A couple of years ago, Bromley and the Smithsonian Astrophysical Observatory's Scott Kenyon worked out two scenarios for the existence of a large planet in the outer solar system.
A planet might have been formed from material that was already out there, and such a planet would tend to have a circular orbit. Or, in a scenario now supported by the research of Caltech's Konstantin Batygin and Mike Brown, a large planet might once have been ejected from the inner solar system by one of the gas giants.
Brown told The Associated Press on Wednesday that he expects the ninth planet to be discovered within the next five years. Utah astronomer and NASA ambassador Patrick Wiggins said Wednesday that it's important to note that the planet has not been discovered.
"What has been discovered is evidence that it could exist," Wiggins said, adding that the evidence is "pretty decent."
Caltech's research, the result of a year-and-a-half collaboration, is based on mathematical evidence and computer modeling that appear to illustrate what Brown said in a news release is "the most planet-y of the [solar system's] planets" because of its profound gravitational influence.
It would be a stretch to say it's in Earth's neighborhood..
"We're talking, like, 200 times further away than the outermost planet now, Neptune," Wiggins said, adding that at its farthest from the sun, Planet X might be 1,200 times as far away as Neptune, and its orbit may take as long as 20,000 years.
That distance may make it a challenge to observe. Although the naked eye can see stars many times farther away than Planet X's theoretical orbit, any light from Planet X would be very faint.
If the planet is near what's thought to be the closest point in its orbit to the sun, Caltech's news release said astronomers may be able to spot it on images from previous surveys. If it's in its most distant orbit, only the world's largest telescopes will do.
"We just may not have a telescope big enough to capture enough light to be able to get a picture of the thing," Wiggins said.
Neptune was found using a similar method, Wiggins said: by looking where the math said it should be. But Bromley said astronomers tried for a long time to find a ninth planet by observing its effects on other planets. Neptune was given away by perturbations in Uranus' orbit, for instance. The latest model instead is based on the orbits of "very, very distant things" that appear to be "shepherded" by an unseen body.
"Planet X," Wiggins noted with a chuckle, is "what they always seem to call the mysterious next planet." Pluto, before it was Pluto, was termed the long-sought Planet X.