There are eight known planets in our solar system. Beyond the orbit of Neptune, there are small icy worlds like Pluto and Eris, but none that rival the planets in size. But could there be a planet lurking in the depths of our solar system?
It’s possible that there is a planet in the outer solar system, but it is so far away and so cold that it would be difficult to see. As we continue to make high resolution sky surveys, and as new telescopes such as the James Webb Space Telescope are launched, we will be able to find such planets in time. If any exist. But there is another way to look for these planets, and that’s by the gravitational tug they exert on other bodies.
Neptune was discovered in this way. After the discovery of Uranus, it was noticed that its motion didn’t quite agree with the predictions of Newtonian gravity. But the deflection of its orbit could be solved if it was caused by the pull of an undiscovered planet. In the mid 1800s John Couch Adams and Urbain Le Verrier calculated the position of such a planet independently. Soon afterward Johanne Galle discovered Neptune using Le Verrier’s predictions. This method has also failed, such as when strange motions of Mercury predicted a planet that doesn’t exist. It was later found that Mercury’s oddness is due to the effects of general relativity.
Seeing the gravitational tug from a much more distant world is much more difficult. The distances are greater and the gravitational influences much weaker. However there are hundreds of known objects beyond Neptune (or Trans-Neptunian Objects) and together they give us a statistical tool to look for other planets. In recent years there has been some evidence to support the existence of a super-Earth planet in the outer solar system. This was supported by the fact that the orbits of the most distant Trans-Neptunian Objects (TNOs) seemed to be clustered together. But the most recent survey of outer worlds clearly demonstrates that this clustering was likely due to detection bias, and the distribution of TNOs doesn’t support the existence of a super-Earth planet 9. Ethan Siegel has recently about this, and you should really check it out.
No if there’s no planet 9, is that it for solar system bodies? Maybe not. While the latest survey of TNOs rules out a super-Earth, it leaves open the possibility of a smaller planet, sometimes called Planet 10. The planets of our solar system have orbits that are all roughly along the same plane, known as the invariable plane. Since the solar system as a whole is gravitationally isolated, we would expect the orbits of TNOs to have a similar orientation. Orbits would vary a bit, but the average orientation should be near the invariable plane. But this latest survey found that the orbital orientations of distant TNOs are tilted about 8 degrees from the invariable plane. For a single orbit that isn’t unusual, but it is odd for lots of orbits to have this orientation. This kind of shift can occur due to a gravitational tug from a larger body, through what is known as the Kozai mechanism. Based on the data, this planet would be at least the size of Mars.
As it stands the evidence for planet 10 is relatively weak. There’s a small chance that the odd orbits of TNOs is merely random chance. But this work demonstrates that we are closing in on a final answer to the number of planets in our solar system. If they are out there, we will find them in the next several years, and if not we’ll confirm they don’t exist.
Paper: Kathryn Volk and Renu Malhotra. The curiously warped mean plane of the Kuiper belt. arXiv:1704.02444 [astro-ph.EP] (2017)