Scientists discern internal structure of mysterious dwarf planet Eris

An artist's concept of the dwarf planet Eris and its moon Dysnomia is seen in this undated illustration released by NASA. The sun is the small star in the distance. NASA/JPL-Caltech/Handout via REUTERS

Similar in size to its more well-known cosmic cousin Pluto, the dwarf planet Eris was found hiding in the farthest regions of the solar system in 2005. Despite this discovery, Eris has remained a mystery. Although NASA’s New Horizons spacecraft studied Pluto during a flyby in 2015, Eris, located almost 40% farther from the sun, has never been visited.

But thanks to research that reveals facts about the composition and interior structure of this cold, isolated world based on its orbital interaction with its moon, Dysnomia, scientists are beginning to grasp Eris and its distinctions with Pluto in greater detail.

The scientists said on Wednesday that Eris seemed to have a rocky interior hidden beneath an ice cover. Although Pluto contains rock beneath its icy surface, it has a higher ice concentration and is believed to contain an underground liquid ocean.

Lead author of the study published in the journal Science Advances, Francis Nimmo, is a planetary scientist at the University of California Santa Cruz. “We already knew that Eris is more rock-rich than Pluto, but what we didn’t know was whether Eris had separated the rock from the ice,” Nimmo said.

This indicates that at some point in the past, Eris warmed up to the point where all the rocks sank to the center. The residual heat from the inside is causing the ice to swirl rather than remain still slowly. “It is doubtful that Eris contains a liquid ocean,” Nimmo continued.

Pluto’s diameter is 1,473 miles (2,370 km), while Eris’s is approximately 1,445 miles (2,326 km). In contrast, the moon’s diameter on Earth is approximately 2,160 miles (3,475 km). Eris is almost 25% heavier than Pluto due to a higher percentage of rock, which is denser than ice.

“Eris is what you get when you take Pluto and add all of the asteroid belt’s members, as I like to conceive of it. “One of the three scientists who discovered Eris, Mike Brown, a Caltech astronomer and research co-author, said that Pluto is mostly ice with a small amount of rock on its outer side.

Eris, named after the ancient Greek goddess of discord, takes 557 years to complete an orbit, averaging roughly 68 times farther from the sun than Earth. Pluto’s typical orbital radius around the sun is approximately 39 times Earth’s.

Named after Eris’s legendary daughter, Dysnomia is primarily made of ice and has a circumference of around 440 miles (700 km).

“Tides on Eris gradually push Dysnomia away and slow down Eris’s rotation, just like in the Earth-moon system. This process is complete: Eris and dysnomia consistently exhibit identical facial features,” Nimmo stated.

While the Earth-moon system differs, Pluto and its moon, Charon, have the same layout. “The moon always presents the same face to the Earth, but the Earth does not return the favor,” Nimmo stated.

Because of Eris’s extreme distance from the sun, which would seem like nothing more than an exceptionally bright star in the sky, it would appear at midday when standing on the planet. “On Eris, you could block out the sun with the head of a pin,” Brown stated.

Beyond Neptune, the furthest planet among the eight planets in our solar system, are Pluto and Eris. In addition to Eris and Pluto, the International Astronomical Union, which establishes standards for planetary science, acknowledges five dwarf planets: Ceres, Haumea, and Makemake. However, hundreds more may be eligible. The largest of them is called Eris.

The new information on Eris fills in some gaps. “It helps to put Eris in the context of all of the information that we’ve learned about Pluto, with its big mountains and giant impact basin, and forces us to remember: each of the largest dwarf planets is unique, and we should be cautious about inferring too much from what we know about Pluto,” Brown stated.


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