Evidence mounts for hidden ninth planet

Gravitational signature hints at massive object that orbits the Sun every 20,000 years
A century after observatory founder Percival Lowell speculated that a ‘Planet X’ lurks at the fringes of the Solar System, astronomers say that they have the best evidence yet for such a world. They call it Planet Nine.

Orbital calculations suggest that Planet Nine, if it exists, is about ten times the mass of Earth and swings an elliptical path around the Sun once every 10,000–20,000 years. It would never get closer than about 200 times the Earth–Sun distance, or 200 astronomical units (au). That range would put it far beyond Pluto, in the realm of icy bodies known as the Kuiper belt.

No one has seen Planet Nine, but researchers have inferred its existence from the way several other Kuiper belt objects (KBOs) move. And given the history of speculation about distant planets, Planet Nine may end up in the dustbin of good ideas gone wrong.

Astronomers have long speculated about the existence of additional large planets in the outer Solar System, but none has yet been confirmed.

1846 Johann Gottfried Galle discovers Neptune, guided by predictions from perturbations of Uranus’s orbit.

1905 Percival Lowell starts hunting for a ‘Planet X’, which he predicted would lie beyond Neptune, just as Neptune lies beyond Uranus. His calculations led astronomers at Lowell’s namesake observatory to find Pluto in 1930, but the object is not massive enough to be Planet X.

1984 On the basis of periodic extinctions in the fossil record, scientists propose that a dwarf star, later named Nemesis, passes through the Solar System every 26 million years, flinging comets on a path to impact Earth.

1999 Perturbations in comet orbits lead astronomers to propose that a brown dwarf (bigger than a planet but smaller than a star) exists in the outer Solar System. It is named Tyche, the good sister of Nemesis.

2014 A search with the Wide-Field Infrared Survey Explorer satellite rules out the existence of both Nemesis and Tyche. But the discovery of an object in the distant Kuiper belt prompts Chadwick Trujillo and Scott Sheppard to propose a large planet in the Kuiper belt.

2016 Orbital calculations by Konstantin Batygin and Mike Brown strengthen the concept of this unseen planet, which they name ‘Planet Nine’.

“If I read this paper out of the blue, my first reaction would be that it was crazy,” says Mike Brown, an astronomer at the California Institute of Technology in Pasadena who was part of the research team. “But if you look at the evidence and statistics, it’s very hard to come away with any other conclusion.”

Brown and his colleague Konstantin Batygin propose Planet Nine in a paper published on 20 January in the Astronomical Journal.

Alessandro Morbidelli, an orbital-dynamics specialist at the University of the Côte d’Azur in Nice, France, who has reviewed the paper in detail, says he is “quite convinced” that the planet exists. Others are not so sure.

“I have seen many, many such claims in my career,” says Hal Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “And all of them have been wrong.”

Claims of Planet Nine’s existence recall a period in the nineteenth century when astronomers predicted and then discovered Neptune by studying tiny perturbations in the orbit of Uranus. The gravity of some unseen body must be tugging on Uranus, they said- and they were right. “In some sense we’re hoping to relive history a little bit,” says Batygin.
*Culled from Nature