Observations of one of the closest star-forming regions revealed the largest population of free-floating planets discovered to date. These planets do not orbit stars but freely roam the Galaxy. This new sample will help astronomers investigate how stars and planets form.

Artist’s impression of a Jupiter-mass free-floating planet in a star forming region. Approximately 100 free-floating planets have been detected in the Upper Scorpius region in this study. They are not circling around a host star but are isolated like shown in this figure. (Credit: Univ. of Bordeaux)

Like Earth and Jupiter in our Solar System, planets are very low-mass objects compared with the Sun. Although small and light, they have attracted the attention of many people since long ago. Now more than 4,500 planets have been discovered beyond our Solar System (exoplanets). These exoplanets are usually circling around their host stars. On the other hand, it has been known since around the year 2000 that there are isolated planets, not circling around the stars. Astronomers call these free-floating planets (FFPs). The nature and origin of FFPs has remained controversial since their discovery: do they form like stars through the gravitational collapse of small clouds of gas? Or are they formed around stars like other planets and are then dynamically ejected or stripped off? While it is known that both mechanisms can produce FFPs, their respective contributions are still an open question due to the lack of a large homogeneous sample.

Identifying FFPs within a star cluster is a major challenge, in many ways, similar to the "needle in the haystack" parable. 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. Second, one must identify the rare planetary-mass members (the "needles," typically a few hundred) within the overwhelming multitude of field stars and background galaxies (the "haystack," millions of interlopers).

To solve this challenge, an international team composed of French, Japanese, and Spanish astronomers combined the vast number of images available in public astronomical archives with new deep wide-field observations obtained with the best infrared and optical telescopes in the world (including the powerful Subaru Telescope’s mosaic cameras, Hyper Suprime-Cam and Suprime-Cam) to measure the tiny motions, colors, and luminosities of tens of millions of sources in a large area of the sky around the Upper Scorpius OB young stellar association.

Combining proper motions (i.e., motions across the plane of the sky) and multi-wavelength photometry is the most efficient and robust way to identify all the members of an association over large areas. Proper motions are an extremely effective method to identify members of an association since all the members were born from the same molecular cloud complex and thus, have similar motions to the parent cloud. Unrelated field stars have almost random proper motions, and distant galaxies have no measurable proper motions. Luminosities and colors are useful to refine the selection and reject the few remaining interlopers.

Using over 80,000 wide-field images adding up to around 100 TB taken over 20 years, the team has identified about 100 (70-170, depending on the models used) FFP members of the Upper Scorpius association from among the vast amount of background stars and galaxies. This is by far the largest sample of FFPs in a single association, and almost doubles the number of FFPs known to date over the entire sky. This number exceeds the number of FFPs expected if they only form like stars from the collapse of a small molecular cloud, indicating that other mechanisms must be at play. From the current knowledge of exoplanetary systems (frequency, configuration, dynamics), the team speculates that dynamical ejection from planetary systems is an important mechanism of FFP formation.

(Credit: Hervé Bouy (University of Bordeaux), Teun van der Zalm and the COSMIC-DANCE team)

Núria Miret-Roig, first author of this study, remarks, "The discovery of this large population of young FFPs also bears important implications on 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 of the region of 3-10 million years in order to contribute to the population of FFPs."

Hervé Bouy, responsible for the European project which funded this study, emphasizes, "The FFPs that 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 allowing greater detail. The comparison with atmospheres of planets orbiting stars will provide key details about their formation and properties."

Motohide Tamura, Professor at the University of Tokyo and Director of the Astrobiology Center, comments, "The wide-field and sharp imaging capability of the Subaru Telescope has played an important role for this study. The combination of astrometry and photometry is essential to reject interlopers that have been a problem in the previous photometry-only based searches."

Assuming the fraction of FFPs that they measured in Upper Scorpius is similar to that of other star forming regions, there could be several billions of Jupiters roaming the Milky Way without a host star. This number would be even greater for Earth-mass planets since they are known to be more common than massive planets.

These results were published in Nature Astronomy on December 22 2021 as Núria Miret-Roig et al. "A rich population of free-floating planets in the Upper Scorpius young stellar association."

This research is part of the ERC Consolidator program COSMIC-DANCE led by H. Bouy (Univ. of Bordeaux) and the main result is by N. Miret-Roig for her Ph.D., in the context of the ERC project. M. Tamura is supported by JSPS KAKENHI grant Nos.18H05442, 15H02063, and 22000005. This study has been possible thanks to the extensive use of data from ESO, NOAJ, NOAO, ING, VISIONS, and from the ESA mission Gaia.

Note 1: Sun is about 1000 times heavier than Jupiter, while Jupiter is about 320 times heavier than Earth. The objects between 13 and 80 Jupiter masses are called brown dwarfs or failed stars. Both planets and brown dwarfs are too light weight to burn hydrogen to produce their own nuclear energy.


About the Subaru Telescope:
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.