No Planet Nine? Small-body Pile-up Could Explain Odd Orbits

New research shows that interactions between small objects beyond Neptune’s orbit —and not a hypothetical Planet Nine — could be the reason some far-flung solar system objects “detach” from their original orbits.

Astronomers have been struggling to explain the orbits of 30 or so bodies at the outer rims of the solar system, called “detached objects.” These worlds are smaller than Pluto and travel in elliptical trajectories around the Sun.

Sedna is one of the most well-known detached objects: a reddish world found in 2003, it’s one-third the size of the Moon and has an orbital period of 11,400 years — the longest of any object known in the solar system. At closest approach it passes 76 times farther away than the distance between the Sun and Earth. At its farthest, it goes more than 900 times that distance.

The orbits of Sedna and the other detached objects appear to be completely removed from Neptune’s gravitational pull. Yet their trajectories share similarities that seem to point to a common but unknown source of gravitational influence. For that reason astronomers have pointed to the influence of a yet-to-be-found ninth planet of the solar system, hiding far beyond Pluto’s orbit. Astronomers have proposed the existence of this alleged planet to explain not only the detachment of these objects’ orbits, but also other characteristics, such as their orbits’ tilt relative to the plane where most of the planets of the solar system reside.

But Planet Nine isn’t the only plausible source of disruption in our system’s outskirts. A group of researchers led by Ann-Marie Madigan and Jacob Fleisig (both at University of Colorado, Boulder) have found that the combined gravitational pull of the many smaller bodies beyond Neptune’s orbit (known as trans-Neptunian objects, or TNOs) could do the trick. Thanks to computer simulations, they have found that the combined gravity of the smaller TNOs could push the larger members of their family — such as Sedna — into detached orbits. The researchers presented their findings on June 6th at the 232nd meeting of the American Astronomical Society.

According to Fleisig, the key to this mechanism is the precession of the TNO orbits. If all the objects were the same size, their orbits would move at the same rate and stay stable. But the team’s simulations showed that the larger bodies’ orbits revolve around the Sun more slowly than their smaller counterparts. The motion is similar to the hands of a clock, where the minute hand catches up with the hour hand. When that happens, the larger body feels the gravity of its smaller counterparts piling up behind it. Their combined pull changes the largest object’s path, pushing it away from the Sun.

“For this mechanism to work you need a significant amount of objects out there,” Fleisig says.

That raises the question: Is there enough mass in the outer solar system to make this happen? To Fleisig and his team, the answer is yes. “If we look at the objects that have been detected so far, there’s just a handful of them,” Fleisig says. “We wouldn’t have found them in the time scales that humans have been looking at the sky if that was all there was out there.”

Hal Levison (Southwest Research Institute) says he likes this idea but will remain cautious until the group publishes their work in a peer-reviewed paper, which is currently in preparation. “This could be a really big deal,” Levison says. Nevertheless, he warns, there might be something in the simulations that is “not likely to happen in nature.”

One of the main proponents of the existence of “Planet Nine,” astronomer Mike Brown (Caltech), agrees that the new work puts forward a way to detach objects’ orbits. But he doesn’t think the work rules out the existence of “Planet Nine.” The detachment of the orbits was more a “side effect” of the existence of the rogue planet than the reason to look for it, he argues. “The mechanism discussed here doesn't actually create any of the main effects of Planet Nine, including lining up the orbits, tilting the orbital planes, and generating a population of high-inclination nearby objects,” Brown says. According to Brown, Planet Nine is still required to cause all of these major effects.

However, findings like this may put a bit more of pressure on the supporters of Planet Nine and those actively looking for it. “It’s become a moving target,” Levison says, “I’m very skeptical.” Then he adds, “but not to the point to say we shouldn’t look for it.”