22 Dec 2021

Using observations and archival data from several telescopes around the world and in orbit, including the Isaac Newton Telescope (INT), astronomers have discovered at least 70 new free-floating planets (FFPs) — planets that wander through space without a parent star to orbit — in the Upper Scorpius OB stellar association, which is the nearest region of star formation to our Sun. This is the largest sample of such planets found in a single group and it nearly doubles the number known over the entire sky.

Identifying FFPs within a star cluster is a major challenge, like trying to find a needle in a haystack. 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 hundreds) within the overwhelming multitude of field stars and background galaxies (the “haystack").

To solve this challenge, the astronomers combined proper motions (i.e., motions across the plane of the sky) with multi-wavelength photometry. 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 background galaxies have no measurable proper motions. Photometric luminosities and colours are useful to refine the selection and reject the few remaining interlopers.

The astronomers also combined the vast number of images available in public astronomical archives with the new deep wide-field observations obtained with the best infrared and optical telescopes on the ground and in space. Using over 80,000 wide-field images adding up to around 100 terabytes and spanning 20 years, they identified at least 70, and up to as many as 170 of these Jupiter-sized planets, as members of the Upper Scorpius association among the 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.

The nature and origin of FFPs remains unknown: do they form like stars through the gravitational collapse of small clouds of gas? Or do they form like planets around stars and are then dynamically ejected or stripped off? The number of FFPs discovered in the Upper Scorpius association 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.

Núria Miret-Roig, first author of this study, remarks that "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."

FFPs, lurking far away from any star illuminating them, would normally be impossible to image. However, the astronomers 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.

Moreover, Hervé Bouy, project leader of the new research, emphasises that "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. The comparision 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".

"Assuming the fraction of FFPs that we 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."

[Image]

(A) This artist’s impression shows an example of an FFP detected in the Rho Ophiuchi region.

(B) This image shows the locations of 115 potential FFPs in the direction of the Upper Scorpius and Ophiuchus constellations, highlighted with red circles. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study.