It has been described as hiding in plain sight, a planet 10 times as massive as Earth and orbiting the Sun beyond Neptune. Predicted in a series of studies over the last few years, Planet Nine of the solar system — if it exists — continues to elude, and yet intrigue, with clues suggesting that it is indeed out there.
The latest pointer comes from another object in the outer Solar System, called 2015 BP519. This one is certain: it was discovered during the international project Dark Energy Survey. In a paper on a preprint archive reporting the discovery, a large team of scientists has concluded that 2015 BP519 “adds to the circumstantial evidence for the existence of this proposed new member of the Solar System”.
The object orbits the Sun at an extreme tilt — its orbital plane is inclined at 54° to that of the eight planets. This, the researchers believe, is probably because of the influence of the gigantic Planet Nine. Otherwise, the extreme inclination of the BP519 orbit did not make sense in simulations of the known Solar System.
“The computer simulations we use takes all objects in the Solar System and evolves them forward or backwards in time, and looks at how the orbits of the objects change over time,” lead author Juliette Becker, a PhD student at the University of Michigan, told The Indian Express by email. “When we ran a simulation without Planet Nine, we found it was very hard to make objects like BP519. When we ran a different simulation including Planet Nine, we found that it was very easy to make objects like BP519,” she said.
Over the years, scientists have sought to explain other puzzling aspects of the solar system by attributing these to the influence of Planet Nine. In a 2016 paper in The Astronomical Journal, California University of Technology researchers Konstantin Batygin and Michael Brown made out a case for the existence of the planet by arguing that it could be responsible for the peculiar alignment of objects in the Kuiper belt, an expansive field of icy debris on the outskirts of the Solar System.
The same year, another Caltech team attributed a well-known feature of the Solar System to Planet Nine. The Sun’s equatorial plane is aligned six degrees off from the orbital plane of the planets, something that had long puzzled scientists. According to the Caltech team, it is not the Sun that is out of alignment but the eight planets; Planet Nine’s mass has caused their orbital plane to wobble.
Batygin and Brown had predicted that Planet Nine’s gravity would push Kuiper belt objects into higher inclinations. Asked about the latest findings, Batygin said by email: “The discovery of Becker et al is a fantastic result — they have detected the first highly inclined Kuiper belt object that securely resides in the Planet Nine-dominated domain of the Solar System. I could not be happier about their detection.”
All the evidence, however strong, does not prove conclusively that Planet Nine exists. How much longer will that take? Batygin said: “The false-alarm probability with the current set of objects is about 0.1%. Additional findings will lower this value even further.” But Becker felt: “The only way to prove the existence of Planet Nine is to directly detect it (to take a picture and see it there). All these pieces of indirect evidence will help us figure out where to look for it, though!”