Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. These are the planets most of us grew up with. Throw in Ceres and you've got every planetary-shaped object directly orbiting the sun inside Neptune's orbit that I know of. Of course as we all know Pluto was demoted about 10 years back to the status of dwarf planet. Part of the reason for this lies with another planet-like object out in the outer reaches of the solar system: Eris, the subject of this post.

Eris is, in everything but official designation, the most remote planet in the solar system (although technically a dwarf planet, this designation is totally arbitrary - I personally think it's a somewhat silly distinction used because people don't want to have to memorize 30 planets). It is the most massive inhabitant of the distant Kuiper Belt (essentially an icy asteroid belt far bigger than the asteroid belt we are used to), being even more massive than Pluto, although Eris is slightly smaller than Pluto in physical size. This makes it the largest dwarf planet in the solar system yet discovered.

Eris was originally announced as the 10th planet because it is essentially an even more remote Pluto, which resulted in the eventual dwarf planet debate.

Artist's rendition of Eris
Credit

Location and Orbit of Eris

Eris is currently the second most distant known object in the solar system, sitting over 96 AU away from the sun (the Earth lies around 1 AU from the sun). 96 AU is a big deal: That's over 14 billion kilometers! This is nearly three times farther away than Pluto. Eris' insane distance from us makes it likely that the only reason it was discovered at all is because of its large size and brightness.

Eris' orbit is unlike that of any of the planets. Rather than being approximately circular, Eris' orbit is highly eccentric (e = 0.44). At close approach, Eris comes to around 37 AU from the sun, similar to Pluto. But at Eris' aphelion, its furthest point along its orbit, the dwarf planet reaches the incredible distance of 97 AU. This means that Eris is currently near aphelion - although it takes so long to go around the sun that it will be near this point for a long time. A year on Eris would take 557 Earth years. As such, Eris experiences absurdly long "seasons" caused by the varying level of sunlight the world receives along different points in its orbit.

Eris' orbit is also massively inclined, meaning that it travels far away from the ecliptic, the plane in which most of the planets approximately reside.

To visualize how extreme Eris' orbit really is, I've attached an image of it compared to the other planets and Pluto.

The ridiculous orbit of Eris
Credit

Due to the vast distance between Earth and Eris, a mission here is not plausible without a way to shorten the transit time. This unfortunately makes a flyby mission the only mission even somewhat feasible, as an orbiter would have to carry an impossible amount of fuel to enter orbit around Eris and still arrive in less than a few centuries. New Horizons was forced to only flyby Pluto due to the distance - and Eris is 3 times farther away.

The surface of Eris

Like Pluto, Eris is enormous. There is more land on Eris than the entire country of Russia, the largest country on Earth. Eris has more land than the continent of Europe and almost as much as South America. As usual I'd like to point out that this is a real place: If you could get there, you could stand on the surface and walk around. It is as real as Rhode Island or Germany or Antarctica. Often we tend to think of other (dwarf) planets as abstractions, but I want to be very clear that they are very real places. As such, the surface likely has much to explore and discover.

Being both more massive and physically smaller than Pluto, you would weigh more on the surface of Eris than you would on Pluto. Eris is extremely bright for its size, indicating that the surface is very reflective and white-colored.

This in Enceladus, moon of Saturn. It has a similar brightness to Eris, so Eris likely looks something like this does color-wise.
Credit

The surface of Eris is likely entirely ice, similar to Pluto. In particular, methane (CH4) ice has been detected on the surface of the dwarf planet. The presence of this ice lines up with the incredibly cold temperatures on Eris: The surface is likely somewhere around -230 degrees Celsius. This is only about 40 degrees above absolute zero! Or, that temperature is about 150 degrees colder than the coldest ever recorded temperature on Earth. This place is not somewhere you want to be if you'd like to keep yourself at room temperature.

Eris likely has a significant atmosphere that is currently frozen to its surface due to its current huge distance from the sun. As Eris approaches its perihelion several centuries from now, it is possible that its atmosphere will reform as the surface temperature rises and incident sunlight increases. This would give Eris an atmosphere similar to the one Pluto currently has. Since Eris is currently near aphelion, it likely has little to no atmosphere, as the gas has frozen down onto the surface ice.

Dysonmia: Eris' Moon

But Eris is not alone out there. Astronomers have discovered a moon around this dwarf planet, called Dysnomia. Dysnomia is much smaller than Eris, and unlike Pluto's large moon Charon, is small enough that it directly orbits Eris. Dysnomia's size is contested, but it is likely several hundred kilometers across. Make no mistake, even though this is a "small" moon, it's not actually small on scales we are used to: Hundreds of kilometers across means that this "tiny" moon still has tons of surface area (land).

Dysnomia is likely much darker than Eris, making its surface red or brown rather than the bright surface of Eris. How dark the surface will determine whether the diameter estimate is high or low. This moon orbits Eris in a mostly circular orbit, and is so far the only moon discovered in this remote corner of the solar system.

High-Resolution Images of Eris

As you've seen, Eris is really not a very small place at all. There's several continents worth of space up there, covered in vast sheets of methane and nitrogen ice. But that does not mean that we have high-resolution images of Eris. Remember that the nice pictures you see of Pluto today did not exist until New Horizons flew past the dwarf planet in 2015 - prior to that, the best picture of Pluto was an incredibly blurry multi-colored smudge from the Hubble Space Telescope, in which nothing was really visible. So, without further pause, here is the best picture ever taken of Eris and its companion moon:

The best picture of Eris and Dysnomia to date. The little speck is Dysnomia.
Credit

Disappointing, isn't it? Despite Eris being thousands of kilometers across, around 2/3 the size of our Moon, the massive distance between us and Eris makes it essentially impossible to get a decent image from an Earth-based telescope. Unfortunately, in order to really see what lies out there, we will have to send a spacecraft, and that is no easy feat. I recently wrote about New Horizons, which took almost a decade and a lot of speed to briefly touch the Pluto system. Such a mission to Eris would have to travel three times farther, and endure an even colder environment to reach the distant world. A mission to Eris would truly be one of the greatest feats of our time if it was attempted. Unfortunately, no such mission is in the works, and even if it were, it would take something around 30 years from launch to arrival. Even worse, because said hypothetical mission would need to be a flyby mission, it would only see one side of Eris at best.

Needless to say, we will not see what this place really looks like for a very long time. Honestly, that makes me a little bit sad. Hopefully one day we can send something out there to see this weird place for ourselves.

---

I hope you were able to learn something about Eris and its companion moon from this. Unfortunately, there is less information on these objects than I would like due to their extreme distance.

By the way, don't think for a moment that Eris and Pluto are the only things out there beyond Neptune and the planets. In addition to countless discovered and undiscovered mega ice asteroids, numerous other dwarf planets (all somewhat smaller than Pluto/Eris) exist out there in the Kuiper Belt. Some of these include 2007OR10, which despite being way bigger than anything in the asteroid belt has no official name, and Haumea, an egg-shaped fast-spinning big dwarf planet. In my opinion the Kuiper Belt is the most fascinating place in the solar system, and it isn't even close to being fully discovered. New stuff is being discovered out there all the time.

Thanks for reading!

Credit

Sources for Additional Reading:
NatGeo article on Eris' Atmosphere
Eris Wikipedia Entry
Dysnomia Wikipedia Entry
Mike Brown's Dwarf Planet Classification (Dr. Brown is responsible for leading the team that discovered Eris)
Caltech Eris Page