New evidence for the mysterious Planet Nine

Scientists studying eight of the most distant known objects in the solar system have concluded that many of them wouldn’t even exist if their orbits weren’t stabilised by an as-yet undiscovered planet lurking far out beyond the orbit of Neptune.

The existence of this planet, colloquially known as Planet Nine, has been suspected for several years due to the tilted orbits of these objects, which suggests that they have gravitationally interacted with a planet the size of Neptune several hundred times further out from the Sun than the Earth. 

The bodies in these tilted orbits are a population of large Trans-Neptunian Objects (TNOs), which lie on extremely elliptical orbits out beyond Neptune and Pluto. One of the best known is Sedna, a 1000-kilometre world discovered in 2003.

But the new work suggests that Planet Nine may also exert a stabilising influence on these bodies, keeping them from being ejected from the solar system by interactions with Neptune, whose orbit they sometimes approach.

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Sedna itself is in a stable orbit, says Juliette Becker, a doctoral student at the University of Michigan, Ann Arbor, but others are on orbits so easily affected by Neptune that they should have been knocked out of orbit (and potentially been thrown into the Sun, collided with another planet, or been kicked entirely out of the Solar System) in as little as 10 million years. And yet, 4.5 billion years after the birth of the Solar System, they are still here for us to see.

It’s a mystery, she says, until you add Planet Nine into the equation. Then, interactions with Planet Nine dampen the effect of Neptune’s occasional gravitational kicks. “Instead of getting kicked out of the Solar System it hops to a new orbit,” she said at a meeting of the American Astronomical Society’s Division for Planetary Sciences in Provo, Utah, on 7 October. “Planet Nine enhances the dynamic stability of these objects.”

Furthermore, she says, not all orbits for Planet Nine will have this stabilising effect. 

And since it is also known that Planet Nine must be in an orbit that tilts these same TNO orbits, the two effects – tilting orbits and stabilising them – can be used to further narrow the parts of the sky astronomers need to search for the elusive planet.

Further assisting the quest, Becker announced that astronomers reviewing images from a sky-watch known as the Dark Energy Survey have found a ninth TNO similar to the eight others. Nicknamed Caju (the Portuguese word for “cashew”) it lies in a highly eccentric orbit that averages about 450 times farther out than the Earth. Based on its brightness, it is probably on the order of 480 kilometres across, Becker added – though she noted that this is just an estimate.

The new finds are exciting because they will help astronomers find Planet Nine, if indeed it exists, adds Mike Brown, a planetary scientist from California Institute of Technology in Pasadena, California, who was not part of the study team. 

As soon as the technical data is released, he says, his team will fold it into their calculations of where best to look for Planet Nine. And the discovery of Caju, he says, will almost certainly help refine the primary search region because there are so few known Planet-Nine-affected TNOs that the addition of one more will definitely help.

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