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Does Planet Nine really exist, or is it deep space fantasy?

Johns Hopkins planetary scientist Hal Weaver discusses new evidence suggesting a giant planet lurks beyond Pluto

Large, dark planet illuminated by distant sun

Image caption: An artistic rendering shows the distant view from the hypothetical Planet Nine back towards the sun.

Image credit: Caltech/R. Hurt (IPAC)

New evidence suggesting the existence of a massive ninth planet at the outer reaches of our solar system made headlines Wednesday and captured the imagination of astronomers and space enthusiasts alike.

Hal Weaver

Image caption: Hal Weaver

The theory was put forth by California Institute of Technology astronomer Mike Brown—the scientist responsible for popularizing a revised classification of planets that excluded Pluto (which earned him the nickname "Pluto Killer")—and his colleague, Konstantin Batygin. The researchers describe their work in an article published in The Astronomical Journal.

For some perspective on the new evidence, the Hub turned to Hal Weaver, a planetary science expert at Johns Hopkins University, project scientist at the Johns Hopkins Applied Physics Laboratory and on NASA's New Horizons mission to Pluto, and part of the team responsible for the discovery of the extraterrestrial object 2014MU69, a distant object beyond Pluto that is New Horizons' next destination.

Weaver discussed the possibility of the existence of the so-called Planet Nine—which he prefers to call the "fifth giant planet"—what its presence could mean to our understanding of the solar system, and his objections to the demotion of Pluto from planet to dwarf planet.

First, you say this isn't the ninth planet, because we already have a ninth planet. So you obviously disagree with the new classification of planets that excludes Pluto?

There's no doubt, after the New Horizons flyby, that Pluto is a planet. It has planetary processes coming out the wazoo. It's just a matter of how you define "planet." Just about everybody agrees that there's a category of objects called dwarf planets in the solar system. Most planetary scientists like to define planets by their geophysical processes instead of the rather esoteric definitions having to do with their orbital dynamics. From the perspective of some of us, it's a rather artificial definition that's almost specifically invoked to exclude a category of planets in order to keep the numbers down, but that's sort of an absurd proposition. Why do we need to do that?

So a lot of us favor a geophysical definition, which is still very different from what we call the minor bodies of the solar system: the asteroids and the comets. There are all kinds of incredible things taking place on Pluto and [its moon] Charon that I think are planetary processes. Mike Brown has made a name for himself as the "Pluto Killer," and Mike is an incredible scientist—and I'm friends with him—but it's just provocative.

So what about this new planet that we began hearing about yesterday?

There's been no discovery of this bigger-than-Earth object, but there's intriguing evidence for the existence of such a body that was already known a couple years ago by Chad Trujillo and Scott Sheppard. They were the ones who proposed that maybe something interesting is going on—a large object in the outer solar system that may be creating an alignment of these other objects. They proposed several other explanations, but Brown and Batygin have gone significantly beyond what Sheppard and Trujillo had proposed with more simulations and have built a stronger circumstantial case. But it's still circumstantial because we don't have a discovery of the object yet.

There have been many examples over the past century of people proposing Planet X out there for various reasons. In fact, Percival Lowell, around the turn of the 20th century, built a whole observatory to search for Planet X, and Clyde Tombaugh discovered Pluto as part of that quest. People were thinking that they saw perturbations of Neptune's orbit that could potentially indicate that there was a planet beyond Neptune's orbit, but it turned out that the data were wrong. There were no actual perturbations of Neptune's orbit.

In this particular case, as you go farther out, you see this kind of strange handful of objects at the most outer regions of the solar system that we're dubbing the inner Oort cloud that indicate there does seem to be something going on, maybe.

One potential explanation for how you get this particular alignment of these objects that have been discovered in the past decade is the existence of a fifth giant planet.

What are some of the other possible explanations?

Well, I'm not an expert in orbital dynamics, but one of the explanations that Sheppard and Trujillo investigated was that a passing star—because we're in the Milky Way galaxy—could have perturbed the orbits just temporarily as it passed through and produced the current distribution of orbits that we see. I don't think that's been ruled out. Or galactic tides from where we are and where our solar system is in the galaxy. The differential forces—because of the asymmetry of the galaxy—could have produced an effect that has this result. They didn't see that as a good explanation for what happened, but there are several other potential possibilities.

Theorists who work on these projects come up with all kinds of ideas. I'm sure there are lots of people out there now who work on orbital dynamics that are investigating potential ways of producing what we're observing right now.

If [the planet is] there, people will come up with an explanation for what makes it physically possible. This is by no means the only possible explanation, and until you actually see it, it's just a fantasy. It may turn out to be true, but we don't know that yet. And the cool thing about it is that it generates a lot of interest in both the theoretical scientists as well as the observational scientists who are out there looking for these things. Mike Brown is a part of both of those groups, actually, and he's one of the most important and prodigious surveyors of the outer solar system. He's certainly looking for it, and maybe he'll be the first to discover it. It'll be really neat if that happens.

But the problem is that the orbit that they're proposing goes way out there. It's really far away, and space is huge, so maybe we just haven't looked in the right place. But people are going to be even more vigilant in these surveys that we're doing. There will be a couple of surveys using larger telescopes that can survey the whole sky and maybe it'll show up in the next few years. That'll be exciting if it does.

There has just been a revolution in our view of how the solar system formed based on the discovery of the Kuiper belt, which started with Pluto in 1930. Nobody understood the real implication of the discovery of Pluto in 1930. It wasn't until the 1990s when we started seeing that there are other objects out there and real evidence for them. The three different populations of objects in the Kuiper Belt spurred a whole new generation of theoretical work on the formation of the solar system and it has completely changed our view.

When I was taught in school about the architecture of our solar system, the giant planets—Jupiter, Saturn, Uranus, Neptune—we thought they were always in their current positions where you see them today. Now we know there was a lot of what we call radial migration of the planets in our solar system. Jupiter may have come in almost as close as Mars' orbit and then moved back out again. Uranus and Neptune may have flipped spots. As the giant planets start moving in and out, they interact with each other in bizarre ways and can produce all kinds of crazy configurations. Uranus' and Neptune's positions relative to the sun may have flipped several hundred million years after the formation of the solar system.

What sort of things could the existence of Planet Nine reveal?

Well first of all—how did it form and how did it get to where it is now?

Those are the next questions scientists will try to answer?

Yes, as we put together a theoretical picture of how our solar system formed—this has to be a part of it. You have to explain it. If we actually discover this thing, then we have to work backwards and say "OK, we have five giant planets in our solar system. How did we get there?"

Similarly, the discovery of Pluto and all the other Kuiper belt objects spurred an investigation into how the solar system formed and how it evolved and produced a new idea about the radial migration of the planets during the early stages of the solar system's formation. All of that came out of the discovery of the objects in the Kuiper belt and the need to explain such an architecture in the solar system. The idea of radial migration—can that also explain the existence of this new object? Can we put together a completely consistent picture that explains all of the observational evidence?

How does a group of scientists go about searching for this thing?

The problem is the orbit of this fifth giant planet is huge! And it's inclined to be ecliptic, so we don't know where it is. It'll be brightest when it's at its closest spot to the sun, but it's probably not there because it spends most of its time at other points in its orbit. I'm sure Mike Brown and his team are already searching. They have theoretical orbits, but as you're looking out from Earth, the Kuiper belt takes up a lot of area.

The observational revolution that has been happening is that we've got bigger and bigger telescopes with bigger and bigger cameras that can search larger regions of the sky at once. It also increases the data volume that you have to process, and we're figuring out ways to do that. It's pretty remarkable how much data you have to crunch through to search. It really is like searching for a needle in a haystack. The object could be in a relatively dense star field or a relatively sparse star field, and that also affects your ability to pick it out. Even though it's a very large object, it can be very faint and you have all these bright stars. It's like picking out a little firefly next to a bright lighthouse. If it were easy, it would've already been done.

We're always trying to push forward that frontier. This theoretical work is motivated by the discovery of the objects in the bizarre region of the inner Oort cloud and the need to explain that in the context of the solar system's formation. They sort of motivate each other. The new observational discoveries motivate theoretical work, and then this theoretical work by Brown and Batygin is motivating further observational searches.

So what ramifications would there be for your work if this object is discovered?

The New Horizons mission, which I'm working on, is an in situ reconnaissance mission of this new region of the solar system. Seeing these objects as little points of light is one thing, but actually going out there and seeing them close up and personal helps us understand what are these objects. The exploration of Pluto and Charon and small objects in the Pluto system—this is the first time we've seen them as anything other than little pixelated blobs, and we've seen the complexity of these objects and how they're very different from the other objects. These are some of the most primitive objects in our solar system. Any comprehensive model of the solar system is going to have to explain not only the orbital dynamics but also the physical properties of the objects out there.

The discovery of this fifth giant planet could influence our understanding of the inner Oort cloud objects. Our observation of the Kuiper belt is a different sort of investigation, telling us something more detailed about the composition and the true shapes of these bodies that you can only do in in situ investigations: through spacecraft flybys. That's the lesson we've learned, and we've now been doing it for 50 years. The very first flyby of an extraterrestrial object was in 1965, 50 years before New Horizons flew by the Pluto system. The flyby is an opportunity to see close up what the nature of some of these objects are. Otherwise we're relegated to observing them from 3 billion miles away.

Unfortunately, because these objects are so far away, it takes a long time to get to them. It took us 10 years to get to Pluto even though we were the fastest spacecraft ever to leave the Earth. But we've already sampled one of the resonant objects, the Rosetta mission is looking at one of the scattered disc objects, and now, if we're able to do the extended mission phase on New Horizons, it'll be the opportunity to see a cold classical object. This is really the only opportunity to do that in our lifetime.