The hunt for 'Planet X'

Will new tools shed light on this hypothetical planet?
17 January 2025

Interview with 

Matt Bothwell, University of Cambridge

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Planet X - which is often called Planet Nine - is a hypothetical planetary body that some scientists believe exists out beyond Pluto and Neptune. Regular mathematical wobbles in the orbits of some of the distant bodies in the solar system point towards something we can’t account for exerting a periodic gravitational tug on them. So is there a giant planet out there? Here’s Matt Bothwell, Public Astronomer at Cambridge University’s Institute of Astronomy…

Matt - This sometimes gets called Planet X or Planet Nine. Planet X, meaning ten, of course, in Roman numerals. When we used to have Pluto, the idea of there being a mysterious, massive planet beyond the orbit of Pluto would have made it the tenth planet. Since Pluto got kicked out of the Planet Club, this mysterious planet that some people think might exist in the dark outer reaches of our solar system, if found, would be Planet Nine. The question is: is there a fairly massive planet that we haven’t found yet, lurking in the freezing darkness of the outer solar system?

Chris - Why do space scientists think there might be?

Matt - The best way of answering this is to go back in time 250 years. Astronomers had just discovered the planet Uranus in 1781. In the decades following Uranus’s discovery, astronomers noticed that it was speeding up and slowing down in ways that didn’t align with our understanding of how gravity worked. The best explanation we had was that there was another planet outside the orbit of Uranus, and it was the gravity of this unseen planet pushing Uranus around.
A mathematician did some calculations, pinpointed the planet, and it turned out to exist. That planet was Neptune. The way we found Neptune wasn’t just because we spotted something in the sky and thought, "Huh, that’s funny." It was because we saw Uranus—something we knew existed—being pushed around by something invisible.
In short, that seems to be happening again. In the outskirts of the solar system, where Pluto is, there’s a whole population of objects called TNOs—trans-Neptunian objects. When you look at the orbits of all these different TNOs, they seem to be shepherded or herded by something massive that we haven’t seen yet. Their orbits appear to be aligning in a way that goes beyond coincidence but makes perfect sense if there’s a missing planet out there.

Chris - How big would that missing planet need to be to produce those effects?

Matt - Estimates vary, but it would need to be at least a few times the size of Earth—possibly even as large as an ice giant like Uranus or Neptune.

Chris - And just one planet? Or could there be a swarm of them out there? Can the maths resolve that?

Matt - Our best guess is one, simply because it’s the simplest explanation. Once we have some new telescopes online, we’ll get a better idea of how these TNOs are behaving and be able to refine our estimates. But for now, one planet seems the most likely scenario.

Chris - You’ve sort of anticipated my next question, which is about seeing it. What’s it going to take to resolve this quandary?

Matt - It’s really tricky. In theory, there’s nothing stopping us from seeing this planet. The problem is we don’t know where it is. This is a more complicated problem than the one astronomers faced when they found Neptune. Uranus was behaving predictably, and we were able to pinpoint Neptune’s position accurately.
The TNOs that seem to be lining up in this particular way are harder to pin down. The "haystack" we’re searching for our needle in is enormous. We’re looking at a huge swathe of sky, and the planet is likely to be incredibly small and faint.
Luckily, a telescope coming online soon is perfect for this search: the Vera Rubin Observatory. The amazing thing about Rubin is that it’s doing something almost no telescope has done before.
Most telescopes are incredibly powerful but focus on tiny patches of sky. For example, the James Webb Telescope is extraordinarily powerful and can see across the universe, but it looks at an astonishingly small area of sky. If we tried to scan the possible region where this planet might be using James Webb, it would take a thousand years.
Rubin, however, will photograph the entire sky every few days at an incredible level of faintness. If Planet Nine exists, Vera Rubin will almost certainly find it.

Chris - Where is this amazing telescope, and when will it begin operating?

Matt - The telescope is in Chile, under some of the clearest skies in the world. Its "first light," which is what we call a telescope’s first glimpse of the sky, is due in July 2025.

Chris - So we’ll find out some pretty exciting stuff soon. Obviously, it wasn’t conceived to solve this particular problem—it’s just something that might be explored using it. What’s its primary purpose?

Matt - It’s designed for what we call time-domain astronomy, which is astronomers’ lingo for anything that changes over time. That includes transient events like exploding stars, gamma-ray bursts, or black holes changing how fast they consume dust and gas.
Because Vera Rubin will photograph the entire sky every few days, we’ll effectively be able to make "videos" of the sky. We’ll see stars exploding and fading, and black holes growing as they consume matter—almost in real time.
Rubin will also help us make massive strides in understanding dark energy and dark matter. It will create enormous, detailed maps of the cosmos, which are exactly what we need to study the universe on its largest scales and uncover more about dark energy and dark matter.

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