Ever since William Herschel discovered Uranus in 1781, astronomers have been eager to find new planets on the outer edge of the solar system. But after the discovery of Neptune in 1846, we have not found any other large planets. Sure, we've discovered Pluto and other dwarf planets beyond it, but nothing the size of Earth or larger. If there is a Planet Nine or “Planet X” lurking out there, we have yet to find it.
But there is some preliminary evidence for this. As we found more Pluto-like bodies known as Trans-Neptunian Objects (TNOs) and even more distant bodies known as Kuiper Belt Objects (KBOs), we noticed that there appeared to be a strange orbital clustering between them gives. The orientation of their orbits is not as randomly distributed as we would expect, which could be caused by the small gravitational forces of a super-Earth at the edge of the solar system. If we assume that this is the solution to the orbital distortion, then there could be a planet with the mass of five Earths that is ten times further from the Sun than Neptune.
Astronomers have searched for the planet but found nothing. This has led some to speculate that Planet's evidence is simply not that convincing and that there are other possible explanations for the cluster. So a new paper argues for a new way to collect evidence for Planet X, and it's remarkably clever.
The idea is based on a phenomenon known as occultation. This happens when an asteroid or planetary body passes in front of a star. By observing the star while the object obscures it, astronomers can measure things like the body's orbit and shape. Through an occultation, we discovered that the asteroid Chariklo has a ring system. Amateur astronomers have used occultation events to map the shapes of small asteroids.
Occultations from different angles show the shape of an asteroid. Photo credit: IOTA
The authors propose building 200 40-cm telescopes spaced 5 kilometers apart to create a 1,000-kilometer-wide field of coverage. Because each telescope would have a slightly different vantage point, occultations would be perceived differently by different telescopes, allowing astronomers to map the orbit and size of trans-Neptunian objects. They estimate that they could discover about 1,800 new TNOs over the course of a 10-year study. Based on simulations of TNO orbits and clusters, the authors show that such a system should find clear evidence of any body with five Earth masses within 800 AU of the Sun. In other words, if Planet X is out there, this study could prove it.
The entire facility would only cost about $15 million, which is surprisingly inexpensive for a project like this. Even if the study fails, Planet [sunlight can shift the orbits of small solar system bodies.](https://briankoberlein.com/blog/super-breakout/)
Reference: Gomes, Daniel CH and Gary M. Bernstein. “An automated occultation network for gravity mapping of the trans-Neptunian solar system.” arXiv preprint arXiv:2410.16348 (2024).
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