Mysterious 'rogue' objects discovered by James Webb telescope may not actually exist, new simulations hint

Mysterious "rogue" pairs of Jupiter-size objects spotted by the James Webb Space Telescope (JWST) are a tiny fraction of those that originally formed, a new study suggests. The finding hints that these enigmatic entities, dubbed "JuMBOs," are even rarer than previously thought — and casts doubt on their very existence.

JuMBOs, short for "Jupiter-mass binary objects," are pairs of planet-like, Jupiter-size objects that JWST spotted in the trapezoid region of the Orion Nebula Cluster in 2023. Each JuMBO comprises two gas giants between 0.7 and 30 times Jupiter's mass. The members of a JuMBO don't orbit stars; instead, they twirl around each other at distances of approximately 25 to 400 astronomical units, making them free-floating or "rogue." (One astronomical unit is approximately 93 million miles, or 150 million kilometers, the average distance between Earth and the sun.)

The objects' paired status and their apparent lack of tethering to any star have challenged existing notions of how planets are born. That hasn't stopped scientists from floating several ideas about JuMBO formation, including that they formed around a star, just like the solar system's planets, but were jointly lured away by another star. An alternate hypothesis is that JuMBOs are the eroded cores of embryonic stars, suggesting they formed like stars.

However, some researchers are skeptical that JUMBOs even exist. For instance, in 2024, Kevin Luhman, a professor in the Department of Astronomy and Astrophysics at Penn State, reanalyzed the JWST observations and suggested that the purported pairs aren't planets after all. Instead, he proposed that they're distant background objects that had been serendipitously captured in JWST's snapshots of the Orion Nebula Cluster.

In fact, Richard Parker, a senior lecturer in astrophysics at the University of Sheffield in the U.K. and the lead author of the new study, told Live Science via email that it was a discussion about Luhman's work that prompted the new study. During this group meeting, Simon Goodwin, a professor of theoretical astrophysics at the University of Sheffield and the new study's second author, suggested that simulations could help identify how susceptible JUMBOs are to destruction. Indeed, no previous research had examined how long these planetary pairs persist in interstellar space. Such environments are packed with growing stars that could fragment the duos via their powerful gravitational pulls.

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To figure out how effectively JuMBOs tolerated the turbulence of their birth environment, Parker, Goodwin and Jessica Diamond, an integrated Masters student at the University of Sheffield, created a computer model of a nebula containing a mixture of stars and JuMBOs that totaled 1,500 components, in an arrangement that likely mimicked the Orion Nebula Cloud's original composition, Parker explained.

The researchers then generated five copies of this model that differed in various internal parameters, such as the distance between members of a given planetary duo and how crowded the nebula was overall. For each model copy, the team conducted 10 rounds of N-body simulations.

"These computer simulations calculate the force due to gravity on each object from all of the other objects,” Parker said, adding that such calculations, performed repeatedly, can reveal how different components of the model nebula interact over time.

The researchers found that the simulated JuMBOs were extremely ephemeral. In a dense nebula, for instance, nearly 90% of the planet pairs were destroyed by neighboring stars within a million years. Even in the best-case scenario — when there were fewer stars overall in the nebula and the JuMBOs waltzed in tighter orbits — only half of the planet pairs resisted any disruption. The analyses also revealed that the more widely separated a planet pair was, the more likely it would get disrupted.

Parker said that since he and his colleagues had previously found that star-planet systems are very fragile in environments chock-full of stars, he wasn't particularly surprised by the findings, noting that "[b]ecause the planet-planet binaries are less massive, they have a lower energy and are even more susceptible to destruction."

The results, published May 2 in the journal Monthly Notices of the Royal Astronomical Society: Letters, show that the observed JuMBOs are extremely rare. But Parker said this hints at the same disturbing possibility proposed by Luhman: that they don't really exist. That's because, to explain the JWST-detected JuMBO numbers, the planet pairs would have had to have been produced in much larger numbers than currently thought. According to Parker, this result likely adds support to the interpretation of JuMBOs as background noise.

"I think the next steps are for someone else to take the original JWST data and to analyse it again," he added.