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’ve found no other large planets. Sure, we discovered Pluto and other dwarf planets beyond it, but nothing Earth-sized or larger. If there is some planet nine, or “Planet X” lurking out there, we have yet to find it.
But there is some tentative evidence for it. As we have found more Pluto-like bodies known as Trans-Neptunian Objects (TNOs) and even more distant bodies known as Kuiper Belt Objects (KBOs), we’ve noticed that there appears to be an odd bit of orbital clustering among them. The orientation of their orbits isn’t as randomly distributed as we’d expect, which could be caused by the small gravitational tugs of a super-Earth at the edge of the solar system. If we assume that is the solution to the orbital bias, then there could be a five Earth-mass planet orbiting ten times farther from the Sun than Neptune.
Astronomers have searched for the planet but have found nothing. This has led some to speculate that Planet X might be a primordial black hole, while more skeptical minds argue it must not exist. The evidence just isn’t that strong, and there are other possible explanations for the clustering. So a new paper argues for a new way to gather evidence of Planet X, and it’s remarkably clever.
The idea is based on a phenomenon known as occultation. This is when an asteroid or planetary body passes in front of a star. By observing the star as the object occults it, astronomers can measure things such as the orbit and shape of the body. 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.
Ocultations different vantage points reveals the shape of an asteroid. Credit: IOTA
The authors propose building 200 40-cm telescopes spaced 5 kilometers apart to create an occultation array 1,000 km wide. Since each telescope would have a slightly different vantage point, occultations would be seen differently by different telescopes, allowing astronomers to map the orbit and size of Trans-Neptunian Objects. They estimate that over the course of a 10-year study they could detect about 1,800 new TNOs. Based on simulations of TNO orbits and clustering, the authors show that such a system should find clear evidence of any 5 Earth-mass body within 800 AU of the Sun. In other words, if Planet X is out there, this study could prove it.
The whole array would only cost about $15 million U.S. dollars, which is surprisingly cheap for such a project. Even if the study failed to find Planet X, it would add to our understanding of the distant solar system and also allow us to study how [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 gravitational mapping of the trans-neptunian solar system.” arXiv preprint arXiv:2410.16348 (2024).