Collective gravity, not Planet Nine, may explain the orbits of ‘detached objects’

Bumper car-like interactions at the edges of our solar system—and not a mysterious ninth planet—may explain the the dynamics of strange bodies called “detached objects,” according to a new study.

CU Boulder Assistant Professor Ann-Marie Madigan and a team of researchers have offered up a new theory for the existence of planetary oddities like Sedna—an icy minor planet that circles the sun at a distance of nearly 8 billion miles. Scientists have struggled to explain why Sedna and a handful of other bodies at that distance look separated from the rest of the solar system.

One theory suggests that an as-of-yet-unseen ninth planet lurking beyond Neptune may have kicked up the orbits of these detached objects.

But Madigan and her colleagues calculated that the orbits of Sedna and its ilk may result from these bodies jostling against each other and space debris in the outer solar system.

“There are so many of these bodies out there. What does their collective gravity do?” said Madigan of the Department of Astrophysical and Planetary Sciences (APS) and JILA. “We can solve a lot of these problems by just taking into account that question.”

The researchers presented their findings today at a press briefing at the 232nd meeting of the American Astronomical Society, which runs from June 3-7 in Denver, Colorado.

The project takes a deep dive into the outer solar system, a busy place filled with minor planets, such as Pluto, icy moons and other space debris.

It’s also an unusual place, gravitationally speaking. “Once you get further away from Neptune, things don’t make any sense, which is really exciting,” Madigan said.

Among the things that don’t make sense: Sedna. This minor planet takes more than 11,000 years to circumnavigate Earth’s sun and is a little smaller than Pluto. Unlike that original ninth planet, Sedna and other detached objects complete humongous, circle-shaped orbits that bring them nowhere close to big planets like Jupiter or Neptune. How they got out there on their own remains an ongoing mystery.

Enter the hypothetical Planet Nine. Astronomers have been searching for such a planet, which would be about 10 times the size of Earth, for about two years but have yet to spot it with telescopes.

Madigan’s team didn’t originally intend to look for another explanation for those orbits. Instead, Jacob Fleisig, an undergraduate studying astrophysics at CU Boulder, was developing computer simulations to explore the dynamics of the detached objects.

“He came into my office one day and says, ‘I’m seeing some really cool stuff here,’” Madigan said.

Fleisig had calculated that the orbits of icy objects beyond Neptune circle the sun like the hands of a clock. Some of those orbits, such as those belonging to asteroids, move like the minute hand, or relatively fast and in tandem. Others, the orbits of bigger objects like Sedna, move more slowly. They’re the hour hand. Eventually, those hands meet.

“You see a pileup of the orbits of smaller objects to one side of the sun,” said Fleisig, who is the lead author of the new research. “These orbits crash into the bigger body, and what happens is those interactions will change its orbit from an oval shape to a more circular shape.”

In other words, Sedna’s orbit goes from normal to detached, entirely because of those small-scale interactions. The team’s findings also fall in line with recent observations. Research from 2012 noted that the bigger a detached object gets, the farther away its orbit becomes from the sun—exactly what Fleisig’s calculations showed. Alexander Zderic, a graduate student in APS at CU Boulder, also co-authored the new research.

The findings may provide clues around another phenomenon: the extinction of the dinosaurs. As space debris interacts in the outer solar system, the orbits of these objects tighten and widen in a repeating cycle. This cycle could wind up shooting comets toward the inner solar system—including in the direction of Earth—on a predictable timescale.

“While we’re not able to say that this pattern killed the dinosaurs,” Fleisig said, “it’s tantalizing.”

Madigan added that the orbit of Sedna is one more example of just how interesting the outer solar system has become.

“The picture we draw of the outer solar system in textbooks may have to change,” she said. “There’s a lot more stuff out there than we once thought, which is really cool.”

This research was supported by NASA Solar System Workings and the Rocky Mountain Advanced Computing Consortium Summit Supercomputer.

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