A distant planet may lurk far beyond Neptune

Out beyond Neptune, the solar system resembles the deep ocean: dark, remote and largely unexplored. To an Earth-bound observer, even the brightest objects, such as Pluto, are 4,000 times as faint as what the human eye can see. An undiscovered planet could easily lurk out there unnoticed, a possible fossil from a time when the giant planets jockeyed for position 4 billion years ago, scattering planets and asteroids in their wake. But even the largest telescopes would struggle to find such a faint spot of light. Most likely, the clues would be entangled in the distorted orbits of faraway ice boulders tumbling around the sun.

Astronomers Chad Trujillo and Scott Sheppard provided a hint about how such a world might reveal itself last March when they announced the discovery of a 450-kilometer-wide dwarf planet just outside the Kuiper belt — the icy debris field past Neptune (SN: 5/3/14, p. 16). 

Their find, designated 2012 VP113, is on a course that loops around the sun in a vastly elongated orbit far from the known planets. It has thousands of neighbors but shares its odd trajectory only with Sedna, another dwarf planet, discovered in 2003.

“They’re kind of in a no-man’s-land,” says Sheppard, of the Carnegie Institution for Science in Washington, D.C. “These objects couldn’t get out there with what we currently know.”

Something had to drag the two dwarf planets from their original, smaller orbits. Except nothing is close or massive enough to take the credit. At least, nothing astronomers are aware of.

The discovery of 2012 VP113 confirmed that Sedna is not a fluke but is possibly the first of a large population of icy bodies distinct from others in the rest of the solar system. So Trujillo and Sheppard continued to poke around the Kuiper belt, and the mystery deepened. They noticed that beyond 150 astronomical units (150 times the distance from the sun to the Earth), 10 previously discovered objects, along with Sedna and 2012 VP113, follow orbits that appear strangely bunched up.

“That immediately piqued our interest,” says Sheppard. Could an unseen planet, a Planet X, be holding the orbits of all these far-out bodies in place?

“The idea’s not crazy,” says David Jewitt, a planetary scientist at the University of California, Los Angeles. “But I think the evidence is slim.” The trail of bread crumbs leading to an undiscovered planet is sparse: just 12 chunks of ice lead the way. But it’s enough to get some researchers wondering about a ninth (or 10th, depending on your attitude regarding Pluto) planet roaming the outer solar system and how it might have arrived there.

Kuiper belt clues

“The exciting thing for me is that 2012 VP113 exists,” says Megan Schwamb, a planetary scientist at Academia Sinica in Taipei, Taiwan. “Whatever put Sedna on its orbit should have put a whole bunch of other objects out there.”

The enormous, stretched orbits of Sedna and 2012 VP113 are unlike anything else in the solar system. Both are too far from Neptune to feel its effects. And they’re too far from the Oort cloud, the distant shell of ice boulders thought to envelop the solar system. Their trajectories could be a relic of a passing star, or the changing influence of the Milky Way’s gravity as the sun moves around the galaxy — or of a massive planet, long-gone or yet to be detected.

The case for an additional planet got stronger when Trujillo and Sheppard realized that Sedna and 2012 VP113 had something in common with 10 other objects. All the objects beyond 150 astronomical units come closest to the sun, a point known as perihelion, at nearly the same time that they cross the plane of the solar system. There’s no reason for these perihelia to bunch up like that. Billions of years of evolution should have left the perihelia scattered, like the rest of the Kuiper belt — unless something was holding the perihelia in place.

Trujillo and Sheppard estimated that a planet about two to 15 times as massive as Earth, at a distance of  250 astronomical units (about eight times as far from the sun as Neptune) could explain why these 12 perihelia were bunched together. But the astronomers admit that’s not the only possibility. A closer planet as massive as Mars would have the same effect as a Neptune-mass body much farther away.

“A few years ago everyone thought that nothing relevant other than just plain asteroids and comets inhabited that region,” says physicist Carlos de la Fuente Marcos. “Now the observational evidence indicates that probably we were wrong.” He and his brother Raúl, both at the Complutense University of Madrid, took a closer look at the orbits. The brothers claim, in the Sept. 1Monthly Notices of the Royal Astronomical Society Letters, that not one but two planets are needed to explain the perihelion clustering.

Around the same time, physicist Lorenzo Iorio at the Ministry of Education, Universities and Research in Bari, Italy, offered a different take. He says that the planet proposed by Trujillo and Sheppard, if it exists, must be much farther out — at least twice as far as the original prediction. By looking at gradual changes in the orbits of a few of the known planets, Iorio calculated that a planet twice as massive as Earth must be at least 500 astronomical units from the sun, according to research in the Oct. 11Monthly Notices of the Royal Astronomical Society Letters.

Others are more cautious. “The outer solar system can be full of all sorts of unseen and interesting things,” Jewitt says, “but the argument ... for a massive perturber is a bit puzzling.” First, 10 of the 12 bodies with peculiar perihelia dive far enough into the Kuiper belt to possibly feel Neptune’s gravity. And, second, he says, 12 objects is a tiny sample — the apparent perihelion clustering may just be an illusion caused by where researchers point their telescopes.

The recent speculation about additional planets has a familiar ring, says Jewitt. In the late 1800s and early 1900s, astronomers relied on apparent hiccups in Neptune’s motion and a handful of comets to kick off a search that eventually led to the discovery of Pluto. “Not much has changed since then,” he says. In fact, musings of a planet beyond Neptune have been around since before anyone knew Neptune existed.

Planet hunters

In 1834, German astronomer Peter Andreas Hansen allegedly suggested to a colleague that two planets were needed to explain oddities in the motion of what was then the farthest known planet, Uranus — oddities that led to the discovery of Neptune in 1846. Two years later, French astronomer Jacques Babinet claimed that Neptune also stumbled along its orbit, hinting that a ninth planet must have been causing Neptune to speed up and slow down as it ran around the sun.

Over the next half century, the search for more planets went in and out of vogue. Like a game of solar system Whac-A-Mole, new predictions popped up after each claimed discovery was trounced. The predictions relied on observations of Neptune as well as a handful of comets that reached their farthest point from the sun at nearly the same distance, a clue that a massive planet was bringing the comets all to the same point before the comets returned to the sun.

In the early 1900s, Boston-born polymath Percival Lowell got into the planet-hunting game. It’s not clear if Lowell was the first person to use the phrase “Planet X,” but he certainly popularized it. Lowell calculated where Planet X should be, based on observations of Uranus and Neptune. Thirteen years after Lowell died in 1916, American astronomer Clyde Tombaugh picked up the torch, using Lowell’s calculations as a guide. Tombaugh’s systematic observations led him to Pluto in 1930, close to where Lowell predicted Planet X would be.

Pluto’s fall from grace started months after its discovery. Unlike the other eight planets, which travel on flat, circular orbits, Pluto speeds through Neptune’s orbit along a stretched, cockeyed trajectory. The diminutive planet didn’t appear massive enough to push the outer planets around. Estimates for Pluto’s mass dwindled until astronomer James Christy discovered Charon, Pluto’s largest moon, in 1978. Christy used the moon’s motion to weigh Pluto and found that the outermost planet had only 0.2 percent of Earth’s mass. If something was tugging on Uranus and Neptune, Pluto was much too small to be the culprit.

NASA’s Voyager 2 spacecraft settled the Planet X question — for a time. When the probe flew past Uranus and Neptune in the 1980s, it gave astronomers better data to revise the masses of the planets. The new data revealed that nothing was pulling on them. Tombaugh finding Pluto where Lowell’s calculations pointed was just a coincidence.

The search for Planet X quieted, never disappearing, but not taken seriously. Planet X became a favorite of the tinfoil-hat crowd, who were convinced that NASA was hiding knowledge of a planet that would either crash into Earth or hurl a barrage of comets our way. The end, as always, was near.

Modern planetary research has been plagued by rough estimates and few objects to study. “What this means,” Schwamb says, “is the observers need to go back to work.” Only a detailed accounting of the darkness beyond Neptune will help researchers figure out if something is there.

If that something exists, it can’t be as massive as Jupiter or Saturn. Kevin Luhman, an astronomer at Penn State, looked for Jupiter doppelgängers in images from NASA’s WISE satellite, a 10-month mission to scan the entire sky twice with an infrared telescope. Massive planets are best seen in infrared light because they’re still cooling off from their formation. Jupiter, for example, radiates more heat than it receives from the sun.

Reporting in the Jan. 20 Astrophysical Journal, Luhman found no evidence for a Jupiter-mass planet within 82,000 astronomical units. Likewise, there’s no sign of something as massive as Saturn out to about a third as far. But Luhman says he can’t rule out a small, rocky planet, which would be too cold for WISE to pick up.

The best bet is to look for reflected sunlight, which is how scientists discovered Pluto and the Kuiper belt. But even large worlds at such enormous distances are extremely dim. If Pluto was twice as far from the sun, it would be one-sixteenth as bright because the sunlight not only has to get out there but also has to come back.

“We would not yet have detected the Earth,” says Jewitt, “if it were more than 600 astronomical units from the sun.” And that’s assuming researchers knew where to look. “That gives you an idea of the darkness of the outer solar system.”

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Instead of trying to directly observe a planet, researchers are looking for more Kuiper belt objects, whose orbits might bear the gravitational signature of something unseen. Trujillo and Sheppard discovered 2012 VP113 as part of an ongoing effort to scan the sky from Cerro Tololo Inter-American Observatory in Chile. Undoubtedly, more interesting objects will turn up.

Most telescope searches, however, are akin to mapping the universe while staring through a drinking straw. Telescopes see a tiny fraction of the sky, and observatories dole out access just a few days at a time. Sheppard says that it will take them several years to cover just 20 percent of the sky.

Enter the Large Synoptic Survey Telescope, or LSST, an 8.4-meter-wide telescope being built in northern Chile with full operations planned for late 2023. Unlike other telescopes, it will have an enormous field of view and will make a decade-long movie of the sky, perfect for looking for moving points of light.

Lynne Jones, an astronomer at the University of Washington in Seattle, says the LSST could find 20,000 to 40,000 more bodies in the Kuiper belt. With about 20 times as many Kuiper belt objects in hand, astronomers should be able to see if there are more objects with bizarre orbits and determine if the bunching of perihelia is real or just an artifact of having found only a few.

Plus, says Jones, LSST could detect an Earth-sized planet out to between 300 and 500 astronomical units, depending on how reflective its surface is.

For planetary scientists, if a remote Planet X exists the question is: How do you form a planet that far from the sun? Renu Malhotra, a planetary scientist at the University of Arizona in Tucson, says the problem is time. At that distance, the planet building materials would have been smeared over a ring several hundred billion kilometers around. “To make a planet the size of the Earth,” says Malhotra, “could take longer than the age of the solar system.” The only solution, she says, is to steal the planet from somewhere else.

Uranus and Neptune are the most likely thieves, pilfering planets from the space between their orbits. Malhotra says that a close encounter with either of those giants could slingshot an Earth-sized ball of rock to well beyond the Kuiper belt.

Planet X might also be extrasolar, says planetary scientist Rodney Gomes of the National Observatory in Rio de Janeiro. The sun was born in a nebula along with several thousand other stars, and many of those probably had planets of their own. As stars jostled each other, planets could have been torn away from one star and captured by the gravity of another. Perhaps Planet X is just the result of a brief game of interstellar catch.

The Large Synoptic Survey Telescope in Chile, illustrated here, will make a 10-year-long movie of the night sky starting in 2023.

“The jury’s still out on whether you need to have a planet there or not,” Schwamb says. A close encounter with a passing star could have lured Sedna and 2012 VP113 away from their siblings, like an astronomical pied piper. Stellar flybys are rare, however, and the star must pass close enough for the two dwarf planets to notice but not so close that it disrupts the entire Kuiper belt and possibly the outer planets.

The odds go up if the star is a relative, born in the same nebula as the sun. In addition to tossing planets around, stellar siblings could have tugged on the debris swirling around the sun. The distorted orbits would have frozen in place after the sun’s brothers and sisters drifted away.

For 168 years, the lure of planets hiding beyond Neptune has never faded. The remoteness of the outer solar system, Jewitt says, “leaves open the door to all sorts of wild speculation.”

The hunt for Planet X “is one of those things that’s very high risk,” Luhman says, “but if it was found, it would be a huge discovery.” Astronomers have discovered more than 1,800 planets orbiting other stars, and yet our own backyard is still largely a mystery. “We haven’t explored all of the solar system yet,” Sheppard says, “so people always want to believe that there’s something else out there.”