Arizona Public Radio | Your Source for NPR News
Play Live Radio
Next Up:
0:00 0:00
Available On Air Stations
Science and Innovations

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


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.

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.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
Related Content