“Nomads of the Galaxy” –Unmoored, Rogue Planets May Outnumber Milky Way’s Stars


Rogue PLanet


Imagine a galaxy filled with roaming, dark, lifeless island worlds – rogue, free-floating planets (FFPs) without the life-giving light of an alien host star. Welcome to the Milky Way Galaxy. Astronomers suggest that there may be more rogue planets than there are stars in the Milky Way.

“The universe could be teeming with rogue planets and we wouldn’t even know it,” said Scott Gaudi, a professor of astronomy and distinguished university scholar at The Ohio State University and chair of NASA’s Exoplanet Exploration Analysis Group.

70 New Rogue Planets Discovered in Our Galaxy

Not many such objects were known until now, but a team of astronomers, using data from several European Southern Observatory (ESO) telescopes and other facilities, have just discovered at least 70 new rogue planets in our galaxy. This is the largest group of rogue planets ever discovered, an important step towards understanding the origins and features of these mysterious galactic nomads.

“We did not know how many to expect and are excited to have found so many,” says Núria Miret-Roig, an astronomer at the Laboratoire d’Astrophysique de Bordeaux, France and the University of Vienna, Austria, and the first author of the new study published today in Nature Astronomy.

Fast Facts About the Milky Way Galaxy

Miret-Roig and her team took advantage of the fact that, in the few million years after their formation, these planets are still hot enough to glow, making them directly detectable by sensitive cameras on large telescopes. They found at least 70 new rogue planets with masses comparable to Jupiter’s in a star-forming region close to our Sun, located within the Scorpius and Ophiuchus constellations. All of the newly discovered rogue planets have masses less than 13 times that of Jupiter, which is the minimum mass to fuse deuterium (heavy isotope of hydrogen) and the canonical division between planet and brown dwarf. 

To spot so many rogue planets, the team used data spanning about 20 years from a number of telescopes on the ground and in space. “We measured the tiny motions, the colors and luminosities of tens of millions of sources in a large area of the sky,” explains Miret-Roig. “These measurements allowed us to securely identify the faintest objects in this region, the rogue planets.”

TESS Detects Mystery Objects –“Neither a Brown Dwarf or a Star”

ESO Observatory Data

The team used observations from ESO’s Very Large Telescope (VLT), the Visible and Infrared Survey Telescope for Astronomy (VISTA), the VLT Survey Telescope (VST) and the MPG/ESO 2.2-metre telescope located in Chile, along with other facilities. 

“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,” explains Hervé Bouy, an astronomer at the Laboratoire d’Astrophysique de Bordeaux, France, and project leader of the new research. “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 team also used data from the European Space Agency’s Gaia satellite, marking a huge success for the collaboration of ground- and space-based telescopes in the exploration and understanding of our Universe.

Several Billions Roaming the Milky Way?

The study suggests there could be many more of these elusive, starless planets that we have yet to discover. “There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star,” Bouy explains.

By studying the newly found rogue planets, astronomers may find clues to how these mysterious objects form. Some scientists believe rogue planets can form from the collapse of a gas cloud that is too small to lead to the formation of a star, or that they could have been kicked out from their parent system. But which mechanism is more likely remains unknown.

“The absolute number of FFP planets that have been found is less interesting than the fact that at least some of these are likely to be FFP with mass as low as ~5 Jupiter masses.  This poses a challenge for theories where these objects form like stars,” Scott Gaudi told The Daily Galaxy..

Further advances in technology will be key to unlocking the mystery of these nomadic planets. The team hopes to continue to study them in greater detail with ESO’s forthcoming Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert and due to start observations later this decade. 

“These objects are extremely faint and little can be done to study them with current facilities,” says Bouy. “The ELT will be absolutely crucial to gathering more information about most of the rogue planets we have found.”  The infrared James Webb Space Telescope, currently scheduled to launch on Christmas Day Dec. 25, will also be able to more fully characterize these cool, faint objects.

Fast Facts About Brown Dwarfs

The Last Word

“The high mass boundary, hence definition, of the word “planet” is still very much under investigation,” says astrophysicist and dailygalaxy.com editor, Jackie Faherty. “What is clear is that mass is not a good delineator between planets and brown dwarfs. The real question is how the objects formed: either through the fragmentation of a giant molecular cloud or the accretion of material around a newly formed star. 

“ In the case of this paper which is claiming a large over abundance of ‘free-floating planets’, Faherty says, “the argument can be made that there is some crossover between formation mechanisms that can create an identical object through multiple means. It remains to be seen what the masses of these sources are (its highly model dependent and age of the association dependent) and they need to be verified as bonafide members through spectroscopic and astrometric follow-up. But the finding is enticing and certainly worthy of dedicated follow-up.”

Image credit top of page: artist’s rendition of a Jupiter-sized rogue planet, floating freely through interstellar space without a parent star.  NOIRLab/NSF/AURA/J. da Silva)

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Jackie Faherty and  ESO


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