These Rogue Worlds Upend the Theory of How Planets Form

The original version of this story appeared in Quanta Magazine.

When Galileo Galilei, a mathematician at the University of Padua, trained a spyglass of his own creation on the sky, he was overwhelmed with what he saw—more than 500 new stars in the constellation Orion, in addition to the familiar three in the hunter’s belt and six in the sword.

In October, astronomers used the James Webb Space Telescope to zoom in on one of the middle stars in the sword and identified another 500 or so previously unseen spots. The worlds are so small and dim that they blur the line between star and planet. It’s an ambiguity that plagued Galileo, who referred to the moons of Jupiter as both “stars” and “planets” in the same page of his 1610 astronomical treatise, and it continues to trouble astronomers today.

“When we look at the solar system it’s all nice and neat. You get the sun, and you get planets,” said Samuel Pearson, an astronomer with the European Space Agency (ESA). There’s nothing in the middle. But “when you actually go and have a look,” Pearson said, “you realize there’s a full spectrum of [objects with] basically every mass in between.”

The JWST observation bolsters a growing catalog of isolated objects occupying this gray zone between giant planets and tiny stars. Sometimes called “free-floating” or “rogue” planets, these solitary worlds drift freely through space. While astronomers can estimate the mass of these dark, Jupiter-mass balls of gas, their origins remain mysterious. Are they actually planets—“Jupiters” that once orbited stars but were somehow spit out? Or are they more like micro-stars that failed to ignite?

Rather than answering this question, the JWST observation adds to the mystery: The telescope’s infrared eye found that dozens of the worlds appear to be in pairs orbiting each other—a puzzling arrangement that, if confirmed, would defy expectations.

“We’re missing something,” said Nienke van der Marel, a researcher who studies planet formation at Leiden Observatory in the Netherlands, “and we don’t know what it is.”

These improbable duos cannot be easily explained by any known formation theories of either stars or free-floating planets. But within a week of the JWST announcement, researchers published a daring new idea describing how giant planets might be ejected from their home system in pairs—an event most researchers had thought all but impossible. Whether or not the proposal can fully account for the entire zoo of dim, starless worlds remains to be seen. But researchers expect that a refined understanding of free-floating worlds, and the star systems that create them, is at hand.

“If indeed [this discovery] is confirmed,” said Peter Plavchan, an astrophysicist at George Mason University who was not involved in detecting the pairs of Jupiters, “it will truly be groundbreaking.”

Dark Worlds Everywhere

Free-floating worlds escaped the notice of astronomers for centuries because they are extremely dark. To fuse hydrogen and shine brightly, stars need to be at least 80 times as massive as Jupiter. Rogue worlds are much lighter and are commonly defined as weighing less than 13 Jupiters. (Anything between 13 and 80 Jupiters can fuse a heavier variant of hydrogen and is classified as a brown dwarf, or what astronomers sometimes romantically call a “failed star”).

In fact, the relative invisibility of free-roaming planets once prompted some astrophysicists to wonder whether there might be enough of these objects to account for dark matter—the unidentified bulk of mass that appears to hold galaxies together. This question motivated astronomers to search for signs of such worlds in the 1990s, which they did by looking for the subtle ways in which their gravity would distort the appearance of stars they had passed in front of. The indirect nature of these “microlensing” surveys wasn’t well suited for identifying individual free-floating objects, but they showed that there wasn’t enough of whatever was out there to make up the dark matter.

The first images of rogue worlds came in the 2000s, when astronomers spotted a few objects still glowing in infrared light from the heat of their formation. Based on those observations, one possible origin emerged. In 2010, astrophysicists including Sean Raymond at the University of Bordeaux in France simulated the evolution of planetary systems and found that when one gas giant planet punts a sibling from their home system, as sometimes happens, the expulsion stretches the survivor’s orbit into an ellipse. Astronomers had seen these skewed orbits, which Raymond’s group and other researchers interpreted as scars of past interplanetary trauma.

The first substantial catalog of free-floating worlds came not from planet hunters but from star hunters searching for starlike objects with even less heft than brown dwarfs. Núria Miret Roig of the University of Vienna and Hervé Bouy of the University of Bordeaux were looking for the most dwarfish of brown dwarfs in the constellation Scorpius, which hosts a gassy nebula that cranks out lots of stars and planets. Amid more than 26 million pinpricks of infrared light in 80,000 images, they searched for dimly glowing objects that moved across their field of view in observations spanning 20 years. In 2021, they announced that they had found a bounty of around 100 candidate objects between 4 and 13 Jupiter masses—increasing the number of known rogue worlds by about a factor of five.

With more than just a handful of free-floating objects to analyze, the researchers could then begin to ask basic questions about where these worlds had come from. One possibility was that they had coalesced from the disc-shaped detritus that surrounds a newly born star, as planets do. And then some chance encounter with a neighbor had ejected them, in the style of Raymond’s 2010 simulations.

The second possibility was that they had formed alone, when an isolated cloud of hydrogen and helium became dense enough to collapse into a ball. This is how stars are born, and it would make these worlds less like planets and more like the galaxy’s tiniest brown dwarfs.

Miret Roig and Bouy concluded that their candidates likely contained worlds that had formed in both ways. The lightest objects were probably punted planets, although the astronomers had found too many of them to easily explain using planetary ejection models alone.

“There are many free-floating planets,” Miret Roig said, “and they probably form by different mechanisms.”

A mixture of both origins seemed likely. But how many of the 100 free-floating worlds were planets, and how many were starlike, the researchers couldn’t say.

Three days after Miret Roig and Bouy posted their results, JWST launched, along with a new era for free-floating-planet hunting.

Drops of Jupiters

Astronomers had suspected that JWST would be a free-floating-planet-finding machine. It sits far beyond the interfering murk of Earth’s atmosphere. Its giant mirror gives it far more sensitivity to the fine features of the universe than its forerunner, the Hubble Space Telescope. And it picks up infrared light, which makes it perfect for spotting dimly glowing worlds.

Pearson partnered with Mark McCaughrean, an ESA astronomer, to look more deeply for free-floating worlds than had previously been possible. They were fascinated by star formation and planet formation, and wanted to target objects—like brown dwarfs—in the “chaotic gray area” between the two. There, “you get the crossover of both worlds,” Pearson said. In October 2022, Pearson and McCaughrean spun the space telescope toward a central star in the sword hanging from Orion’s belt. For 35 hours.

It took Pearson months to align the resulting 12,500 JWST images of the Orion nebula, pixel by pixel. The formidable task was frustrated by the telescope’s exquisite sensitivity: Many of the faint objects typically used as landmarks blinded JWST’s ultra-sensitive eye.

“The brown dwarfs that are normally difficult to see were wiping out bits of the detector,” he said. It was “just not a problem I’ve ever encountered with any other telescope.”

After completing the cosmic mosaic, Pearson was rewarded with an abundance of the mysterious worlds he sought: More than 500 free-floating objects of a few Jupiter masses speckled the Orion nebula. But the real surprise was that, when he looked closely, he saw something that initially didn’t make much sense. Some of the blobs of light were pairs of Jupiter-mass objects. In all, he counted 42 pairs of whirling Jupiters—a striking number.

“Hang on, why is there all this faint stuff in pairs?” Pearson recalled wondering. “Then the penny dropped, and we realized we should look at this very carefully.”

From a theoretical perspective, these duos seemed nearly impossible. They were unlikely to be punted planets; when one planet kicks another out of a stellar system, the ejected planet almost always flies off alone. But they couldn’t be stars either, since many of them weighed as little as a single Jupiter—a mass too light for the object to have formed directly from a collapsing gas cloud. The team dubbed their mystery duos Jupiter Mass Binary Objects, or JUMBOs for short, and described them in a preprint posted on October 2.

The JUMBOs caught experts in both star and planet formation flat-footed. “This has not been predicted at all. There are no existing theories where we would have expected these wide, free-floating planetary objects in these numbers,” said Matthew Bate, an astrophysicist at the University of Exeter specializing in star formation.

Astronomers had previously observed that although many massive stars twirl through space with partners, the percentage of coupled-off stars falls with their mass. “We usually expect trends to continue,” van der Marel said. Thus, she said, the percentage of Jupiter-mass objects in pairs “should go to zero.” Leaping up to 10 percent hadn’t been on anyone’s JWST bingo card.

The catch is that at least some of the JUMBOs are probably mirages. The deeper an object lies in a dusty environment (and the Orion nebula is extremely dusty), the tougher it is to distinguish it from a distant, more massive star behind the nebula, which would be expected to have a partner. In previous studies, between 20 percent and 80 percent of what looked like free-floating worlds turned out to be vanilla stars. “One needs to be a bit cautious at the moment,” Miret Roig said.

In the spring, Pearson and McCaughrean will use JWST to again observe their batch of free-floating worlds, this time in a richer spectrum of colors. These follow-up observations will help confirm which JUMBOs are real by looking for traces of methane or water in their atmospheres, a telltale signature of Jupiter-mass worlds.

“Once you’ve got spectra,” Pearson said, “there’s basically no place to hide.”

Speedy Simulations

Even without confirmation, theorists are already racing to explain these perplexing worlds.

Rosalba Perna, an astrophysicist at Stony Brook University, heard about Orion’s JUMBOs in the news, before she even read Pearson’s paper. Perna and Yihan Wang of the University of Nevada, Las Vegas, had been studying what happens when a star flies past another solar system. They had focused mainly on simulating systems with a single giant planet. But the JUMBOs got Perna wondering: What if there were two giant planets? She called up Wang and asked him to see what would happen if he stuck a second Jupiter into the simulations.

Wang set up his program to hurl digital stars at countless two-Jupiter stellar systems from all angles. He also set up the software to notify him if the “intruder” star sent both planets careening off into space together—creating a JUMBO. Then he sent the code off to a computing cluster at his university and went to lunch.

When Wang returned to his office and checked his computer, he found a list of alerts reading “binary planet formed!!!”

From tens of billions of simulations, the team saw that booting pairs of Jupiters was relatively easy if the planets happened to be quite close together when the marauding star swept by. This happened especially often for neighbors with tightly spaced orbits (think Uranus and Neptune). In such cases, up to 20 out of 100 ejections produced JUMBOs (the other 80 produced single planets)—more than enough to account for the 10 percent rate Pearson had seen in Orion. But for planets with more distantly spaced orbits (think Jupiter-Neptune), almost all ejections resulted in lone planets.

With input from Wang’s colleague Zhaohuan Zhu, the group worked around the clock (and in one case during a flight to Europe). The trio wrote up their results and posted a preprint on October 9, one week after the JUMBO find.

“The speed at which they wrote that is slightly frightening,” Pearson said.

Other theoretical astrophysicists have yet to fully digest the new results, but they find them plausible—and surprising. “I didn’t think [making a free-floating pair of planets] was possible to do from an ejection point of view,” Raymond said. “But then this paper came out.”

Still, some details of the stellar-intruder theory will need further study. The Orion nebula is a dense place with lots of stars whizzing around, but is it chaotic enough to first make solar systems and then break them apart, all within a few million years? Also, many of Pearson and McCaughrean’s JUMBOs orbit one another at great distances; they are multiple times farther from one another than Pluto is from Earth. But according to Wang’s simulations, the only way to get such widely spaced JUMBOs is to start with similarly spaced-out solar systems, which astronomers rarely see.

“We know from direct imaging searches of young stars that very few stars have giant planets in [wide] orbits,” Bate said. “It is difficult to accept that there were many large planetary systems in Orion to disrupt.”

Rogue Objects Abound

At this point, many researchers suspect there’s more than one way to make these strange in-between objects. For instance, with some fiddling, theorists might find that supernova shock waves can compress smaller gas clouds and help them to collapse into pairs of tiny stars more readily than expected. And Wang’s simulations have shown that booting giant planets in pairs is, at least in some cases, theoretically unavoidable.

While many questions remain, the multitude of free-floating worlds discovered in the past two years has taught researchers two things. First, they form quickly—over millions of years, rather than billions. In Orion, gas clouds have collapsed and planets have formed, and some, perhaps, have even been dragged into the abyss by passing stars, all during the time in which modern humans were evolving on Earth.

“Forming a planet in 1 million years is hard with current models,” van der Marel said. “This [discovery] would add another piece to that puzzle.”

Second, there are a ton of untethered worlds out there. And the heavy gas giants are the hardest to evict from their systems, much as a bowling ball would be the hardest object to knock off a billiard table. This observation suggests that for every Jupiter spotted, numerous free-floating Neptunes and Earths are going unnoticed.

We likely live in a galaxy teeming with banished worlds of all sizes.

Now, nearly half a millennium after Galileo marveled at the myriad pinpricks of light—moons, planets, and stars—in Earth’s skies, his successors are getting acquainted with the brightest tip of the iceberg of darker objects adrift between them. The tiny stars, the starless worlds, invisible asteroids, alien comets, and more.

“We know there’s a whole bunch of crap between stars,” Raymond said. This kind of research is “opening a window on all of that, not just free-floating planets but free-floating stuff in general.”

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.