There might be a ninth planet in the solar system after all, and it is not Pluto.

Two astronomers reported on Wednesday that they had compelling signs of something bigger and farther away — something that would satisfy the current definition of a planet, where Pluto falls short.

“We are pretty sure there’s one out there,” said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology.

What Dr. Brown and a fellow Caltech professor, Konstantin Batygin, have not done is actually find that planet, so it would be premature to start revising mnemonics of the planets.

In a paper published in The Astronomical Journal, Dr. Brown and Dr. Batygin laid out a detailed circumstantial argument for the planet’s existence in what astronomers have observed: a half-dozen small bodies in distant elliptical orbits.