Astronomers Find 70 Planets Without Stars Floating Free in the Milky Way

The field of extrasolar planet studies continues to reveal some truly amazing things about our Universe. After decades of having just a handful of exoplanets available for study, astronomers are now working with a total of 4,884 confirmed exoplanets and another 8,288 awaiting confirmation. This number is expected to increase exponentially in the coming years as next-generation missions like the James Webb Space Telescope (JWST), Euclid, PLATO, and the Nancy Grace Roman Space Telescope (RST) reveal tens of thousands more.

In addition to learning a great deal about the types of exoplanets that are out there and what kind of stars are known to give rise to them, astronomers have also made another startling discovery. There is no shortage of exoplanets in our galaxy that don’t have a parent star. Using telescopes from around the world, a team of astronomers recently discovered 70 additional free-floating planets (FFPs), the largest sample of “Rogue Planets” discovered to date, and nearly doubling the number of FFPs available for study.

The research team responsible for the discovery was led by Nuria Miret-Roig, a postdoctoral researcher with the Laboratoire d’Astrophysique de Bordeaux (LAB) and the University of Vienna. She was joined by multiple researchers from the LAB, the National Institutes of Natural Sciences (NINS) in Kyoto, the Centre National de la Recherche Scientifique (CNRS) in France, and the Centro de Astrobiología (CAB) and Departamento de Inteligencia Artificial (DIA) in Spain. The study that describes their findings was recently published in Nature Astronomy.

To break it down, astronomers have speculated about the existence of FFPs (also known as “Rogue Planets“) for decades, and numerical simulations have indicated that they may be entirely common. In fact, some research has shown that there may be billions of these planets floating around in interstellar space – potentially outnumbering stars in the Milky Way! The exact mechanisms for how planets go rogue remain a mystery, but several theories exist.

Among them, astronomers have conjectured that planets regularly form in interstellar space, that they are pulled away by gravitational interactions with passing stars, that supernovae kick them out, or that they free float into space after their sun dies. As Roig and her colleagues indicated in their study, previous research has identified FFPs in young stellar clusters and within the Galactic Field. Still, the samples were always small or heterogeneous in age and origin.

Moreover, rogue planets are usually impossible to image in visible light, much like trying to discern exoplanets that orbit stars several thousand times brighter. To do this, astronomers need to have access to very sensitive telescopes and instruments. Second, they also need to identify planetary-mass members within an overwhelming multitude of field stars and background galaxies. This is equivalent to finding a needle in a haystack, but where the needle is the least-shiny object.

To overcome this, Roig and her team combined the proper motions of objects in the night sky with multi-wavelength photometry obtained by multiple observatories over 20 years. These included the Isaac Newton Group (ING) on the island of La Palma (off the coast of Spain), the Canada-France-Hawaii Telescope in Manua Kea, Hawaii, and the ESO’s Very Large Telescope (VLT), Visible and Infrared Survey Telescope for Astronomy (VISTA), VLT Survey Telescope (VST) and MPG/ESO 2.2-meter telescope, all of which are located in the Atacama Desert in northern Chile.

They also relied on astrometric observations by the European Space Agency’s (ESA) space-based Gaia Observatory. As Hervé Bouy – the project leader of the new research – said in a recent ING press release.

“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.”

Lastly, the team took advantage of how younger rogue planets are still warm from formation, allowing direct detection by sensitive telescopes and cameras. This is where the new deep wide-field observations by infrared and optical telescopes came into play, which provided the team with over 80,000 wide-field images (100 terabytes of data). From this, the team found at least 70-170 new FFPs comparable in mass to Jupiter and located in the Scorpius and Ophiuchus constellations, the closest star-forming region to our Solar System.

As Miret-Roig said in a recent ESO press release, this was the largest single-sample of FFPs ever discovered:

“We did not know how many to expect and are excited to have found so many. We measured the tiny motions, the colors 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.”

This discovery also means that astronomers will have nearly twice the data set they previously had, which will come in handy when follow-up observations happen in the near future. This large sample is already helping astronomers refine their theories about the nature and origin of rogue planets. Basically, the number of FFPs observed in the Upper Scorpius association exceeds what astronomers would expect if they only formed as stars do in the interstellar medium.

This suggests that there could be many more mechanisms at play and that previous estimates that suggested there could be billions of FFPs in our galaxies are correct. Assuming the fraction of FFPs that they observed in Upper Scorpius is similar to that of other star-forming regions, said Bouy, there would be several billion Jupiter-mass planets roaming the galaxy and even more Earth-mass planets – many of which have been observed in the Milky Way:

“There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star. 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 ESO’s Extremely Large Telescope (ELT) is currently under construction in the Atacama Desert and is expected to gather its first light by 2027. With its 39-meter (128-foot) primary mirror and advanced suite of spectrometers, coronographs, and adaptive optics, the ELT will be able to directly image exoplanets, rogue planets, and characterize their atmospheres. That same year, NASA’s Nancy Grace Roman Space Telescope will also launch for space and begin conducting exoplanet surveys that could include FFPs as small as Mars.

“The FFPs we identified are also excellent targets for follow-up studies. In particular, they will be essential to study planetary atmospheres in the absence of a blinding host star, making the observation far easier and more detailed,” Bouy added. “The comparison with atmospheres of planets orbiting stars will provide key details about their formation and properties. Additionally, studying the presence of gas and dust around these objects, what we call ‘circumplanetary discs,’ will shed more light on their formation process”.

Another implication of this study is what it could mean for models of planet formation and evolution, which are key to understanding the origin of habitable planets and life. Said Miret-Roig:

“The discovery of this large population of young FFPs has important implications for the formation and early evolution of planetary systems and, specifically, on the timescale of the processes involved. Our observations suggest that giant-planet systems must form and become dynamically unstable within the observed lifetime (3-10 million years) of the region to contribute to the population of FFPs. Current studies suggest that dynamical instability among the giant planets in our Solar System may also have occurred at early times, although it was much less violent than the instability needed to eject planets as massive as the ones we have found.”

There’s also the exciting possibility that FFPs could host life, possibly tucked away in subterranean pockets where the slow decay of radioactive elements or convection provides the necessary heat. Another possibility is that FFPs could have moons that possess thick atmospheres and water on their surface, raising the possibility of life again. Could any of these possibilities be real? With hundreds or thousands of FFPs available for study in the coming years, we’ll find out one way or another.

Further Reading: ESO, Isaac Newton Group, Nature Astronomy