If the solar system does have a Planet Nine, it is a world that was bullied by its larger siblings, exiled to the far reaches of space and only rescued from oblivion thanks to the intervention of passing stars.
Planet Nine is a hypothesised planet that is five to 10 times the mass of Earth, orbiting in the outer solar system at about 400 to 800 times the Earth-sun distance. The best evidence for its existence comes from a clustering of objects orbiting far from the sun, suggesting the gravitational influence of a planetary body. But if Planet Nine is real, we aren’t sure how it got there.
We know closer-in planets form in a disc of dust and gas surrounding a star, but, like Planet Nine, some exoplanets have been seen orbiting much further from their star, up to thousands of times more remote than the Earth-sun distance, which is well outside where the disc formation process should be possible.
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To investigate, at Rice University in Houston, Texas, and his colleagues calculated how likely it was that a world like Planet Nine could end up in such a wide orbit around our sun. They showed that, early in the solar system, the jostling of the giant planets Jupiter, Saturn, Uranus and Neptune could have resulted in Planet Nine being kicked out after initially forming closer to the sun, potentially removed from the system for good. But if this occurred early enough, in the first 3 million to 5 million years, nearby stars born in the same cluster as our sun could have then prevented the planet from being completely lost by giving it repeated gravitational kicks back towards our star, before the stars spread out over time into the galaxy.
“If you just eject a planet, this planet is going to become lost,” says Izidoro. “But if the sun is in the cluster, sometimes you’re lucky enough that you have a star approaching the sun and the encounter is just perfect, and it gives a little kick to the planet. Then the planet might have a safe orbit from that point on.”
The researchers also looked at the possibility of this happening in other star systems, and found that it occurs in only 1 out of every 1000 stars, because giant planets appear to be quite rare. “The solar system is not common at all,” says Izidoro, with current observations suggesting that only 10 per cent of sun-like stars have giant planets. But for our solar system, the chance of a wide-orbiting world like Planet Nine being created could have been as high as 40 per cent, provided the jostling of the planets occurred early enough in the cluster phase. “So if this planet is there, it has been there for a long, long time,” says Izidoro.
at the California Institute of Technology, who co-proposed the existence of Planet Nine in 2016, called the work an “extraordinary calculation” in how it modelled the early solar system. “This study puts the pieces together with an unprecedented level of detail,” he says. “The fact that Planet Nine comes out with a probability of as much as 40 per cent is really quite remarkable.”
The search for Planet Nine continues, and will be aided by the upcoming Vera C. Rubin Observatory in Chile, which will begin a detailed survey of the sky later this year. Other possibilities for the planet’s formation exist, including that it was born in our sun’s disc or it was captured from another star, but those scenarios remain unlikely, says Batygin.
“I think with a high degree of certainty, if we find Planet Nine, we will conclude that it formed in a similar way as what’s described in this paper,” he says.
Nature Astronomy
