Since 1992 and the declassification of Pluto for a dwarf planet, our solar system has only eight planets. However, astronomers are still looking for a ninth planet, the so-called planet X, analyzing the trajectories of all the objects beyond Neptune’s orbit. At this distance, a large population of small, rocky objects is having an anomalous collective structure meaning they are possibly interacting with a massive object. Many of these rocks appear to occupy a region close to the plane containing the eighth known planets, leading to this region being called the Kuiper Belt. It is difficult make observation in this region far remote from any light source and astronomers have yet discovered only a small fraction of the objects orbiting beyond Neptune.

In 2016, Caltech researchers found new evidence about “Planet X”. This hypothetical Neptune-sized planet would be orbiting our sun in a highly elongated orbit far beyond Pluto. This object which would be “Planet Nine,” could have a mass about 10 times that of Earth and orbit about 20 times farther from the sun on average than Neptune. It may take between 10,000 and 20,000 Earth years to make one full orbit around the sun.

The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us. This is not, however, the detection or discovery of a new planet. It’s too early to say with certainty there’s a so-called Planet X. What we’re seeing is an early prediction based on modeling from limited observations. It’s the start of a process that could lead to an exciting result.

Jim Green, director of NASA’s Planetary Science Division

In March 2017, interesting new results were presented by a team from the University of Michigan. They showed new evidence confirming the possibility of a planet Nine in our solar system. For that, they evaluated the dynamical stability of a selection of outer solar system objects in the presence of the proposed new solar system member Planet Nine. They used numerical simulations to study a variety of orbital for this new planet and evaluate the dynamical stability of eight trans-Neptunian objects in the presence of Planet Nine. The results showed that Planet Nine would be acting as a stabilizing influence.

The ultimate goal would be to directly see Planet Nine—to take a telescope, point it at the sky, and see reflected light from the sun bouncing off of Planet Nine. Since we haven’t yet been able to find it, despite many people looking, we’re stuck with these kinds of indirect methods.

Juliette C. Becker, Astronomy Department, University of Michigan

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