This image of Pluto was taken after the New Horizons spacecraft flew past the dwarf planet on July 14, 2015, from a distance of 80,000 kilometers (50,000 miles). (Image credit: NASA)
Pluto is something of a loner. This dwarf planet is no longer considered a regular planet; its orbit does not lie in the same plane as the eight planets of the Solar System; its orbit is highly elliptical and highly inclined.
In fact, its orbit is much more similar to that of its neighbors in the Kuiper Belt—the doughnut-shaped region beyond Neptune that also contains other dwarf planets like Makemake and Eris, as well as millions of icy objects. But even compared to other Kuiper Belt objects, Pluto's orbit is quite unusual.
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First, let's compare Pluto with Earth and the other planets in the Solar System. The dwarf planet's orbital eccentricity (its deviation from a perfect circle) is 0.25. For comparison, Earth's orbital eccentricity is 0.0167, making it nearly circular. The eccentricities of Saturn and Mars are 0.054 and 0.093, respectively.
Pluto's orbit is tilted 17.4 degrees, compared to Earth's 1.5 degrees and Mercury's about 2 degrees.
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According to Renu Malhotra, a planetary scientist at the University of Arizona who has extensively studied Pluto's orbit, Pluto's unusual eccentricity and tilt are likely due to its interactions with neighboring Neptune and other giant planets.
Scientists believe that Neptune's migration due to gravitational interactions with its giant neighbors plays a role in explaining Pluto's orbit. At some point, the planets migrated, and Pluto “ended up in this orbit because Neptune's orbit shifted outward… and pulled Pluto into this resonance,” which is when the gravity of orbiting bodies periodically influences each other as they pass close to each other, Malhotra told Live Science.
“Imagine you have a flat surface,” Malhotra said. “If you randomly throw rocks at it, they'll end up anywhere. But if the surface has depressions, the rocks will end up in them.” In the case of planets, Neptune's migration has created a gravitational well for resonant objects like Pluto.
Compared to the eight planets in the solar system, Pluto's strange orbit simply defies description.
Neptune controls Pluto, despite its odd orbit. For 20 of its 248-year orbit, Pluto is inside Neptune's orbit. The two planets are in a 3:2 orbital resonance, meaning Pluto completes two orbits for every three Neptune completes.
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“This configuration is very stable and protects Pluto from further jolts,” said Will Grundy, an astronomer at the Lowell Observatory in Arizona and a co-investigator on NASA's New Horizons mission to Pluto.
There's another strange 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. This is “really weird,” Malhotra said. Typically, planets and other dwarf planets dip above and below the plane over time, something Pluto has never done, according to simulations. This is caused by a dance between Pluto and the planets Jupiter and Uranus, which prevents Pluto from descending into chaos.
“We used to think it was just Neptune and Pluto, but it turns out that other planets play a very important role in this other characteristic of Pluto,” Malhotra said.
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However, Pluto isn't the only one with such a strange orbit. For example, the dwarf planet Eris has an eccentricity of 0.45 and an inclination of about 43 degrees. “That's a much more extreme orbit,” Grandi told Live Science.
We still have much to learn about how and how quickly the planets migrated. And Pluto's cosmic environment, the Kuiper Belt, likely holds even more mysteries.
“There are more landscapes on objects [100 kilometers, or about 60 miles, or larger] in the Kuiper Belt than on solid surfaces in the rest of the solar system combined,” Grandi said. “It's a really exciting area for exploration.”
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Sarah Hashemi, Live Science contributor
Sarah Hashemi is a journalist and fact-checker covering environmental justice, climate, and the intersection of science and society. Her work has appeared in Sierra, Smithsonian Magazine, Maisonneuve, and other publications. She holds a master's degree in science journalism from New York University.
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