Eris, the goddess planet of discord

Eris is the name of the Greek goddess of chaos and discord, famous for being the one who caused the Trojan War by throwing the a golden apple with the inscription “To the fairest” during the wedding of Peleus and Thetis, where she had not been invited.  download (1)Eris is, also, the name of the Kuiper Belt object that more than any other has contributed to demote Pluto from its planet status.

Pluto’s icy twin was discovered quite recently – only in 2005, after many other similar dwarf planets in the same neighbourhood, like Sedna, Haumea and Makemake and Quaoar (for an interesting article about all these other planets’ names, read this Mike Brown’s post.)

Differently from the other TNOs companions, however, it has two important characteristics: one, it’s roughly the size of Pluto. Eris was initially estimated larger, then discovered slightly smaller: its diameter is 1,445 miles. There’s a reason for this mistake: no matter its considerable distance from the Sun, it’s a very bright object, reflecting all the light thrown at it. It’s also interesting to notice that the discussion about size between the two is not over yet. Pluto has an atmosphere, that can create measurement issues – another mystery New Horizons is going to solve.Trans-Neptunian

One of the things I enjoy about space exploration is the fact that many of the questions that motivate space science research are very easy to understand. But their answers can be surprisingly difficult to obtain. An apparently simple question about the solar system that has turned out to be hard to answer: What is the biggest thing in the Kuiper belt? We know for certain that Eris is more massive than Pluto, but that’s not how schoolkids usually order the sizes of things in solar system: what they care about is diameter. And we don’t presently know which one has a bigger diameter, Pluto or Eris. It’s going to be a while before we find out, if at all.” (Emily Lakdawalla, The Planetary Society, April, 30, 2014).

Two, Eris has a moon of its own, aptly called Dysmonia (in Greek mythology, the daughter of Eris whose name “lawlessness”).

As I have pointed out before, Eris was the main reasons for the IAU to arrive in 2006 at the decision to the famous Resolution 5A that defined what a planet was once for all – and considered Pluto no longer belonging to the cohort (incid260px-Eris_and_dysnomia2entally: both are, according to these guidelines.)

I’ve also mentioned Eris’ distance from the Sun before: it’s huge – 96.4 AUs at its aphelion, which translate in 4.5 billion miles from our star and about three times that of Pluto. However, since it has – like many of the TNOs – an eccentric orbit, it can come as “close” as 37.9 AU, therefore closer than Pluto (but still at a safe distance from Neptune). With an orbital period of 558 years, Eris and its moon are nonetheless, apart from some comets, the most far away known objects in the Solar System. 

(PHOTO CREDITS: NASA, Hubble Space Telescope)


  1. Aquileana

    Excellent post! … You reminded me of Eris and her role in the Judgement of Paris, which would lead to the Trojan War! .
    All my best wishes Aquileana 😀

    1. Stephen P. BIanchini (Post author)

      Thanks a lot, I love your posts about Greek mythology 🙂


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