The mysterious pairs of planets we still can't explain

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.

"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.

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?'"

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."

"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.

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".

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.

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.

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.