Similar in size to Pluto, Eris has been an enigma since its discovery in 2005, in the far reaches of the Solar System. While the first, its better-known cosmic “cousin”, was explored by NASA’s New Horizons spacecraft during a flyby in 2015, the second — about 40% further from the Sun — has never been visited.
But scientists are gaining more information about this dwarf planet and its differences with Pluto. And this is thanks to research, carried out based on the orbital relationship with the moon Dysnomia, which reveals details about its internal structure and composition.
According to researchers’ announcement last Wednesday (15), Éris appears to have a rocky interior below a layer of ice. Pluto also has an icy surface with rocks below, but it has a higher ice content and is believed to house an underground ocean.
“We already knew that it is richer in rocks than Pluto, but what we didn’t know was whether it had separated the rock from the ice,” said Francis Nimmo, a planetary scientist at the University of California, Santa Cruz and lead author of the study published in the journal Science Advances.
“This means that it became hot enough at some point in its history to melt, causing all the rock to sink to the center. The ice is not motionless, but is experiencing slow, choppy movement, driven by the remaining heat from the interior . There probably isn’t an ocean in it,” Nimmo added.
The dwarf planet has a diameter of about 2,326 km, slightly smaller than Pluto’s 2,370 km. In comparison, the diameter of Earth’s Moon is about 3,475 km. Due to its greater concentration of rock, denser than ice, Eris is about 25% more massive than Pluto.
“As I like to think of it, take Pluto and add every asteroid in the asteroid belt, and you have Eris. Pluto is inflated by ice, while Eris is mostly rocky with some ice on the outside,” said the Caltech astronomer ( California Institute of Technology) and study co-author Mike Brown, 1 of the 3 scientists who discovered it.
Named after the ancient Greek goddess of discord, Eris orbits an average of 68 times further from the Sun than Earth, taking 557 years to complete one orbit. Pluto orbits an average of 39 times the distance from Earth to the Sun.
Dysnomia, named after the mythological daughter of Eris, is approximately 700 km in diameter and is composed mainly of ice.
“Just like the Earth-Moon system, the tides on Eris slowly push Dysnomia away and slow the dwarf planet’s rotation. This process has been completed: Eris and Dysnomia always present the same face to each other,” said Nimmo.
Pluto has this same arrangement with its moon Charon, while the Earth-Moon system is different.
“The Moon always presents the same face to the Earth, but the Earth does not return the favor,” Nimmo said.
On the dwarf planet, it would appear like midday twilight due to its great distance from the Sun, which would be no more than an especially bright star in the sky. “You could block the Sun with the head of a pin,” Brown said.
Eris and Pluto lie beyond Neptune, the outermost of the eight planets in our Solar System. The International Astronomical Union, which establishes definitions for planetary science, recognizes five dwarf planets — the others are Ceres, Haumea and Makemake — although dozens of others may qualify.
The new discoveries fill in some gaps about Eris, the most massive of them.
“This helps put it in the context of all the information we’ve learned about Pluto, with its great 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 based on what we know about Pluto,” Brown said.