Astronomers Find 70 Rogue Exoplanets Wandering the Milky Way in 'Breakthrough' Discovery

Astronomers have discovered at least 70 new "rogue" exoplanets wandering the Milky Way unbound to a parent star. Few of these cosmic orphans, with masses similar to the planets of our solar system, had been discovered until now, and this latest batch represents the largest find of such worlds.

Researchers hope the discovery of so many of these freely-drifting planets could help us better understand how they evolve and the processes that divorce them from the planetary system and stars.

"We did not know how many [rogue planets] to expect and are excited to have found so many," Núria Miret-Roig, astronomer at the Laboratoire d'Astrophysique de Bordeaux, France, and the University of Vienna, said.

Miret-Roig led the investigation, which discovered 170 rogue exoplanets and confirmed at least 70. She is the first author on a paper discussing the findings published in the journal Nature Astronomy.

"Unlike planets in our solar system or in exoplanetary systems, rogue planets do not orbit a star but wander alone," Hervé Bouy, astronomer at the Laboratoire d'Astrophysique de Bordeaux and architect of the investigation, told Newsweek. "They are special because their properties contain imprints of the mechanisms involved in star and planet genesis that are in many ways much more legible and informative than those of more massive counterparts."

Bouy continued: "We have at least doubled the total number of free-floating planets directly detected to date, and this is the largest sample known in a single stellar association, with other examples the spread in the sky.

"We believe the discovery is a breakthrough for both exoplanet and planetary research and star formation studies."

Bouy expressed surprise at the discovery of such a large batch of nomad planets. He said to Newsweek: "We have known they [rogue planets] existed since the early 2000s and previous studies based in indirect detections have suggested that they could be numerous.

"So it was half a surprise, but still a surprise, because they are indeed hard to detect."

The reason rogue planets are so difficult to spot is that they exist at great distances from Earth and don't reflect light from a parent star, as they are not close to such a stellar body most of the time.

Miret-Roig, Bouy, and their team took advantage of the fact that, for a few million years after their formation, rogue planets are still hot enough to glow. This means that they are directly detectable by sensitive cameras on large telescopes.

"Identifying free-floating planets within a star cluster is a major challenge, in many ways, similar to the 'needle in the haystack' adage," Bouy said. "First, one needs eyes sensitive enough to detect the 'needles.' While stars are relatively bright and easy to spot, planetary-mass members are several thousand times fainter and can only be detected with large aperture telescopes and sensitive detectors."

In order to spot so many rogue planets, the team used data spanning about 20 years from a number of ground and space-based telescopes. From this data, they measured tiny motions, colors, and luminosities of tens of millions for cosmic sources.

These measurements allowed the team of astronomers to securely identify the faintest objects in this region, revealing at least 70 new rogue planets. These nomad worlds have masses comparable to Jupiter's and are drifting through a star-forming region located in the Scorpius and Ophiuchus constellations

There are still many questions to answer about these nomad planets, primarily, how have they come to wander the Universe alone. Bouy said "Up to now two possibilities were proposed. Firstly, that they form like stars, from the collapse of a small molecular clump.

"Secondly, they form as 'regular' planets within planetary systems, and are later ejected or stripped off from their parent star."

Bouy added that the team's findings indicate both these mechanisms may be at play in creating these cosmic nomads, but the ejection model results in a significant contribution to this population.

The team aims to study rogue planets in greater detail with the ESO's Extremely Large Telescope, currently under construction at Cerro Armazones in Chile, when it comes online in around 2027.

"The fact that billions of rogue planets must be roaming the galaxy is quite fascinating!" Miret-Roig told Newsweek.

Locations of Rogue Exoplanets
An image showing the location of 115 potential rogue exoplanets in Upper Scorpius and Ophiucus. Thes are from a wider batch of orphan worlds, of which 70 the team confirmed as star-less planets N. Risinger/ESO

Nikku Madhusudhan, professor of Astrophysics and Exoplanetary Science at the Institute of Astronomy, University of Cambridge, was not involved in the research. He said about the discovery: "The findings are very interesting. If the estimates are representative they could imply a notable fraction of the giant exoplanet population as free-floating worlds.

"They also suggest that planet formation in the outer reaches of planetary systems might be more unstable than previously thought, which provides important constraints on planet formation models.

"Some of these newly found planets could be good targets for atmospheric observations which can provide important clues about their compositions and formation histories."

Caltech planet formation and evolution researcher Przemek Mroz, who was not involved in the research, also spoke to Newsweek regarding the team's findings, describing the work as "an important study"

Mroz added the authors may need to further determine that the rogue planets are not actually brown dwarfs, so-called failed stars which never ignited, that exist in the mass boundary between a planet and a star.

A Rogue Planet
An illustration of a rogue planet with the Rho Ophiuchi cloud complex visible in the background. Researchers have discovered a batch of at least 70 new orphan exoplanets wandering the Milky Way alone. M. Kornmesser/ESO