census of them.

It's likely to be ten times more sensitive to these objects than current ground-based telescopes.