The search for planets beyond Pluto

Not many years ago, the idea our Solar System might contain undiscovered planets beyond Pluto would have been considered science fiction. But now, with NASA’s New Horizons spacecraft three months away from giving us our first close-up view of Pluto, astronomers are beginning to wonder. The more we study the dwarf planet and its surrounds, the less lonely it appears. Are there as-yet undiscovered worlds on the far edge of the Solar System?

Some astronomers are beginning to believe there could be hundreds, or even a thousand distant bodies lurking out there – not only undiscovered Plutos, but perhaps even larger bodies, so distant our best telescopes have yet to spot them.

We already know Pluto isn’t entirely alone. In the mid-2000s four more distant worlds were discovered. One, Eris, is within a few kilometres of Pluto’s size. Three others – Makemake, Haumea and Sedna – are 40% to 60% the size of Pluto. All are in a distant region of the Solar System known as the Kuiper Belt.

They are also why Pluto was demoted from planet to dwarf planet in 2006. Part of the criteria for being a planet is that an object must sweep its orbit clear of other large objects. But the elliptical orbits of those distant worlds skim Pluto’s orbit.

Pluto itself provides the biggest clue that other larger bodies are out there.

In the meantime, researchers including Alan Stern, a planetary scientist from the Southwest Research Institute in Boulder, Colorado and principal investigator for the New Horizons mission, argue that more large objects on the edge of the Solar System are waiting to be found.

These far distant objects are only visible by reflected sunlight. And in the outer reaches of the Solar System the light is thousands of times dimmer than it is on Earth. Stern argues that Pluto itself provides the biggest clue that other larger bodies are out there.

Since 1978 it’s been known that Pluto has a large moon called Charon, which is roughly the size of Makemake, Haumea and Sedna. In 2005 astrophysicist Robin Canup, also at the Boulder institute, published a study in Science that used computer simulations to show that early in Solar System history, Charon had been an independent world. She calculated that it was captured by Pluto when the two collided. Although the collision was not violent enough to shatter either one, it slowed Charon so much that it was unable to escape Pluto’s gravitational grasp. Canup’s model remains the prevailing explanation for why Pluto has such a large moon.

The outer Solar System is enormous. If Pluto and Charon were the only large objects out there, the chances of them colliding would have been minute. Stern has calculated it would take 10,000 times the age of the entire Universe for any collision between a lonely Pluto and Charon to become likely. But if you had “1,000 Pluto-sized objects” in the region, then the meeting becomes more probable said Stern at a meeting last year of the American Geophysical Union in San Francisco.

“It’s mind-boggling to think of the Solar System that way, but that is what the data is telling us,” he said. “We might even find Mars and Earth-sized things.”

Other scientists agree. One is Carlos de la Fuente Marcos, an astrophysicist at the University of Madrid, Spain. In a pair of 2014 studies in the Monthly Notices of the Royal Astronomy Society, his team looked at the orbits of the 13 “extreme trans-Neptunian objects” (planetoids whose orbits carry them at least five times farther out than Neptune, or three times farther than Pluto’s greatest distance from the Sun).

These bodies probably aren’t large enough to count among Stern’s missing dwarf planets, although they are so dim and distant it’s hard to be sure of their size. They are the most remote objects yet found in the Solar System. Similarities in their orbits suggest they may have been shepherded into their current paths by gravitational interactions with much larger bodies.

According to de la Fuente Marcos’s computer models these extreme trans-Neptunian orbits may be revealing the presence of not one, but two objects that are more distant than Pluto. He says not yet published calculations suggest these planets might be “super Earths”, two to 15 times more massive than our own planet.

He’s careful to point out that his team is not claiming to have discovered new planets. “Our studies are theoretical: statistics and simulations,” he says. But when it comes to understanding the orbits of these far-distant bodies, he says, “this is the most simple and straightforward explanation”.