Why does Pluto have such a weird orbit?

Pluto is a bit of a loner. The dwarf planet is no longer considered a regular planet; it does not orbit on the same plane as the solar system’s eight planets; and its orbit is both highly elliptical and extremely tilted.

In fact, its orbit is much more similar to that of its neighbors in the Kuiper Belt, a doughnut-shaped region beyond Neptune’s orbit that’s also home to other dwarf planets like Makemake and Eris, as well as millions of icy objects. But even compared with the rest of the objects in the Kuiper Belt, Pluto’s orbit is peculiar.

So, how did Pluto’s orbit end up this way?

First, let’s compare Pluto to Earth and other planets in the solar system. The dwarf planet’s orbit has an eccentricity — how much it deviates from a perfect circle — of 0.25. For comparison, Earth’s orbit has an eccentricity of 0.0167, meaning it’s nearly circular. Saturn and Mars have eccentricities of 0.054 and 0.093, respectively.

Pluto’s orbit is tilted 17.4 degrees, compared with Earth’s 1.5 degrees and Mercury’s roughly 2 degrees.

Pluto’s unusual eccentricity and tilt is likely due to its interactions with neighboring Neptune and other giant planets, said Renu Malhotra, a planetary scientist at the University of Arizona who has studied Pluto’s orbit extensively.

Related: Why aren’t all orbits circular?

Scientists believe Neptune’s migration due to gravitational interactions with its giant neighbors plays a part in explaining Pluto’s orbit. At some point, the planets migrated, and Pluto “was in this kind of orbit because Neptune’s orbit had migrated outward … and swept Pluto up into this resonance,” or when the gravities of orbiting bodies periodically influence each other as they pass nearby, Malhotra told Live Science.

Say you have a flat surface, Malhotra said. If you randomly toss rocks on it, they’ll end up anywhere. But if you have dips in the surface, the rocks will end up in those dips. In the case of the planets, Neptune’s migration created a gravitational well for resonant objects like Pluto.

Neptune keeps Pluto in check despite its strange orbit. For 20 years of 248-year orbit, Pluto is inside Neptune’s orbit. The two planets are in a 3:2 orbital resonance, which means that Pluto completes two orbits every time Neptune completes three.

“That configuration is very stable and it protects Pluto from getting jostled further,” said Will Grundy, an astronomer at the Lowell Observatory in Arizona and co-investigator on NASA’s New Horizons mission to Pluto.

There’s another odd aspect of Pluto’s orbit. When the dwarf planet reaches perihelion — its closest point to the sun — it’s always above the plane of the planets. That’s “really peculiar,” Malhotra said. Generally, planets and other dwarf planets dip above and below the plane over time — something that simulations suggest Pluto has never done. This is caused by a dance between Pluto and the planets Jupiter and Uranus that works to keep Pluto from launching into chaos.

“We used to think it was just the fact of Neptune and Pluto, but it turns out that the other planets are also very important in this other characteristic of Pluto,” Malhotra said.

Pluto isn’t alone in having a strange orbit, though. For instance, the dwarf planet Eris has an eccentricity of 0.45 and an inclination of about 43 degrees. “It’s a much more extreme orbit,” Grundy told Live Science.

There’s still a lot to learn about how the planets migrated and by how much. And Pluto’s cosmic neighborhood, the Kuiper Belt, is certain to offer even more mysteries.

“There’s more landscape on objects [100 kilometers, or about 60 miles, and bigger] in the Kuiper Belt than there is in the entire rest of the solar system combined on solid surfaces,” Grundy said. “It really is a ripe area for exploration.”

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