Earlier in this series, we spent some time looking at the major objects in the solar
system, with the sun and each of the eight planets getting their own tutorials.
We also briefly mentioned some of the smaller objects in the solar system, from asteroids
and comets to dwarf planets.
As we recall, dwarf planets are objects which don’t quite fulfill all of the criteria
required for planetary status.
Like planets, they are objects with enough mass so as to be crushed into a sphere by
their own gravity, and also like planets, they orbit the sun rather than another object.
But unlike planets, a dwarf planet does not dominate its region of space, meaning that
it has not cleared its orbit of debris.
The most famous dwarf planet, Pluto, discovered in 1930, was demoted from planet to dwarf
planet in 2006, when the criteria for planetary status were revised.
Made of ice and rock, this object is quite a bit smaller than the smallest planet, Mercury,
and even smaller than Earth’s moon, though it has at least five moons of its own.
Pluto sits in the Kuiper belt, and is therefore referred to as a trans-Neptunian object, as
are most of the other dwarf planets we are aware of.
Those include Haumea, discovered in 2004, Makemake, discovered in 2005, and Eris, also
discovered in 2005, and named after the Greek goddess of chaos, strife, and discord.
Eris is very far away from the sun, approximately three times as far as Pluto at its farthest.
It is a rocky body surrounded by a thick mantle of ice, with a surface that is brighter than
fresh snow, second in brightness only to Saturn’s moon Enceladus.
It is actually more massive than Pluto, making it the most massive object in the solar system
after the sun and the 8 planets.
It has a moon as well, Dysnomia, named after the daughter of Eris, which is about a quarter
of the size of Eris, and it was the orbit of Dysnomia that was used to calculate Eris’s
mass.
So given that Eris is so far away, how are we able to get such precise information about
it?
Well the size of the dwarf planet was determined in 2010 by observing a phenomenon called stellar
occultation.
What this means is that Eris was observed passing in front of a distant star, such that
the light from the star disappeared briefly.
This occultation event was observed by three different telescopes in Chile.
One witnessed no change in brightness for the star, while the other two saw the light
from the star disappear, though at different moments and for differing durations of time,
a little under thirty seconds and a little over a minute, respectively.
These latter two observations can be used to trace chords across the shadow of Eris.
So by combining this data and performing some simple geometry, researchers were able to
calculate that Eris has a radius of about 1,163 kilometers.
Observations of stellar occultation are difficult to pull off, especially for objects as far
away as Eris, but the success with Eris demonstrated a reliable way to measure the sizes of trans-Neptunian
objects.
In fact, precisely the same technique was used to measure the radius of Makemake just
the following year.
Again the observations occurred in Chile, and acted as further demonstration of stellar
occultation as an effective way to gather information about distant objects in our solar
system.
So that’s a bit of information on one the most interesting trans-Neptunian objects,
the dwarf planet Eris, and its moon Dysnomia.
Hopefully we will one day get a closer look at Eris and the other dwarf planets that sit
so far away from the sun, and perhaps we will even discover a few more such objects way
out there in the Kuiper belt
and beyond.