Any More Big Planets Out There?

Pluto is not the only large object beyond Neptune.

A menagerie of known trans-Neptunian objects (TNOs), shown to scale. (NASA/STScI)
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It’s possible that many large objects are still lurking in the outskirts of our solar system, still undiscovered. So far, we know of only a handful that are half Pluto’s size or bigger. Finding more will be a considerable challenge, even though scientists believe such bodies are key to understanding how the solar system formed.

It’s believed that planets were created by collision, when small rocks smashed into each other and gradually formed bigger objects. Pluto and other Kuiper Belt objects beyond Neptune, which are typically about 30 to 55 astronomical units (AU, or sun-Earth distances) away from us, represent the leftovers of planet formation.

Today the Kuiper Belt contains many thousands of asteroids and comets, and in recent years astronomers have discovered a few large objects close to Pluto’s size (or in the case of Eris, maybe bigger July 13 note: New Horizons just confirmed that Pluto is larger). Other worlds, like Sedna (discovered in 2003) and a similar-sized object called 2012 VP113 announced in a Nature paper in 2013, have been spotted even farther out.

But the pace of discovery is slow for several reasons, says Scott Kenyon, an astronomer with the Harvard-Smithsonian Center for Astrophysics. First, a Pluto-sized object at those distances is faint, and its motion is hard to detect.

“These objects are not moving as rapidly [as closer planets] and there are more competing objects that you have to look through,” Kenyon says. “There are a lot of background stars and galaxies, and more and more, as you get fainter and fainter, it’s a harder problem to identify objects at that faint limit—let alone see if they’re moving.”

Pluto travels an egg-shaped orbit that sometimes brings it closer to the sun than Neptune. It also orbits at an angle to the ecliptic—the plane of the solar system where most planets circle the sun. Since each orbit takes 248 years, Pluto hasn’t even done half a lap since its discovery in 1930. Other objects tilted to the ecliptic, and at similar distances, would also appear to move at a slow pace, only showing themselves as faint glimmers of light, moving in unpredictable directions in the sky.

And many of them would be darker. Pluto currently reflects about 60 percent of the sun’s light, making it relatively bright; our own moon only reflects 12 percent. This makes Pluto an anomaly, and could mean the dwarf planet contains lots of ice, Kenyon says. Charon, Pluto’s largest moon, reflects 40 percent of the sun’s light at most. A typical Kuiper Belt object would reflect about five to 10 percent, he says.

NASA’s heat-seeking WISE (Wide-field Infrared Survey Explorer) telescope has been used to search the outer solar system for large planets; survey results released in 2014 showed that WISE couldn’t find anything the size of Saturn or larger out to a distance of 10,000 AU. No Jupiter-sized planets were found out to 26,000 AU.

“WISE is really good for finding a Jupiter or a Saturn, but there’s no way it could detect an Earth-mass planet at these large distances,” says Kenyon. That’s because a smaller object wouldn’t give off enough heat for WISE’s sensors to detect.

Other limitations arise from the way telescopes are used by astronomers. Space telescopes such as Hubble or the upcoming James Webb have a small field of view—not so good for surveying the sky. And competition for viewing time with large ground-based telescopes (those with mirrors 6.5 meters and larger) is fierce. That’s why the best chance of doing planet-hunting is with telescopes four meters and smaller, according to Kenyon.

Astronomers will continue scanning the skies for more Plutos, and this week’s New Horizons flyby will help. “It will give the investigators who are searching for more Kuiper Belt objects and more Plutos the ammunition they need, or the support they need, to continue these surveys,” Kenyon says.

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