Science & Technology

There's New Evidence for the Mysterious Mega-Earth Known as Planet 9

Is there a planet several times more massive than Earth hiding in plain sight — a mysterious Planet Nine? A new paper gives more evidence that Planet Nine is a real place. Astronomers have discovered a tiny world far away from Neptune, and that world's path in space could point to a gravitational tug from something big — say, a Planet Nine-sized planet.

The unusually closely spaced orbits of six of the most distant objects in the Kuiper Belt indicate the existence of a ninth planet whose gravity affects these movements.

What Is Planet Nine?

First of all, Planet Nine is not Pluto. Many people still think of Pluto as the ninth planet, but for complicated reasons in part related to Pluto's small size, it was officially renamed a dwarf planet in 2006. That leaves eight known planets in our solar system today: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The term "Planet Nine" refers to a hypothetical distant world in our solar system that is bigger than our own planet, but smaller than the planet Neptune. If Planet Nine is out there, it orbits the sun at roughly 20 times farther out than Neptune does. This means Planet Nine would take an extraordinary 10,000 to 20,000 years to go around the sun just once. Earth, by contrast, takes just 365 days to complete a year.

Researchers first suggested Planet Nine's existence back in 2016 to explain the odd orbits of several icy objects in the Kuiper Belt, which is a region of small bodies far out in the solar system. The evidence was so compelling that astronomers all over the world quickly picked up on the Astronomical Journal paper and began doing searches of their own. But so far, Planet Nine — if it's out there — is still hiding from us.

"I would love to find it," stated co-author and dwarf planet discoverer Mike Brown, an astronomer at the California Institute of Technology, when Planet Nine was announced. "But I'd also be perfectly happy if someone else found it. That is why we're publishing this paper. We hope that other people are going to get inspired and start searching."

Here's Why It's Important

There have been many studies about Planet Nine in the past few years, ranging from the mundane to the incredible. One 2016 study said that Planet Nine could have tilted the sun. Another set of research from January 2017 suggested that Planet Nine might have been a free-flying body in space until it got too close to the sun's gravity and was captured.

This new paper is talking about a small object that researchers found far away in the solar system, beyond the orbit of Neptune. The tiny world has a bizarre orbit that makes it very likely that Planet Nine's gravitational pull shepherded it through its path on space.

The case for Planet Nine isn't closed yet, because the solar system is a big place. Who knows how many thousands of tiny objects are pulling on each other, and what sorts of orbits this would produce over billions of years? But the paper does give hope to astronomers continuing to search in the darkness, looking for Planet Nine's location. And you can join in if you like!

A citizen science project at is recruiting volunteers for the Planet Nine search. No special knowledge is required. "Models of this planet imply that we may have already imaged it with NASA's WISE telescope, and simply not recognized it," Zooniverse writes. "Together, we're going to look through the data from this telescope in a powerful new way to try to spot it."

Did the Sun Steal Planet Nine From Another Solar System?

Written by Elizabeth Howell May 21, 2018

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