Dozens of planet-sized objects have been discovered in the Orion Nebula via observations that could herald the existence of a new astronomical category.
The free-floating entities, which have been named Jupiter-mass binary objects, or Jumbos, appear in spectacular images taken by the James Webb space telescope. The objects are too small to be stars, but also defy the conventional definition of a planet because they are not in orbit around a parent star.
The discovery also appears to confound existing theories of star and planetary formation, which suggest it should not be possible to form Jupiter-sized objects through the process that gives rise to stars inside the clouds of dust and gas found in a nebula.
Prof Mark McCaughrean, a senior adviser for science and exploration at the European Space Agency (ESA), said the observations were inspired after data from ground-based telescopes hinted at the existence of the mysterious class of object.
“We were looking for these very small objects and we find them. We find them down as small as one Jupiter mass, even half a Jupiter mass, floating freely, not attached to a star,” he said. “Physics says you can’t even make objects that small. We wanted to see, can we break physics? And I think we have, which is good.”
The large, hot, gassy objects appear to be planet-like in their composition, with analysis revealing steam and methane in their atmospheres, but they are not technically planets. The team settled on the name Jupiter-mass binary objects because, out of the hundreds of planet-like objects identified, dozens came in pairs.
“Most of us don’t have time to get wrapped up in this debate about what is a planet and what isn’t a planet,” McCaughrean said. “It’s like my cat is a chihuahua-mass pet. But it’s not a chihuahua, it’s a cat.”
The Jumbos are about 1m years old – babies in astronomical terms – and have infernal surface temperatures of roughly 1,000C. Without a host star, though, they will rapidly cool and will briefly feature temperatures in the range of habitability before becoming incredibly cold. However, as gas giants, their surfaces would not harbour liquid water, even during their brief temperate window, meaning they are not likely to be strong contenders for hosting alien life.
The observations focus on the Orion Nebula, which can be seen with the naked eye as the fuzzy middle “star” in the “sword” of the Orion constellation. At 1,344 light years away, it is the closest region of massive star formation to Earth and in the latest images it appears as a celestial masterpiece, with roiling clouds of dust and gas, explosions and star beams.
Stars form when the dust and gas clouds in a nebula cool, progressively fragment and eventually collapse under their own gravity. The smallest stars are about 80 Jupiter masses, below which the core is not dense enough to fuse hydrogen, but smaller objects can coalesce through the same process, including dimly glowing brown dwarfs – sometimes called failed stars – and, below about 13 Jupiter masses, planetary-mass objects. But theoretical predications suggest that the lower boundary for an object forming through a star-like gravitational collapse is about three to seven Jupiter masses.
Smaller free-ranging objects have occasionally been sighted, but it was unclear whether they had formed in situ or had been ejected from a planetary disc around another star. The latest observations are more challenging to explain because, out of the hundreds of roughly Jupiter-sized objects found, dozens are in binary pairs. “How can you throw two things out [of a star’s orbit] in a chaotic interaction and get them to stick back together again?” asked McCaughrean.
The findings are published as a preprint, but are yet to be peer reviewed.
Prof Matthew Bate, the head of astrophysics at the University of Exeter, who was not involved in the research, said: “I don’t know how to explain the large numbers of objects they’ve seen. It seems we’re missing something in all of the theories we’ve got so far. It seems that there’s a mechanism that’s forming these [objects] that we haven’t thought of yet.”
“It’s pretty rare that this kind of discovery is made,” Bates added. “In the last decade, a lot of us thought we understood star formation pretty well. So this is really a very, very surprising result and we’re going to learn a lot from it.”
Prof Anthony Whitworth, an astrophysicist at Cardiff University, described the observations as “an amazing result”. The binary objects could have formed within a disc and then been kicked out as a pair, he speculated. “This would require interactions with other stars in the dense Orion star field,” said Whitworth. “All possible, in principle. Lots to ponder!”
