Trans-Neptunian objects build case for ninth planet

An artist's conception of Planet 9(Credit: Robin Dienel)

If you grew up knowing that there were nine planets orbiting our sun and were a bit crushed when Pluto lost its status among those celestial bodies, there might be new hope for a nine-pack, as researchers are again putting forth the idea that a giant planet might be lurking somewhere out there on the fringes of our Solar System.

Previous researchers have certainly theorized the existence of a ninth planet, sometimes known as Planet X or Planet 9, based on computer models and the observed behaviors of objects in the Kuiper Belt – a ring of rocky debris beyond the orbit of Neptune. Now, Scott Sheppard from the Carnegie Institution for Science and Chadwick Trujillo of Northern Arizona University believe they have found even more proof of its existence.

They've discovered a handful of satellites orbiting our Sun at vast distances, known as trans-Neptunian objects. Because of the way these bodies cluster together and share similar orbital angles, the duo have concluded that they are under the influence of a large planet. According to a Carnegie report, that planet could be over 200 times farther from the Sun than the Earth is – which would make it five times farther from the sun from Pluto – and that it could be up to 15 times bigger than our own planet.

"We are now in a similar situation as in the mid-19th century when Alexis Bouvard noticed Uranus' orbital motion was peculiar, which eventually led to the discovery of Neptune," said Sheppard.

In making the new trans-Neptunian object discoveries, Sheppard and Trujillo relied on data from some of the world's most powerful cameras and telescopes including the Dark Energy Camera on the NOAO Blanco telescope in Chile and the Japanese Hyper Suprime Camera on the Subaru telescope in Hawaii. The objects they've found have been submitted to the International Astronomical Union's (IAU) Minor Planet Center to receive official designations.

One of the objects they've submitted to the IAU has an orbit that travels 3,000 times farther from the sun than the Earth. Because it travels so far away from the center of our Solar System, the researchers believe that it comes under extra-Solar-System influences such as gravity from other stars, or another planet.

"Right now we are dealing with very low-number statistics, so we don't really understand what is happening in the outer Solar System," Sheppard said. "Greater numbers of extreme trans-Neptunian objects must be found to fully determine the structure of our outer Solar System."

To that end, the team is now marching forward with what Carnegie calls the "largest, deepest survey for objects beyond Neptune and the Kuiper Belt." So far, they have covered about 10 percent of the sky.

In addition to the newly reported trans-Neptunian objects, Sheppard and Trujillo also discovered another in 2014, which they nicknamed "Biden," after the American Vice President.

"Objects found far beyond Neptune hold the key to unlocking our Solar System's origins and evolution," Sheppard says. "Though we believe there are thousands of these small objects, we haven't found very many of them yet, because they are so far away. The smaller objects can lead us to the much bigger planet we think exists out there. The more we discover, the better we will be able to understand what is going on in the outer Solar System."

The work of the duo has been accepted for future publication in the The Astronomical Journal.

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