The nature and origin of free-floating planets (FFPs) remains unclear: are they formed like stars by the gravitational collapse of tiny clouds of gas? Or like planets? Around the stars and then dynamically ripped or ejected?
Although it is known that both mechanisms can produce stray planets, it is not known how they contribute to their formation, as a large homogeneous sample is not available.
Identifying these type of planets within a star cluster is similar to looking for a needle in a haystack, but it is what scientists at the Center for Astrobiology (CAB, CSIC-INTA) and other international centers have achieved. The results are published in the journal Nature Astronomy.
Using more than 80,000 wide-field images, the authors identified up to 170 wandering planets from the Upper Scorpius association among the vast number of stars and background galaxies.
First, you need eyes sensitive enough to detect the ‘needles’. While stars are relatively bright and easy to detect, members of the planetary mass are several thousand times weaker and can only be detected with large-diameter telescopes and sensitive detectors.
To meet this challenge, the authors combined the images available in the public astronomical archives with new wide-field observations from the world’s best infrared and optical telescopes. In this way, they measured the motions, colors and luminosities of tens of millions of sources across a wide area of the sky where the young Upper Scorpius OB stellar association is found.
Combining proper motion (ie, motion in the sky plane) and multi-wavelength photometry is in fact the most efficient and robust way to identify all members of an association over very large areas.
It is the largest sample of wandering planets in a single association and nearly double the number of wandering planets known to date across the entire sky.
Each member of an association was born into the same molecular cloud complex with its own unique impulse. At the end of the formation process, group members move together with spatial movements similar to the parent cloud complex, which is an extremely effective identification method.
Stars in the field that are unrelated have quasi-random eigenfunctions, while galaxies in the background have no measurable eigenfunctions. Therefore, any object that shows its own group-like movement is likely a member. Luminances and colors can be used to refine the selection and reject the few remaining objects.
With over 80,000 images
From more than 80,000 wide-field images totaling up to 100TB and spanning 20 years, the authors have identified up to 170 errant planets from the Superior Scorpio association among the vast number of stars and background galaxies.
It is by far the largest sample of wandering planets in a single association and nearly double the number of wandering planets known to date across the entire sky. That number clearly exceeds the number of wandering planets expected if they form as stars from the collapse of a small molecular cloud, indicating that other mechanisms must be at work.
The number of wandering planets found exceeds expectations if they form as stars from the collapse of a small molecular cloud, indicating that other mechanisms must be at work.
With current knowledge of exoplanetary systems (frequency, configuration, dynamics), we estimate that the dynamic ejection of planetary systems is an important mechanism in their formation.
Núria Miret Roig, first author of the article, states that the discovery of this large population of young wandering planets also has important implications for the formation and initial evolution of planetary systems and, specifically, for the time scale of the processes involved.
Observations suggest that gas giants in planetary systems must form and become dynamically unstable during the observed lifetimes in the region of 3-10 million years to contribute to the roving planet’s population.
Current studies indicate that instability among giant planets in our solar system may have occurred early on as well, although it was far less violent than the instability needed to eject planets as massive as those we’ve encountered.
The wandering planets identified, according to Hervé Bouy, head of the European project that financed this work, are also excellent objects for follow-up studies. For example, studying planetary atmospheres in the absence of a host star to hide them, which would be interesting to compare with the atmospheres of planets orbiting stars.
There may be several billion Jupiter-like planets roaming the Milky Way without a host star.
Furthermore, studying the binary properties and circumplanetary disks around these objects will shed more light on their formation process, according to the researchers.
Assuming that the fraction of wandering planets measured in Upper Scorpio is similar to that of other star-forming regions, there could be several billion Jupiter-like planets roaming the Milky Way without a host star. This number would be even higher for planets with a mass similar to Earth, as they are known to be more common than massive planets.
New observations with James Webb
This week the James Webb Space Telescope is scheduled to be launched, in which the CAB also participates. David Barrado, responsible at INTA for one of its instruments (MIRI) and coordinator of the guaranteed time subprogram dedicated to this type of objects, highlights that the new FFPs can be perfect targets for this new observatory, “because they will allow us a detailed understanding of the phenomena that occur in their atmospheres”.
Reference:
N. Roig et al. “A rich population of planets floating in the young star association of Superior Scorpio.” Nature Astronomy, 2021.
This research is part of the ERC Consolidator COSMIC-DANCE program directed by H. Bouy (Univ. De Bordeaux) and is the main outcome of the doctoral thesis by N. Miret Roig, and was made possible by the extensive use of data from ESO, NOAJ , NOAO, ING, VISIONS and ESA’s Gaia mission.
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