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Objects beyond Neptune provide fresh evidence for Planet Nine

New solar system objects used to be a distraction for Konstantin Batygin, a planetary scientist and theorist at the California Institute of Technology (Caltech) in Pasadena. Each discovery added another complication to his computer models of the solar system, which twirl planetoids around the sun. But now, Batygin is eager to find more of the objects himself, and missed opportunities pain him. In late September and early October, cloudy skies foiled a 6-night run at the Subaru Telescope atop Mauna Kea in Hawaii. All you can do, he says, is “just sit quietly and wait for things to get slightly better.”

What drew Batygin into the hunt is the ultimate prize: a new planet, the first to be added to our solar system in more than a century. Colloquially called Planet Nine, this distant hypothetical world could have 10 times the mass of Earth and take 15,000 years to go around the sun. This past January, Batygin and Mike Brown, a Caltech astronomer, proposed that the giant could explain the peculiarly clustered orbits of six icy bodies beyond Neptune. Now, several teams, including Batygin and Brown’s, are racing to spot Planet Nine directly.

Curious clusters

In January, astronomers announced evidence for Planet Nine, a new giant planet, based on the way it would shepherd six icy worlds into clustered orbits on the other side of the solar system. Three new objects add to the picture.

(Diagram) G. Grullón/Science; (Data) Scott Sheppard/Michelle Bannister

Studies presented last week at a meeting of the American Astronomical Society in Pasadena are giving them extra encouragement. Researchers have found another three transneptunian objects (TNOs) that, like the first six, may corroborate Planet Nine’s existence and help narrow down its putative orbit. The influence of the unseen giant could also explain the strange orbits of two more objects, perpendicular to the plane of the solar system. And it might explain why the sun is tipped slightly on its axis, astronomers say.

The new evidence leaves astronomer Scott Sheppard of the Carnegie Institution for Science in Washington, D.C., “probably 90% sure there’s a planet out there.” But others say the clues are sparse and unconvincing. “I give it about a 1% chance of turning out to be real,” says astronomer JJ Kavelaars, of the Dominion Astrophysical Observatory in Victoria, Canada.

The trail of Planet Nine began in 2003, when Brown spotted one of the most far-flung bodies in the solar system—an oddity known as Sedna whose orbit takes it out to more than 900 astronomical units (AU), the distance between Earth and sun. More important, Sedna doesn’t come any closer than 76 AU—more than twice as far out as Neptune. That puts it beyond the gravitational influence of that last ice giant. Something else had to pull it into its strange elongated orbit: perhaps a passing star, or the gravitational tides of the Milky Way.

We each want to be the one to find it.

Scott Sheppard, Carnegie Institution for Science

Or maybe a giant planet. When Brown and Batygin found five more TNOs curiously clustered in the sky, they realized with extensive modeling that a giant planet’s gravity would have flung any objects away from its path, leaving the orbits of the remaining objects huddled on the opposite side of the solar system.

Now, additional objects may be adding to the pattern. At the conference, Sheppard and his colleague Chad Trujillo of the Gemini Observatory in Hilo, Hawaii, presented the first two new entrants: 2014 SR349 and 2013 FT28. “The big question is do they make the planet case better or worse,” Sheppard says. “And they make it better.”

The first, 2014 SR349, falls right in line with the earlier six objects. The second, 2013 FT28, is on the opposite side of the sky—well within the proposed orbit of Planet Nine, where computer modeling suggests it would be safe from gravitational kicks. L91—the third new TNO and one of the most distant objects in the solar system—looks as if it might fit in with the antialigned group, but astrophysicist Michele Bannister of Queen’s University Belfast, who described the object at the meeting, cited modeling that suggests maybe it does not have anything to do with Planet Nine.

Kavelaars thinks Brown and Batygin’s clustering is unlikely to be real. To spot these objects at all astronomers have to look away from the bright Milky Way. It may be that the odd ones occupy similar parts of the sky because that is the easiest place to look. He expects that as additional distant bodies are discovered, their orbits will start to look more random. Kavelaars’s collaborator Cory Shankman of the University of Victoria has created models with the exact orbits of the six distant objects but found that a massive planet would not maintain the telltale clustering for long periods.

Hide and seek

In order to spot Planet Nine, astronomers need to look away from the bright Milky Way using powerful, wide-field telescopes. Since 2007, one team has ruled out parts of the sky using three different telescopes (colored dots), and is now homing in on a likely part of Planet Nine’s orbit.

(Diagram) G. Grullón/Science; (Data) Scott Sheppard

Not to be deterred, Planet Nine enthusiasts can now invoke two more lines of evidence. As they spin around the sun, the known planets, asteroids, and most TNOs stay in roughly the same plane, known as the ecliptic. But this year yielded some striking exceptions. One new object, known as 2016 NM56, has an orbit tilted so far out of the ecliptic that it essentially orbits backward. Another has a near-perpendicular orbit relative to the ecliptic. In a talk at the meeting,Batygin showed how Planet Nine might create these wonky trajectories. Through what’s known as the Kozai mechanism, a massive object can induce a gravitational ratcheting effect that slowly changes the inclination of smaller worlds and “leads their orbits to flip upside-down,” he says.

Batygin and Brown’s proposed orbit for Planet Nine is itself rather slanted, poking out about 30° relative to the ecliptic. Their graduate student, Elizabeth Bailey, showed how the tilted orbit could potentially explain a curious feature of the sun: Rather than being pointed perpendicular to the ecliptic, its north pole is off by about 6°. Researchers have tried to explain the anomaly, discovered in the 19th century, by invoking interactions between the early sun’s magnetic field and the disk of gas and dust that gave rise to the solar system. Bailey’s simulations showed instead that, over the course of the solar system’s history, the lopsided orbit of Planet Nine would have exerted a gravitational force on the sun that could have pushed it almost exactly 6° to one side. Astronomer Rodney Gomes of the National Observatory of Brazil in Rio de Janeiro and his collaborators independently came up with the same idea in July.

Few will believe in Planet Nine until it is seen directly. Planets spend the most time in the most distant part of their orbit, where they travel slowest. For Planet Nine, that would put it somewhere in the constellation Orion, which is just where astronomers are searching with the largest, widest angle telescopes they can find Batygin, Brown, Sheppard, and Trujillo are all using the 8-meter Subaru Telescope, because its Hyper Suprime-Cam can cover large parts of the sky with each shot. After their recent run in late September, which covered 10% of the most distant part of Planet Nine’s putative orbit,Batygin believes “there’s a 10% chance it’s in the bag.”

Sheppard’s team has also been awarded time on two telescopes in Chile, where Orion is also visible: the 4-meter Blanco Telescope and the 6.5-meter Magellan Telescopes. He says his team should scrutinize about a third of the outlying parts of Planet Nine’s proposed orbit this year and feels there’s a good chance they might see it by the end of 2017. Computational astrophysicist Peter Nugent of the University of California in Berkeley is taking a different tack: His graduate students will trawl through archival data from a survey begun in 2009 on the venerable 5.1-meter telescope on Mount Palomar in California to see whether it happened to spot the planet.

To the victors will go the glory of discovering a new planet. “We’re trying to keep it friendly,” Sheppard says, but adds that it’s definitely a race. Early on, the different groups talked about sharing their intended search fields, so as to avoid duplicative efforts on the sky—but now “we each want to be the one to find it,” Sheppard says. And should that happen, what then? “I’d basically retire at that point,” he says, laughing.

For more coverage on the solar system, visit our Planets topic page.