Hunt is on for new Planet Nine

Evidence builds for big orb in the outer suburbs of our solar system

A giant planet (orange orbit) would explain why the orbits of the six distant objects about the sun (magenta) line up in an unexpected way.


For a planet that hasn’t technically been discovered yet, Planet Nine is generating a lot of buzz. Astronomers have not yet found a new planet orbiting the sun. Yet some remote icy bodies are dropping clues that a giant orb may be lurking on the fringes of the solar system.

Six hunks of ice in the debris field beyond Neptune travel on orbits that are aligned with one another. Planetary scientists Konstantin Batygin and Mike Brown at Caltech in Pasadena, Calif., described the curious orbits last month. Gravitational tugs from the known planets should have twisted their orbits around by now. But analyses using what are known as computer models now suggest their continuing shared alignment could be explained by the effects of a distant planet. It would have to be roughly 10 times as massive as Earth and come no closer to the sun than about 30 billion kilometers — 200 times the distance between the sun and Earth. The Caltech pair described those findings in the February Astronomical Journal.

Evidence for such a planet is scant. What’s more, finding such a stealth world will be tough. Yet finding hordes of other icy nuggets with overlapping orbits could make a stronger case for the planet — and even help point to where in the sky it is. Until then, researchers are intrigued but cautious about the likelihood of a new planet.

“It’s exciting and very compelling work,” says Meg Schwamb. She’s a planetary scientist at Academia Sinica in Taipei, Taiwan. But so far, only six orbiting bodies point toward a new mystery planet. “Whether that’s enough is still a question,” she says.

Hints of a hidden planet go back to 2014. On beyond the orbit of Neptune lies the Kuiper (KY-per) belt. It’s the vast ring of frozen fossils in which Pluto lives. The orbits of 12 Kuiper belt objects cross the plane of the solar system at roughly the same time as those objects make their closest approach to the sun. Some outside force — such as the gravitational tug of a large planet — appears to hold them in place, reported two planetary scientists back then.

This new analysis “takes the next step in trying to find this giant planet,” says one of those scientists, Scott Sheppard of the Carnegie Institution for Science in Washington, D.C. “It makes it a much more real possibility.”

What’s more, Batygin and Brown found, the orbits of six of the Kuiper belt objects look like long ovals that are stretched out in roughly the same direction. Those orbits also lie in nearly the same plane. The likelihood this is just due to chance is a measly 0.007 percent, the pair calculates.

“Imagine having pencils scattered around a desktop,” says Renu Malhotra. She’s a planetary scientist at the University of Arizona in Tucson. “If all are pointing in the same quarter of a circle, that’s somewhat unusual.”

Kuiper belt object
An artist’s idea of a Kuiper Belt object (KBO) in the outer rim of our solar system, some 4 billion miles from the sun. NASA has directed its New Horizons spacecraft (the one that photographed Pluto), to fly by one KBO in 2019. NASA, ESA, and G. Bacon (STScI)
A mystery planet might also explain a couple of other oddities about the outer solar system. Consider the dwarf planets Sedna and 2012 VP 113 . Both orbit far from the known planets. But a still-unseen Planet Nine could have put them there.

The planet also would stir up the orbits of other Kuiper belt residents, making them roughly perpendicular to the rest of the solar system. And indeed, Batygin was surprised to learn, five known objects have such orbits. When he and Brown compared the simulations from their new computer model of an agitated Kuiper belt to the cockeyed orbits of these objects, they found a match.

“If there was one dramatic moment in the past year and a half, this was it,” Batygin says. “We didn’t really believe our own story for the longest time. But here was the strongest line of evidence.”

Planet 9 would likely be a migrant

Given what scientists know about how the solar system formed, the proposed planet would not have been born in its current environment. It probably developed closer to the sun. Later, it would have been kicked from the inner solar system to its outskirts after flirtations with the giant planets (Jupiter, Saturn, Neptune and Uranus).

This wouldn’t be the first time scientists were led to a new world by the odd behavior of another. Astronomer Johann Galle found Neptune in 1846. What pointed his search: Mathematicians Urbain Le Verrier and John Couch Adams calculated that an unknown planet could be causing Uranus to speed up and slow down along its orbit.

Uranus was a more clearly defined problem, says Scott Tremaine. He’s an astrophysicist at the Institute for Advanced Study in Princeton, N.J. Le Verrier and Adams were trying to understand why Uranus appeared to defy the law of gravity. In contrast, Batygin and Brown are piecing together a story of how the solar system evolves.

“When we’re talking about history rather than laws, it’s always easier to go astray,” Tremaine warns.

The orbital alignments are striking, he says. And, he adds, Batygin and Brown have done sensible calculations. Still, he worries about hunting for mathematical support after noting a possible oddity. “That can be very misleading,” he says. “The numbers that won the Powerball lottery are an unusual combo, but that [particular combo] doesn’t mean anything.”

Hunt now underway

In the meantime, “the hunt for Planet Nine is on,” Batygin says.

NASA’s WISE satellite spent nearly nine months mapping infrared emissions across the sky. Data from that satellite rule out the existence of a planet as massive as Saturn out to 4.2 trillion kilometers from the sun. Those data similarly rule out a Jupiter-like world to at least three times that distance. If a smaller, cooler planet is out there, it’s probably in the outer third of its orbit. And that would put it against a dense background of Milky Way stars. Trying to spot it would be like looking for a planetary needle in a galactic haystack. “It’s not going to be impossible,” Batygin says. “It just makes it harder.”

The Victor Blanco telescope in Chile and Subaru telescope in Hawaii are the best scopes for undertaking the search, says Schwamb in Taiwan. Both have cameras that can see large swaths of sky. If scientists don’t mind waiting, the Large Synoptic Survey Telescope will come online in 2023. It’s currently being built in Chile. It will image the entire sky once every three days.

With that LSST, “We would be able to detect Planet Nine even if it was moving slowly,” says Lynne Jones. She’s an astronomer and LSST scientist at the University of Washington in Seattle. “We could look for motion from month to month or over the course of a year and quickly pick it out from the background stars.”

There’s also the possibility, though remote, that a chance picture of the planet already exists.

Uranus, Neptune and Pluto were all seen before anyone realized they were planets — dwarf or otherwise. Most observations don’t record things as faint as Planet Nine. “But there’s lots of archival data,” says the Carnegie Institution’s Sheppard. They have been collected by observatories as astronomers gathered images of stars, nebulas and galaxies. Among these, he says, an image of the mystery planet “could be sitting there somewhere.”

Power Words

(for more about Power Words, click here)

align    (noun: alignment) To place or organize things in a patterned order, following an apparent line.

archive   (adj. archival)To collect and store materials, including sounds, videos and objects, so that they can be found and used when they are needed. People who perform this task are known as archivists.




dwarf planet   One of the solar system’s small celestial objects. Like a true planet, it orbits the sun. However, dwarf planets are too small to qualify as true planets. Prime examples of these objects: Pluto and Ceres.




Kuiper belt    An area of the solar system beyond the orbit of Neptune. It is a vast area containing leftovers from the formation of the solar system that continue to orbit the sun. Many objects in the Kuiper belt are made of ice, rock, frozen methane and ammonia.

migration   (v: to migrate) Movement from one region or habitat to another, especially regularly and according to the seasons. One that makes this move is known as a migrant.

Milky Way  The galaxy in which Earth’s solar system resides.

nebula   A cloud of space gas and dust existing between major adult stars. Telescopes can detect these clouds by the light they emit or reflect. Some nebulas also appear to serve as the nurseries in which stars are born.


orbital eccentricity  The degree to which a planet’s orbit deviates from a perfect circle.

perpendicular  An adjective that describes two things that are situated approximately 90 degrees to each other. In the letter “T,” the top line of the letter is perpendicular to the bottom line.

plane    (in geometry and space) A flat surface (like a sheet or piece of paper) that extends in all directions, infinitely, without end. It has no height (or depth), much the way a line also has just two dimensions.


Pluto   A dwarf planet that is located in the Kuiper Belt, just beyond Neptune. Pluto is the tenth largest object orbiting the sun.




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