On their quest to find the mysterious ninth planet in our Solar System, astronomers are putting all their hopes on the environment surrounding one celestial object with an “abnormal” trajectory: 2015 BP519.

“Planet Nine”

What if the Solar System really had nine planets? Ever since the International Astronomical Union (IAU) redefined the concept of the planet in 2006, Pluto has been considered a “dwarf planet“: a spherical body orbiting the sun, but one that is too small to “clean” its environment of other objects. There’s no changing the nomenclature now. So why have researchers not given up the hope of finding a ninth planet since 2016? And more recently, how do we explain the potential existence of “Planet Nine,” as announced on May 14?

The hunt for a ghost planet

The ninth planet in the Solar System – if it exists, because it has not yet been formally detected – would have a mass about 10 times Earth’s, and would be in an orbit 20 times farther than Neptune’s. To arrive at this hypothesis, we must go back to 2016. Two years ago, two planetologists from the California Institute of Technology (Caltech) hypothesized, based on mathematical models, that “Planet Nine” would take 10,000 to 20,000 years to completely orbit the sun.

A few months after this theory on the existence of a ninth planet came out, Jacques Laskar, the director of CNRS research at the Institute for Celestial Mechanics and Computation of Ephemerides (IMCCE), published an estimate of the planet’s possible position. To do this, he relied on data from the Cassini probe, which allows us to note anomalies in orbits around far away planets.

The scope thus narrowed, the hunt for the “ghost planet” began. After two years, a team of international researchers (American, British, French, Brazilian, Spanish, Swiss, South African and Chilean) announced they’d studied a celestial object codenamed “2015 BP519” whose trajectory may have corresponded to that of the hypothetical ninth planet. The scientists say it’s probably a dwarf planet whose “abnormal” orbit (at a 54° incline to the Solar System’s plane) could only be caused by the presence of a planet four times bigger than Earth.

“This isn’t proof that Planet Nine exists”

It was detected for the first time in 2014 as part of the “Dark Energy Survey“: an international search for clues about the existence of “dark energy” done by mapping hundreds of millions of galaxies. The famous 2015 BP519 is located in one of the most remote areas of the Solar System, in the Kuiper Belt, beyond Neptune’s orbit.

Of course, this is just a hypothesis. Some astronomers remain skeptical and even argue that the unusual orbit of this extreme trans-Neptunian object (ETNO) may be due to the presence and influence of massive objects (about 10,000 dwarf planets) during the formation of the Solar System.

This is not proof that the ninth planet exists,” says David Gerdes, a researcher at the University of Michigan and co-author of a study at Quanta Magazine. “But I would say that the presence of such an object in our Solar System supports the hypothesis of the Ninth Planet,” he adds, optimistically.

We still have to prove the existence of the ninth planet… and find it. This is likely to be particularly difficult: researchers say it is probably 30 billion kilometers away from Earth. A space probe wouldn’t be able to fly over it until the year 2085, at least. But whether Planet Nine exists or not, one thing’s for sure: something is “acting” on the orbit of 2015 BP519.