How Can We Find Planet Nine? (And Other Burning Questions)

A large, undiscovered planet lurking on the fringe of the solar system is a compelling idea—and it’s nothing new. It’s been proposed many times. And those predictions have always been wrong.

So what about this latest prediction, the one that might actually be right?

(In case you missed it, a new study from Caltech suggests there might be a large world way out there—a planet whose gravitational hand is sculpting the orbits of distant, icy worlds and forcing them to take weird paths around the sun.)

How can we figure out if the planet is actually there? And will it even be bright enough to see? Here are some answers to these and other pressing questions.

Are we sure it’s there?

No. The evidence is tantalizing, but it’s circumstantial. UCSC astronomer Greg Laughlin gives the planet a 68.3 percent chance of actually existing (“That’s odds-on, but it’s not huge odds-on. It’s also not a coin flip.”) Konstantin Batygin, who’s half of the Caltech team, says he’d put the planet’s chances at 83 percent (“I made that up right now…I’m just being a little bit more realistic than Greg.”)

Others aren’t quite so sure. “I’m very skeptical of this turning up because I’ve seen so many predictions like this—and so far they’ve never turned out,” says Alan Stern, principal investigator of the New Horizons mission that sent a spacecraft zooming by Pluto this summer. “But I’m sure that they ultimately will. I have no doubt that there are lots of planets out there.”

And then there’s Alessandro Morbidelli, of France’s Côte d’Azur Observatory, who told The New York Timestold The New York Times he’d bet $10,000 the planet is real.

Want to weigh in? You can log your predictions here.

How do we know it would meet all the criteria to be a planet?

If you mean the criteria defined by the International Astronomical Union in 2006, then we need to know whether the potential planet orbits the sun, is round, and has enough gravitational heft to dominate its orbit.

This planet would obviously orbit the sun, and something with 10 Earth masses is more than massive enough to be round. Regarding the last point—the clearing of small bodies and other junk from its orbit—“Planet Nine is forcing any objects that cross its orbit to push into these misaligned positions. It fits that concept perfectly,” Caltech’s Mike Brown, the other half of the Caltech team, said to the Washington Postto the Washington Post.

What would this planet be like?

Cold, for sure. At its closest, the planet is still 200 times farther from the sun than Earth is, and at its most distant is a whopping 600 to 1,200 times farther away than Earth.

At somewhere around 10 Earth-masses, the frigid world will be more like a gassy, mini-Neptune than a rocky planet. This inference is based on information from exoplanets, which tend to show up most frequently in this mass range. So far, though, this type of extremely common planet has been notably absent from the solar system.

Could it have moons?

Possibly. “It would be very interesting to know if it has satellites,” Laughlin says. If those moons are big enough to see, then we could determine the mass of the planet and get a better idea of what it’s made of.

What should we call this planet?

For now, whatever you want, really (but not Nibiru. This is not Nibiru). The world hasn’t been detected yet, and if it ever is, naming the thing will be a formal and lengthy process. The Caltech team is calling it Planet Nine, which is a pointed reference to the controversial reclassification and removal of Pluto from the planetary lineup in 2006, a decision was motivated in part by the work of Mike Brown, who could now be on the ironic cusp of resurrecting a ninth planet.

But…this could also be Planet Ten, based on how you define planet (hi, Pluto!). Or Planet Fourteen (hello, Ceres, Haumea, Makemake, and Eris). Or Planet One Hundred and Something (hey, every round thing). “Apparently, Caltech can’t count,” Stern says.

How could scientists find it?

“Go to a big telescope with a wide field of view, and look at as much sky as possible,” says the Gemini Observatory’s Chad Trujillo, who has spotted a number of very distant solar system objects. “Take three images of the sky, with maybe 1.5-2 hours between each image, and look for things that move. Things that move fast are asteroids and are nearby, and things that are slower are farther out.”

In short, to find Planet Nine, scientists will need to channel Clyde Tombaugh, who spotted Pluto in 1930 after staring at photos of distant starfields and looking for a shifting speck of light.

Yet Planet Nine, if it’s there, will be a bit harder to find than Pluto. It will be dimmer and farther away, and scientists estimate that it takes between 10,000 and 20,000 years to orbit the sun once—so it will move very slowly across the sky.

We don’t know where the planet is in its orbit, but chances are it’s not nearby (because of the way orbits work, this planet spends a lot of time hanging out very far away). Plus, the patch of sky its orbital path covers is huge and crosses the plane of the Milky Way twice. Pulling a planetary needle from a cosmic haystack is difficult under the best of circumstances, but pulling that needle out from a star-studded galactic streamer is even harder.

But it’s not impossible. Scientists could manage to catch a glimpse of something on the fringe using several telescopes on Earth, Trujillo says. He and others say the Subaru telescope on Mauna Kea, as well as a smaller telescope in Chile are capable of finding something that faint and far away. If teams do see something moving, they’ll try to get at least three observations of the object so they can plot a preliminary distance and brightness. And then, over many months of observations, they’ll work to pin down the exact orbit. Brown and Batygin are already looking; Trujillo may start searching next month. “We’re racing, but it’s a friendly race,” he says. “We don’t try to trip each other or anything.”

Wait…how bright is it? 

The real answer is no one knows for sure. A planet’s brightness depends on its size, distance, and composition—and we don’t really know any of those things right now. But there are ways to estimate how bright the world could be, which is something astronomers refer to as magnitude. Because astronomers sometimes like to do things backwards, the magnitude scale is a bit counterintuitive: Objects with higher magnitudes are actually dimmer. For example, Pluto is currently around magnitude 14. The sun? That’s –26.74. If you’re in a big city, you probably can’t see anything dimmer than magnitude 3, maybe 4 at best.

Batygin and Brown have calculated that when it is closest to the sun, this planet is around 18th magnitudethis planet is around 18th magnitudethis planet is around 18th magnitude, which is bright enough to be picked up with high-end backyard telescopes. That’s why it’s unlikely to be close to the sun now—we would have already seen it.

At its farthest, the planet would be around 24th magnitude, or about 10,000 times dimmer than Pluto. That’s not too dim for a telescope to see, but it doesn’t make spotting the planet easy. Remember, this faint speck of light is going to be moving very slowly through a dense field of stars.

How did they do that brightness calculation, given we don’t know how big it is, where it is, or what it’s made of?

Batygin and Brown assumed the planet is between two and four times larger than Earth (more on that in a minute) and has a Neptune-like reflectivity, which is a property that matters when you’re thinking about how much light bounces off an object. Reflectivity is known as albedo, and is reported on a scale from 0 (very dark) to 1 (very bright).

The reason for the Neptune-like assumption is that, at roughly 10 Earth-masses, this planet is likely more of a gas-shrouded mini-Neptune than something with a hard, rocky surface. Not surprisingly, different atmospheres have different reflectivities (and what if it’s covered in bright white clouds?), so there is some uncertainty in this portion of the calculation.

But the biggest uncertainty in the brightness calculation (aside from distance) comes from the planet’s size, which could be between two and four times bigger than Earth, and the area of a reflecting surface scales as the square of the radius. “The size uncertainty of a factor of 2 leads to an area uncertainty of a factor of 4,” says Andy Rivkin of The Johns Hopkins University Applied Physics Lab. “That corresponds to 1.5 magnitudes all by itself.”

Overall, that kind of uncertainty isn’t terrible, and most estimates of this planet’s brightness at its farthest point tend to hover between 23rd and 25th magnitude. “I would say that at a given distance there’s probably 1-2 magnitudes of uncertainty in the prediction,” says Laughlin, who independently reached a conclusion very similar to Batygin’s and Brown’s, assuming a world that is ¾ the size of Neptune and has the same albedo.

Could we use a similar technique to find planets around other stars?

Yes, if we could make such detailed observations of small bodies around other stars (which we can’t). The question would then become, is it easier or harder to prove the existence of an exoplanet based on how it perturbs exo-smallthings?

Laughlin argues that it would be easier. Discoveries in the solar system, he says, are held to a higher standard of proof than discoveries around other stars. “If there was some mechanism for returning very accurate observations of comet orbits in an exoplanet system, and you saw this amount of evidence, I think it would just be a no-brainer that there’s a 10-Earth-mass planet perturbing that stuff.”

Stern agrees. “I’m trying to think about whether that’s sociological or actually scientific,” he says.

NASA’s Natalie Batalha, a member of the planet-hunting Kepler team, notes that planets around other stars have already been discovered based on how they perturb other objects—notably, other planets. In 2011, scientists described a planet called Kepler 19c. The planet hadn’t been directly detected, but its presence was inferred from the way it jiggled its planetary sibling, called Kepler 19b. As the pair orbits its star, 19c’s gravitational hand pulls on 19b, changing the frequency with which 19b periodically blocks out its star’s light. Scientists can use those slight variations, called transit timing variations, to constrain the orbital period and mass of the invisible, perturbing planet, and the method is thought to be sensitive enough to detect low-mass planets.

It’s not clear whether transit timing variations offer more precise constraints than the whacked-out orbits of icy worlds in the faraway solar system, but it’s certainly interesting to think about.

Is this planet the fifth giant that scientists think Jupiter kicked out way back when?

No. First of all, the idea that our solar system once had a fifth giant planet comes from simulations of our neighborhood’s early days. Plunk a fifth world in the realm of the giants (Jupiter, Saturn, Uranus and Neptune), and you end up with a solar system that looks much more like the one we live in today. If that fifth large sibling was there, it would have been punted into space by that jerk Jupiter’s gravity hundreds of millions of years after being born.

Batygin and Brown think their planet would have been thrown outward within several million years after the sun formed. Back then, our star was still in its native birth cluster and the surrounding stars would have helped keep the flying planet from hurtling forever into space (it’s worth mentioning that some scientists, such as Hal Levison of the Southwest Research Institute, don’t consider this scenario to be incredibly likely).

What’s more, this planet would have been less of a full-fledged giant and more of a planetary core—something like a seed from which a larger planet could have grown, had it been given the chance.

“These two events are both ejections, but they are well separated in epoch,” Batygin says. “The planet that was ejected during the giant instability of the solar system—which, by the way, coincided with the formation of the Kuiper Belt—that planet, if it was there, it was just ejected. It doesn’t get to stick around.”

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