Two researchers say they have found evidence that 19 objects orbiting our sun originated around another star, suggesting interstellar objects in our solar system may be more common than previously thought.
Fathi Namouni at the C么te d鈥橝zur Observatory in Nice, France, and Helena Morais at S茫o Paulo State University in Brazil used a supercomputer to simulate how the orbits of objects called centaurs have evolved since the dawn of the solar system. Centaurs are found between Jupiter and Neptune and have similarities to both asteroids and comets.
They found that the orbits of 17 of these objects could be explained by them being captured by our sun early in its life. A further two asteroids orbiting further out past Neptune, known as聽trans-Neptunian objects (TNOs), also appear to have a similar origin.
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鈥淲e followed their motion back in time to 4.5 billion years [ago] and found that all of these objects [were] perpendicular to the plane of the solar system,鈥� says Namouni. 鈥淭his tells us they could not have been part of the solar system and must have been captured.鈥�
The researchers believe that these 19 objects were captured when the early sun was surrounded by a protoplanetary disc of dust and gas. This material could have slowed incoming objects from other young stars born nearby, although the 19 in question aren鈥檛 thought to all have the same origin.
To date, only two interstellar objects have been definitively identified in our solar system: the asteroid 鈥極umuamua in 2017 and the comet 2I/Borisov in 2019. In 2018, Namouni and Morais also claimed to have discovered that 2015 BZ509, an object orbiting the sun that moves backwards compared with the motion of the planets, was of interstellar origin. Some were sceptical of this finding and suggested the object could have originated in the Oort cloud in the outer solar system instead.
It is thought that many such interstellar objects are constantly traversing our galaxy, and some of them also pass through our solar system. However, as these hurtle past our sun at high speed, we usually only have a brief window to study them. If numerous interstellar objects are permanently orbiting the sun, it would open up new avenues to observe objects born around other stars.
Studies of them with telescopes should be possible, but using a spacecraft might be more challenging, according to Colin Snodgrass at the University of Edinburgh in the UK, deputy lead on the European Space Agency鈥檚 Comet Interceptor project. 鈥淎ny mission [would] need a big rocket to go out there,鈥� he says. 鈥淭here鈥檚 a reason why we haven鈥檛 had a mission to any centaur.鈥�
However, Alan Fitzsimmons at Queen鈥檚 University Belfast in the UK says that while the orbits of the two TNOs could be from interstellar space, he isn鈥檛 so sure about the centaurs. 鈥淢ost of them are in unstable orbits that typically only survive a few million years,鈥� he says. 鈥淪o this implies the chance of any of those centaurs coming from the original interstellar birth cloud of the sun is extremely unlikely.鈥�
Kat Volk at the University of Arizona in the US says it is hard to trace the history of specific objects. 鈥淭hese high-inclination objects are interesting and present a challenge for solar system formation and evolution models, but I don鈥檛 think an interstellar origin is convincing based on this work,鈥� she says.
Namouni, however, says that the centaurs are on stable orbits, and he is very confident that all 19 objects are of interstellar origin. 鈥淔rom my point of view, they are confirmed,鈥� he says. 鈥淲hat we need to do is to go and observe them and try to see if they look like solar system asteroids or not.鈥�
Monthly Notices of the Royal Astronomical Society
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