WaPo: New evidence suggests a ninth planet lurking at the edge of the solar system
Astronomers at the California Institute of Technology announced Wednesday that they have found new evidence of a giant icy planet lurking in the darkness of our solar system far beyond the orbit of Pluto. They are calling it "Planet Nine."
Their paper, published in the Astronomical Journal, describes the planet as about five to 10 times as massive as the Earth. But the authors, astronomers Michael Brown and Konstantin Batygin, have not observed the planet directly.
Instead, they have inferred its existence from the motion of recently discovered dwarf planets and other small objects in the outer solar system. Those smaller bodies have orbits that appear to be influenced by the gravity of a hidden planet a "massive perturber." The astronomers suggest it might have been flung into deep space long ago by the gravitational force of Jupiter or Saturn.
Telescopes on at least two continents are searching for the object, which on average is 20 times farther away than the eighth planet, Neptune. If "Planet Nine" exists, it's big. Its estimated mass would make it about two to four times the diameter of the Earth, distinguishing it as the fifth-largest planet after Jupiter, Saturn, Uranus and Neptune. But at such extreme distances, it would reflect so little sunlight that it could evade even the most powerful telescopes.
Confirmation of its existence would reconfigure the models of the solar system. Pluto, discovered in 1930, spent three-quarters of a century as the iconic ninth planet. Then, a decade ago, Pluto received a controversial demotion, in large part because of Brown.
Brown and Batygin initially set out to prove that Planet Nine didn't exist. Their paper builds on earlier research by two other astronomers that revealed a peculiar clustering of the small, icy objects discovered in the past decade or so in the remote regions of the solar system.
In 2014, Scott Sheppard of the Washington-based Carnegie Institution of Science and Chad Trujillo of the Gemini Observatory in Hawaii published a paper in the journal Nature that discussed the potential existence of a giant planet affecting the orbits of those dwarf worlds. Sheppard and Trujillo noted a similarity in the motion of those bodies when they are closest to the sun.
"We thought their idea was crazy," Brown said, explaining that extra planets are always the "go-to suggestion" when astronomers find orbital behavior they can't explain. But he and Batygin struggled to debunk that hypothetical ninth planet. They used mathematical equations and then computer models, ultimately concluding that the best explanation for the smaller objects' clustering was the gravitational effects of something far bigger.
Such clustering is similar to what's seen in some asteroids that are about as close to the sun as the Earth. They wind up in stable orbits that keep them far from Earth and free from any significant disturbance by the Earth's gravity.
"Until then, we didn't really believe our results ourselves. It just didn't make sense to us," Brown said. But their modeling showed that a planet with 10 times the mass of Earth would exert an influence over the orbits of the smaller bodies and keep them from coming as close to the sun as they should. It would also slowly twist these orbits by 90 degrees, making them periodically perpendicular to the plane of the solar system.
"In the back of my head, I had this nagging memory that someone had found some of these modulating objects and not known what to make of them," Brown said. "And sure enough, these objects do exist. And they were exactly where our theory predicts they should be."
That's when the Caltech researchers started to take Planet Nine seriously. "That was the real jaw-dropping moment, when it went from a cute little idea to something that might be for real," he said.
If and when it's spotted, Planet Nine would be evaluated by the same criteria that got Pluto demoted. Brown isn't concerned about that.
"That's not even a question -- it's definitely a planet," he said. One of the trickiest criterion for planet status, based on the standards set by the International Astronomical Union, is that a planet must "clear the neighborhood" around its orbital zone. It needs to have the gravitational prowess to change the orbits of other objects.
"Planet Nine is forcing any objects that cross its orbit to push into these misaligned positions. It fits that concept perfectly," Brown said.
The "Pluto killer" added: "Not to mention the fact that it's 5,000 times the mass of Pluto."