Dwarf planet Eris is squishier than expected
 |
A depiction of Eris’s rocky core and ice shell on an
illustrated background from NASA. Credit: University of
California - Santa Cruz.
University of California, Santa Cruz Professor of Planetary Sciences Francis Nimmo recently co-authored a Science Advances paper about the internal structure of the dwarf planet Eris.
Eris
is about the size of Pluto but around 50% farther from the sun. The discovery
of Eris in the Kuiper Belt
beyond Neptune in 2005 prompted the debate that ultimately reclassified Pluto
as a dwarf planet.
It
was an interest in Pluto that drew UC Santa Cruz researcher Francis Nimmo to
study Eris.
Nimmo
was visiting Michael Brown — one of the discoverers of Eris — at the California
Institute of Technology about six months ago and realized some of Brown’s new,
unpublished data could help reveal information about the properties of Eris.
The
two worked on models for the next several months and published their results in
a Science Advances paper. Two main pieces of information led to their results.
The
first important clue is that Eris and its moon, Dysnomia, always face
the same way toward each other.
“That
happens because the big planet gets spun down by the tides that the little moon
raises on it,” explained Nimmo. “The bigger the moon is, the faster the planet
spins down.”
Researchers
can use the spin and orbital characteristics of planets and their moons to
infer properties of their internal structures. But until recently, scientists
did not have an estimate for the size of Dysnomia.
.jpg)
Artist's conception of Dysnomia's dark surface, with
dwarf planet Eris in the background. Source: ESO/L. Calçada and
Nick Risinger.
Brown’s
then-unpublished data changed that by revealing that Eris’s moon must be below
a certain mass. This upper limit on mass provided the second crucial piece of
information.
“And
so as soon as you know that, then you can actually start to do real
calculations,” said Nimmo.
The
main, unexpected result of Nimmo and Brown’s model is that Eris is surprisingly
dissipative, or
“squishy”.
The
co-authors determined that Eris has a rocky core surrounded by a layer of ice.
This outer shell of ice is likely convecting, unlike the conducting shell of
Pluto.
“The
rock contains radioactive elements, and those produce heat. And then that heat
has to get out somehow,” explained Nimmo. “So as the heat escapes, it drives
this slow churning in the ice.”
Eris
therefore behaves less like a rigid object and “more like a soft cheese or something
like that. It has a tendency to flow a bit,” said Nimmo.
Future
Data
The
upper limit on the mass of Dysnomia came from measurements made by the Atacama
Large Millimeter Array (ALMA) radio telescope. Nimmo hopes that soon, more
exact measurements of the mass will help further refine the model.
“If
Dysnomia is smaller than that, then Eris is even more squishy,” he said.
Additional
data about the shape of Eris will help verify the model Nimmo and Brown created.
“We
make the point that Eris should be pretty smooth because if there’s any surface
topography, the ice is going to flow and that topography will go away,” said
Nimmo. “So it would be nice to get some measurements of what shape Eris is
because if it’s very irregular, that would not agree with our model.”
Eris
is so distant from Earth that it shows up as a single pixel, so in order to
reconstruct its shape, scientists will need to watch the planet pass in front
of stars.
“The
star blinks out and then the star comes back, and that tells you how wide Eris
is at that point. And if you do that with a whole bunch of stars, then you can
actually reconstruct shape,” explained Nimmo. “I’m hoping people are actually
doing it, I just don’t know whether they are.”
