On the edge of the solar system: The dwarf planet Eris has had a hot past
The dwarf planet Eris contains much more rock than icy Pluto, as can be inferred from measurements of its rotation behavior. Was it once a hot core?
The dwarf planet Eris is almost as large as Pluto, but has a much greater mass.
Eris, the most massive and second-largest known dwarf planet in our solar system after Pluto, may have undergone a hot phase shortly after its formation, during which it largely melted. Francis Nimmo and Michael Brown of the University of California, Santa Cruz and the California Institute of Technology derived this insight from the dwarf planet’s rotation behavior. They present their results In the journal “Scientific Progress”..
Because of Eris’s great distance from the Sun – on average, it is more than twice the distance from the Sun to the outer planet Neptune – even the most powerful telescopes do not show any details about this celestial body, so little is known about it so far. It takes the dwarf planet about 557 years to orbit the Sun, more than twice as long as the dwarf planet Pluto, which was discovered in 1930. Eris was discovered in 2005 and, among other celestial bodies, was the reason behind Pluto’s downgrading from planet status. IX to a dwarf planet.
However, Nimmo and Brown now add some aspects to the current state of knowledge. Eris is 2,326 kilometers in diameter, making it only a few kilometers smaller than Pluto, which is 2,372 kilometers across. However, its mass is about a third higher than that of Pluto. By examining the light curve, i.e. the change in brightness over time, a rotation period of 378.9 hours or 15.8 days can be derived. In addition, Eris orbits the approximately 700 km wide moon Dysnomia, which was discovered in 2005. It turns out that Eris and Dysnomia are in a dumbbell rotation towards each other, that is, they always face each other on the same side, as is the case with Pluto and its moon Sharon. On average, Eris and Dysnomia are separated by 37,300 kilometers, which is about one-tenth the distance between Earth and the Moon. They take as long to rotate on their axis as they do to rotate around each other.
Dwarf planet Eris with moon Dysnomia in the foreground | Dysnomia orbits the dwarf planet Eris (bright disk in the background) at a distance of about 37,000 km. Both celestial bodies always face the same side and require 15.8 days to rotate. (clarification)
However, it is surprising that the Iris system exhibits dual rotation, since the dysplasia has only about one percent of the mass of its parent body and can therefore exert only small tidal forces on the Iris. Nimmo and Brown conclude from their simulations that Eris probably melted substantially shortly after its formation 4.5 billion years ago due to the decay of short-lived radioactive elements such as aluminum-26. The rocky component, with a much higher density than water, sank toward the center and formed a rocky core there. He was surrounded by a thick layer of water with an ice crust, and the water jacket was now frozen.
Water has a low viscosity, which makes it easier for convection currents to transfer heat outward. This also allows the heat generated by mutual tidal friction to be dissipated more easily, so that both celestial bodies gradually rotate more and more slowly until in the end they are always facing the same side. In addition, the conservation of angular momentum ensures that the sleep bug moves further and further away from IRIS due to tidal forces. Since the system finally achieved dumbbell rotation more than four billion years ago, it has been in a state of dynamic—and stable—equilibrium for the next several billion years.
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