Largest group of free-floating planets found in the Milky Way

A team led by astronomers of the Laboratoire d’astrophysique de Bordeaux has reported the discovery of about 100 free-floating planets roaming in our galaxy. This study published 22 December 2021 in the journal Nature Astronomy is key to understanding the formation of planetary systems.

  • 05/01/2022

Artist's impression of a free-floating planet lost in deep space © University of Bordeaux Artist's impression of a free-floating planet lost in deep space © University of Bordeaux

Rogue planets are elusive cosmic objects that have masses comparable to those of the planets in our Solar System but do not orbit a star, instead roaming freely on their own. Not many were known until now, but the astronomers of the European project COSMIC DANCE, led by Hervé Bouy, a professor of the University of Bordeaux at the Laboratoire d’astrophysique de Bordeaux (LAB – CNRS and University of Bordeaux) have just discovered between 70 and 170 new rogue planets in the Milky Way. This is the largest collection 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*, a postdoctoral researcher at the University of Vienna, Austria, who received her PhD from the University of Bordeaux, and is the first author of the new study published 22 December 2021 in the journal Nature Astronomy.

Wanderers in the galaxy

Rogue planets, lurking far away from any star illuminating them, would normally be impossible to image. However, Núria 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.

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

This image shows a small area of the sky in the direction of the region occupied by Upper Scorpius and Ophiucus. It zooms in on a recently discovered rogue planet, meaning a planet that does not orbit a star but instead roams freely on its own. The rogue planet is the tiny, bright red dot at the very centre of the image. © ESO / Miret-Roig et al.

The team of the COSMIC DANCE project used observations from several European Southern Observatory (ESO) telescopes located in Chile, along with NSF’s NOIRLab facilities, the Canada France Hawaii telescope and the Subaru telescope.

“The vast majority of our data comes 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. “We used tens of thousands of wide-field images, corresponding to hundreds of hours of observations, and literally tens of terabytes of data.”

Several billion planets without a host star

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.

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,” Hervé 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.

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

*Núria Miret-Roig has been awarded the 2021 PhD Thesis Prize in Science and Technology of the University of Bordeaux for her research.

Source: ESO press release

Bibliographic reference

Miret-Roig, N., Bouy, H., Raymond, S.N. et al. A rich population of free-floating planets in the Upper Scorpius young stellar association.

Scientific contact

Hervé Bouy
Professor of the University of Bordeaux at the LAB and COSMIC DANCE principal investigator