Flagstaff Astronomer: Proposed 'Planet Nine' Raises Questions About Planet Definition

Jan 22, 2016

Two California astronomers presented evidence for a new planet beyond Neptune on Wednesday. The prediction could reopen the debate about how a planet is defined, says an astronomer at Lowell Observatory in Flagstaff.

An artist's rendition of the proposed planet.
Credit Caltech/R.Hurt(IPAC)


The proposed “Planet Nine” is an ice giant perhaps ten times the size of Earth. Scientists at Caltech inferred its existence from a cluster of small objects in the Kuiper Belt with strange, tilted orbits.

Gerard van Belle, astronomer at Lowell Observatory, says, “A discovery like this, if it gets confirmed, is very important because it actually forces us to reevaluate what we think we know about the early history of the solar system.”

And, van Belle says, it could raise questions about the official definition of a planet. In 2006 the International Astronomical Union defined a planet as an object that “clears its orbit” of other objects. Caltech astronomer Michael Brown says Planet Nine fits that definition.

But van Belle says there’s a chance it doesn’t.

“Its orbit is so very long—it’s going to be between ten and twenty thousand years to complete a single revolution around the sun—that there’s really just not enough time for it to have actually cleared its orbit,” van Belle says. “So even though it’ll be huge and a major body in our solar system, it could potentially still just be a dwarf planet, just like our friend Pluto.”

Astronomers still have to find the proposed planet. Van Belle says this data likely will spark new surveys of the sky.