A cloud of purple, green, and pink dust with a dark, starry background.
A total of 712 individual images from the Near Infrared Camera on the James Webb Space Telescope were combined to make this composite view of the Orion Nebula and the Trapezium Cluster. Credit: NASA, ESA, CSA/Science leads and image processing: M. McCaughrean, S. Pearson, CC BY-SA 3.0 IGO

Planetary-mass objects that don’t orbit stars, called rogue or free-floating planets, are sprinkled about the galaxy like glitter. Rogues may form around a star and later be ejected or may instead form like a star out of a collapsing gas cloud, just in miniature.

Astronomers recently discovered a cache of hundreds of potential rogue planets in the heart of the Orion Nebula. Some of these planets are twice the mass of Saturn, smaller than other rogue planets seen before. Strangely, many of these planetary-mass objects are bound to each other as binary pairs. That configuration is typical of stars but is less so for planets. The discovery of so many paired-up planets suggests that they might have formed just as stars do, despite their small size.

“When does the star formation process stop? What’s the smallest thing we can make out of a cloud of gas?”

“When does the star formation process stop? What’s the smallest thing we can make out of a cloud of gas?” asked lead researcher Samuel Pearson, an astronomer at the European Space Agency (ESA) in the Netherlands. “We’ve known theoretical estimates of that for decades, but we’d not been able to see down to low enough masses to test it.”

Astronomers don’t know whether star formation processes can create objects as small as planets, Pearson explained, but if they can, that would have major implications for our understanding of planet formation, stellar astrophysics, and galaxy evolution.

Free-Floating Flock

Rogue planets are notoriously difficult to find because they are small and faint. Most have been discovered because they bend the light of a distant star behind them. Some of the youngest rogue planets still radiate a little heat leftover from their formation and can be seen with a high-powered infrared telescope.

Pearson and his ESA colleague Mark McCaughrean used the James Webb Space Telescope (JWST) to peer at the dense center of the Orion Nebula into the Trapezium Cluster of stars. The cluster contains around 2,000 stars and is less than 2 million years old. Any free-floating planets would likely still shine brightly enough for JWST to see, they postulated.

The image revealed 540 possible rogue planets that range from twice Saturn’s mass to 13 times Jupiter’s mass.

JWST’s Near Infrared Camera detected stars and planets buried within the dense clouds of gas that swirl through the Orion Nebula. The team’s analysis of the image revealed 540 possible rogue planets that range from twice Saturn’s mass to 13 times Jupiter’s mass. The discovery is currently under peer review and was published to a preprint archive in October.

The telescope’s “unprecedented sensitivity and resolution” in infrared wavelengths made these discoveries possible, said Núria Miret Roig, an astrophysicist at the University of Vienna in Austria.

Measurements of the planets’ motion across the sky, which Miret Roig’s team used to discover a different group of rogues, would help confirm whether these are, in fact, planets. Those measurements are not yet possible for Trapezium because JWST has viewed the region only once, Miret Roig said, but JWST imaged these planets at wavelengths where planets, and not much else, glow bright.

The researchers plan to use JWST’s Near Infrared Spectrograph to confirm whether these objects are planets and measure what’s in their atmospheres.

Jumbo-Sized Mystery

It’s not surprising that the Trapezium Cluster contains hundreds of free-floating planets, said astronomer Sean Raymond of the Laboratoire d’Astrophysique de Bordeaux in France. Stars in young clusters are very close together, making the close encounters that tear a planet from its star more likely. “The binaries are the crazy and most interesting part [of the finding],” he said.

A purple haze with small bright white stars. At right are close-up images of planetary-mass object pairs.
Hundreds of candidate planetary-mass objects are embedded within the Orion Nebula’s Trapezium Cluster, 90 of which are Jupiter-mass binary objects (JuMBOs). This image zooms in on five pairs of JuMBOs visible within a recent JWST image of the cluster. Click image for larger version. Credit: Pearson and McCaughrean, 2023, https://doi.org/10.48550/arXiv.2310.01231, CC BY-SA 4.0

Of the 540 planetary-mass objects the researchers discovered, about 50 of them, or 9%, seem to orbit each other in groups of two or three. Although almost all high-mass stars exist in binary pairs, it’s far less common for low-mass stars and even less so for substellar objects such as brown dwarfs. When astronomers have seen pairs of small objects in the past, the pairs typically orbit within a few Earth–Sun distances of each other. However, the Trapezium Jupiter-mass binary objects, or JuMBOs, as the team calls them, are on wider orbits and are of smaller mass than binary star theories suggest they should be.

JuMBOs throw another twist into the debate about how free-floating planets form. Raymond, who was not involved with the new discovery, said the binaries’ “origins are pretty mysterious, although there’s already one idea out there about how they might have formed around stars, then been stripped by passing stars and ended up as wide binaries.”

Miret Roig said she finds it “improbable” that so many binary planets survived being stripped from their host stars but remained gravitationally bound together.

“Our existing models are unable to account for this particular scenario.”

“In contrast, it’s well established that many stars form in pairs,” Miret Roig said. She supports the idea that the binary rogue planets formed in a similar way. “Nevertheless, our existing models are unable to account for this particular scenario,” she added.

“Nine percent is massively more than what you’d expect for the planetary-mass regime,” Pearson said. “You’d really struggle to explain that from a star formation perspective…. That’s really quite puzzling.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2023), Rogues’ gallery comes in pairs, Eos, 104, https://doi.org/10.1029/2023EO230417. Published on 31 October 2023.
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