Most people think the eight planets in our solar system orbit the sun along a straight plane, like a disc on a record player. But actually, that plane is slightly tilted, and now astronomers think they know why: The elusive Planet Nine.

Earlier this year Michael Brown, a professor of planetary astronomy at the California Institute of Technology, presented evidence that there may be a massive planet beyond Neptune orbiting the sun. The so-called Planet Nine is thought to be about 10 times the mass of Earth and about as large as Neptune. At its farthest point it is about 155 billion miles away from the sun.

At that size and distance, this planet’s mass would have large effects on the other planets orbiting the sun.

“It’s so far away that it essentially gets this huge lever arm on the solar system, and it slowly tilts the planets in its direction,” Dr. Brown said Wednesday during a joint meeting of the American Astronomical Society’s Division for Planetary Sciences and European Planetary Science Congress in Pasadena, Calif. “It can tilt the solar system.”