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

By Melissa Sevigny
Published: Friday, January 22, 2016 - 8:30am
Updated: Monday, January 25, 2016 - 8:46am
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(Photo courtesy of Caltech/R. Hurt (IPAC))
A diagram released by the Caltech researchers shows the proposed location of "Planet Nine."

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.

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.

“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," said Gerard van Belle, astronomer at Lowell Observatory.

He said 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 said Planet Nine fits that definition.

But van Belle said 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,” said van Belle. “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 said this data likely will spark new surveys of the sky.