Rogue Planets are Born in Young Star Clusters

Rogue planetary-mass objects, also known as free-floating planets (FFPs) drift through space alone, unbound to any other objects. They’re loosely defined as bodies with masses between stars and planets. There could be billions, even trillions of them, in the Milky Way.

Their origins are unclear, but new research says they’re born in young star clusters.

Some free-floating planets (FFPs) form the same way stars form by collapsing inside a cloud. The International Astronomical Union calls them sub-brown dwarfs. But that can’t account for all FFPs, or isolated planetary-mass objects (iPMOs) as they’re sometimes called.

New research in Science Advances shows how FFPs form in young star clusters where circumstellar disks interact with one another.

The research is titled “Formation of free-floating planetary mass objects via circumstellar disk encounters.” Zhihau Fu from the Department of Physics at the University of Hong Kong and the Shanghai Astronomical Observatory is the lead author, and Lucio Mayer from the University of Zurich is the corresponding author.

“PMOs don’t fit neatly into existing categories of stars or planets,” said corresponding author Meyer. “Our simulations show they are probably formed by a completely different process.”

Astronomers found some of the first evidence of PMOs in the Trapezium Cluster in the year 2,000. The Trapezium is a tight, open cluster of stars in Orion. It’s also relatively young, and half of its stars show dwindling circumstellar disks, a sign that planet formation is taking place. In the research published in 2,000, the authors wrote that “Approximately 13 planetary-mass objects are detected.”

Since then, astronomers have found many more PMOs and hundreds more candidates. Scientists have wondered about their origins, but so far, there are no widely accepted explanations.

“The origin of planetary mass objects (PMOs) wandering in young star clusters remains enigmatic, especially when they come in pairs,” the authors write in their new research. “They could represent the lowest-mass object formed via molecular cloud collapse or high-mass planets ejected from their host stars. However, neither theory fully accounts for their abundance and multiplicity.”

The researchers used hydrodynamic simulations to test another origin for PMOs and found that they have a unique origin story. Instead of forming in a collapsing cloud like stars or in a protoplanetary disk around a young star, they form in the dense environments in young star clusters. The densely packed environments provide another pathway for PMO formation.

In their simulations, the researchers recreated some of the conditions inside young star clusters where stars readily interact with one another. During close encounters between two stars, their circumstellar disks interact. They get stretched into a tidal bridge between the pair of stars, and the gas in the bridge is also compressed into a greater density.

In the simulations, these bridges collapse into filaments, and those filaments collapse even further into dense cores. Eventually, these cores form PMOs of about 10 Jupiter masses. This fruitful process produces many pairs and triplets of PMOs. Astronomers observe a high number of PMO binaries in some clusters, so these simulations appear to match observations.

“Many young circumstellar disks are prone to instabilities due to the self-gravity of disk gas, potentially leading to disk fragmentation and the formation of gaseous planets,” the authors explain in their paper. “Circumstellar disks appear even more unstable when perturbed by a stellar or circumstellar disk flyby.”

Even stable and isolated disks can form PMOs during flybys. However, the formation of PMOs is dependent on the combined velocity of the interactions. “For high- and low-velocity encounters, the tidal bridge is either stretched too thin or torn apart by the stars, and thus, forming isolated cores becomes impossible,” the authors explain. The interaction velocity has to be in the middle range.

Some of their simulations also showed up to four PMO cores forming in the filaments. “The middle part of the tidal bridge contracts into thin filaments with line mass over the critical value for stability, forming up to four cores in one encounter,” the researchers write. They explain that the exact number of cores is determined by the length of the filaments and is “sensitive to random density fluctuations.” These fluctuations are very difficult to predict from the encounter parameters.

The PMOs display some particular characteristics. They’re likely to have their own disks, and they’re likely to be metal-poor because of where they get their dust from. “In addition, PMOs and their hosts are expected to be metal-poor since they inherit materials in the parent disks’ outskirts that are susceptible to dust drift and, thus, are metal-depleted,” the authors explain.

The authors calculate that in just one million years, which is the approximate age of the Trapezium Cluster, each star will experience 3.6 encounters with other stars. “The highly efficient PMO production channel via encounters can, therefore, explain the hundreds of PMO candidates (540 over 3500 stars) observed in the Trapezium cluster,” the authors write.

It’s important to note that the results only apply to dense clusters that force interactions between circumstellar disks. “This process can be highly productive in dense clusters like Trapezium forming metal-poor PMOs with disks. Free-floating multiple PMOs also naturally emerge when neighbouring PMOs are caught by their mutual gravity,” the authors write.

“This discovery partly reshapes how we view cosmic diversity,” said co-author Lucio Mayer. “PMOs may represent a third class of objects, born not from the raw material of star forming clouds or via planet-building processes, but rather from the gravitational chaos of disk collisions.”

PMOs can be difficult to spot, so their population is based on preliminary estimates and understandings. But they’re out there, and we’ll only get better at identifying them.

The Upper Scorpius Association contains the next highest-known population of PMOs. A 2021 study identified between 70 and 170 candidate PMOs in the region.

The soon-to-see-first-light Vera Rubin Observator (VRO) will significantly grow the number of known PMOs. More data is better data, and the VRO’s observations will lead to a better understanding of how they form.

“Future studies of various young clusters can further constrain the population of PMOs,” the authors conclude.

• Press Release: Young Star Clusters Give Birth to Rogue Planetary-Mass Objects
• New Research: Formation of free-floating planetary mass objects via circumstellar disk encounters

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