The Largest Collection of Free-Floating Planets Discovered to Date
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) â planets that wander through space without a parent star to orbit â
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.
This artistâs impression shows an example of an FFP detected in the Rho Ophiuchi region. Credit: ESO/M. Kornmesser. Other formats: eso2120a.
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
âneedlesâ. 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 âneedlesâ, typically a
few hundreds) within the overwhelming multitude of field stars and background galaxies (the
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.
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.
Credit: ESO/N. Risinger (skysurvey.org). Other formats: eso2120c.
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 discsâ, 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."
Artistic 3D animation of free-floating exoplanets in the Upper Scorpius OB stellar association. Credit: Hervé Bouy (University of Bordeaux), Teun van der Zalm and the COSMIC-DANCE team. Large format: MP4 (4K resolution).
About the Isaac Newton Telescope
Based on observations made with the Isaac Newton Telescope (INT) operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.
Núria Miret-Roig; Hervé Bouy, Sean N. Raymond, Motohide Tamura, Emmanuel
Bertin, David Barrado, Javier Olivares, Phillip Galli, Jean-Charles Cuillandre, Luis
Manuel Sarro, Angel Berihuete, Nuria Huélamo, 2021, "A rich population of free-floating planets in the Upper
Scorpius young stellar association", Nature Astronomy, 22 December 2021. Paper: DOI | Nature.