We thought we broadly understood how planets and stars form. But the discovery of dozens of pairs of young planets in a nearby nebula threatens to turn that on its head.
They are worlds that simply defy explanation. Drifting through the Orion Nebula – an enormous cloud of dust and gas relatively close by in our galaxy – are what appears to be dozens of Jupiter-sized planets that don't conform to the conventional understanding of how planetary systems form. Rather than being bound to a star like the Earth is in our own Solar System, these planets are free-floating through space in pairs. Astronomers who spotted them with the help of the James Webb Space Telescope (JWST) could only scratch their heads in awe at the discovery.
"These things shouldn't exist," says Simon Portegies Zwart, an astrophysicist at Leiden University in the Netherlands. "They go against everything we have learned about star and planet formation."
In the subsequent months, efforts have been made to try and explain what's going on. These planets, called Jupiter Mass Binary Objects, or Jumbos, still cannot be fully explained. But we are getting closer to an answer – with crucial observations on the horizon that may solve the mystery once and for all.
Jumbos were discovered by Mark McCaughrean, a former ESA astronomer in the Netherlands who now works at the at the Max Planck Institute for Astronomy in Germany, and Samuel Pearson, an astronomer at ESA in the Netherlands. They had been using data from JWST to study the Orion Nebula, around 1,500 light years away from Earth. They were particularly interested in a 10-light-year-wide region of young star formation called the Trapezium Cluster that is just one million years old.
JWST – which contains the biggest mirror to ever be launched into space – has immense infrared capabilities, allowing it to peer through the cloud and dust of the cluster like no telescope before it. Those observations revealed a myriad of interesting young stars and star-forming regions, where immense swirling masses of gas were condensing under gravity to form stars. But they also discovered something wholly surprising – amidst the cosmic dust were floating Jupiter-sized planets that appeared to be drifting in pairs. "The finding was completely unexpected," says McCaughrean.
The objects ranged in size from about half the mass of Jupiter – the largest planet in our Solar System – to 13 times the mass of Jupiter, suggesting they were likely all to be gas giant planets. Jupiter, which is about 11 times the size of the Earth, is one of four gas giants orbiting our Sun. Such giant worlds don't have solid surfaces, but instead are composed of gas, often around a solid core.
Each Jumbo pair was separated by distances of as little as 2.8 billion miles (4.5 billion kilometres) – the same distance that separates Neptune and our Sun – or up to nearly 400 times that distance. Each pair appears as twin dots of light in the Orion Nebula and seem to orbit one another.
Jessie Christiansen, an astronomer at the Nasa Exoplanet Science Institute at the California Institute of Technology, says the discovery set her team of exoplanet hunters – planets found outside our Solar System – into "crisis" mode. "I was very worried about them at first," she says. "One of our definitions of an exoplanet is 'a planet that orbits another star'. As soon as these planets came out, I was like, 'Oh my God, binary free-floating Jupiter-mass objects. What am I going to do?'"
Free-floating planets themselves have been found before. We have seen planets drifting alone in many regions of space, likely a result of being flung from their home solar system by the gravitational nudge of a passing star. Even our own Solar System may have once lost a giant world in this way early in its history. Larger free-floating objects have also been found that blur the line between planet and star. Known as brown dwarfs, they are some 15 to 75 times the mass of Jupiter, too small to begin hydrogen fusion in their cores as occurs in stars, making them dimmer and cooler.
Jumbo planets would represent a newly discovered class below brown dwarfs. McCaughrean and Pearson detecting the hot, infrared glow of about 100 of these objects not in pairs down to the mass of Jupiter in the Trapezium Cluster. "Nobody's seen those before," says McCaughrean. But it was the discovery of 42 pairs of these objects, and one triple, that really set minds racing. "This was not something we were looking for at all," says Pearson.
Many stars exist as pairs, or binaries, a result of forming in proximity in relatively tight, dense nebulas of dust and gas like Orion. Jumbos are a different problem. If they were planets that once orbited stars but were ejected, it would be difficult to explain how they would end up in pairs. Two ejected planets passing each other would be unlikely to become gravitationally bound as they flew through space. But their masses appear to be too low for them to have formed directly from the collapse of a cloud of gas, like a star. "These objects are way off the bottom [of] where we think this works," says Christiansen, "which is partly why theorists are struggling."
Rosalba Perna, a theoretical astrophysicist at Stony Brook University in New York, and her colleagues have one possible solution. They say the Jumbos could be ejected from stars in pairs if the two objects had been orbiting a star in just the right configuration, both on the same side, just as another star passed by. This might have seen them flung away together in a "soft" binary that would eventually see them separate around a million years later.
"It requires the orbits of the two planets to be relatively closely aligned," says Perna. But in such instances, pairs of Jumbos would be "an unavoidable consequence" of interactions between stars. Another ejection idea is that the pairs were already a planet-planet or planet-moon pair orbiting a young star together before they were ejected.
Portegies Zwart favours a different explanation, where Jumbos form in the same way as stars, directly from the collapse of a cloud of gas. Known as in-situ formation, this would require us to rethink how low the density of a gas cloud can be in order to trigger such a collapse. But for Portegies Zwart, "I think in-situ formation is the only one in which I don't have theoretical problems," he says. "It is the most promising."
One way to trigger the collapse of small clouds of gas into Jumbos might involve high-energy particles called cosmic rays. These pervade the Universe, emitted by explosive events such as supernovae or active black holes. Normally, if a cloud of galactic gas – mostly hydrogen and helium – were too low density, it should have too much angular momentum to collapse. The gas would simply be too dispersed and energetic to form objects as small as a Jumbo.
Cosmic rays could offer a solution, slowing down the motion of the gas and allowing smaller pockets to form objects like Jumbos. "Cosmic rays could act like a very viscous fluid and transport angular momentum out," says Jonathan Katz, an astronomer at Washington University in St Louis in the US, who came up with the idea. "It could have a low mass and make planets." Katz disfavours the idea that Jumbos are ejected pairs of planets. "It's like kicking an egg with a foot that's moving at a kilometre a second (2,240mph)," he says. "You're going to spatter egg white and yolk all over the place. You aren't going to have an intact shell. You can kick one [planet], but you can't kick the two of them as a unit."
Some aren't sure that Jumbos exist at all. Peter Plavchan, an astronomer at George Mason University in the US, says he thinks they could be stars masquerading as planets. He says the dusty nature of the Orion Nebula could disguise the light of the stars, making them appear redder, giving them a planet-like signature. "The more plausible explanation is they're just two stars that are higher mass that appear to have the colours of planetary-mass objects," he says.
McCaughrean says this could be the case for some of the Jumbos, but not all of them. "We've been careful in our analysis to rule out reddened low-mass background stars as potential contaminants," he says. "The statistical chances of all of the Jumbos being background sources [is low]."
Pearson says that "even if just two or three of them are real, it means there's something missing from our entire understanding of how you make planets and stars".
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To find out for sure, we need further observations of the objects. McCaughrean and Pearson are on the case – they have studied them more extensively with JWST this year, using the telescope to pick apart the light of the objects. They haven't yet released their latest findings, but when they do, they will be looking for signs of certain elements in the atmospheres of the Jumbos that could hint at their origin.
If they formed around stars, they should contain heavier elements that would have been present in the disk of planet-forming dust around the stars. "All the dense stuff falls towards the middle where these things are forming," says Pearson. Future observations could even look for clouds of sand-like silicates in the atmospheres of the objects, supporting this explanation, although Orion will not be visible to JWST again until October 2024 as it is too close to the Sun for the telescope to observe until then.
Another option might be to study Jumbos with radio telescopes and track how fast they are moving across the sky. If the pairs are moving at the same speed away from a common star, that might support the idea they are ejected planets. If not, it could hint at the in-situ model being correct.
Luis Rodriguez, an astronomer at the National Autonomous University of Mexico, has already managed to observe radio signals from one of the largest Jumbo pairs. Such radio signals were not unexpected, possibly the result of magnetic fields or aurorae on the objects. "It's probably related to some interaction with the magnetic field," says Rodriguez, with perhaps the objects having particularly strong magnetic fields because of a more powerful dynamo effect from their spinning cores due to their young age.
Rodriguez hopes to have more radio data on Jumbos from a US network of radio telescopes called the Very Long Baseline Array in the coming months, and another US network called the Very Long Array by the end of the year. "Then we will know if they are moving fast or breaking apart, which will favour an ejection mechanism," he says.
An upcoming Nasa telescope called the Nancy Grace Roman Space Telescope, set to launch in 2027, could also study Jumbos. It will perform a survey of the Universe to look for exoplanets, but could also be used to look for objects inside the Orion Nebula, perhaps finding more Jumbos than even JWST can detect. "You could point out really faint objects down to about the mass of Saturn," says Melinda Soares-Furtado, an astrophysicist at the University of Wisconsin at Madison in the US.
Studying other young nebulas for Jumbos could be useful too, confirming if these peculiar pairs of objects are widespread across other regions of star formation. "Any young cluster would be interesting," says Portegies Zwart. There could even be some Jumbos drifting freely through space awaiting discovery, and perhaps quite close to home. "There may be Jumbos near the Solar System but we have never spotted them because we didn't point at them," he says.
Regardless of what they turn out to be, Jumbos could help us "really understand these stellar nurseries and what they have to teach us", says Soares-Furtado. For now the mystery continues.
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2024-05-31 15:00:40Z
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