September 19, 2018

The Discovery of Eris & its Implications

As 2010 draws to a close, a quiet discovery was made recently about a fascinating solar system object. January 5th, 2011 will mark the six year anniversary of the discovery of 2003 UB313, which became provisionally known as the planet Xena and then later demoted to the dwarf planet Eris.

This sparked the whole controversy over what is and isn’t a planet, as Eris appeared to be larger and more massive than the then accepted Pluto. Of course, ever “shrinking” Pluto had been in trouble for about a decade prior, as astronomers had called for its lynching over the years… should the full fledged status of ‘planet’ be a select club, or should we accept a brave new solar system teeming with what could prove to be hundreds of them? Angry text messages and letters from disgruntled fifth graders ensued. Apparently, folks like simplicity, and promptly filled the inbox of astronomers who seemed intent on turning the solar system upside down. Matters weren’t helped when scientists discovered that Eris had a tiny moon, initially dubbed Gabrielle and later Dysnomia. An orbiting object gave us the ability to calculate the mass of Eris, again a win for planet-hood.

Enter the International Astronomical Union. Their ruling adopted in August 2006 left one caveat; a planet must clear its orbit. This somewhat cryptic invocation left Pluto and Eris high and dry, with their eccentric orbits transecting the Kuiper belt. The term Plutoid entered the astronomical lexicon, and the word “Pluto’d” became the word of the year, meaning an undeserved demotion. No one was happy, as elementary teachers scrambled for a new mnemonic for a confusing solar system.

Fast-forward to November, 2010. A scattering of telescopes lay posed to catch a difficult to observe event; the occultation of a distant background star by the faint (+18 magnitude) Eris. One of the three telescopes was the TRAPPIST instrument based at the La Silla Observatory in Chile. They successfully recorded a 27 second ‘wink’ as the tiny world passed in front of the distant star.

Enough  measurements were recorded to give a startling revelation; Eris is almost certainly smaller than Pluto, at a radius of about 1,170 km. This would also mean that the worldlet is more dense, and at a distance of three times Pluto (97A.U.) would have a much higher albedo or brightness of 86%, or similar to that of freshly fallen snow…

So, what does this portend for these distant worlds? Should Eris be in the planetary club, but Pluto out? Should the IAUs original decision stand, or should we eject the lot of them to the Oort cloud? Whatever the case, one thing is true; it’s a strange old universe out there. I highly invite you to follow the controversy on Mike Brown’s (he of the Eris discovery) blog as he delves into some unique facets of this evolving tale. Clearly, the story is not over, as we find more and more worlds that blur the line on both large and small ends of the spectrum. This is science, real and organic, unfolding right in our very own time. A true definition of what is and isn’t a planet will be laid down in our lifetimes, one that is scientific and not purely cultural. The New Horizons spacecraft is now over half way to its 2015 rendezvous with Pluto and Charon, and it’s our suspicion that the controversy will again rear its head in that not so far off year. Unfortunately, reconnaissance of distant Eris will have to wait, as it is in a totally different direction than any currently outward bound spacecraft… and the outer solar system is a simply huge place!

Ultimately, the most satisfying definition may be a simple one. In Star Trek, a quick visual scan always suffices to properly define a new world ahead. Whatever classification scheme we settle on, we need a way to look over an object and state “Captain, we’ve entered the orbit of a class M planet…”

Incidentally, you can see Eris for yourself… at 18th magnitude, the world is in range of large amateur instruments and pristine dark skies. It passed aphelion in 1977, and is current inbound on its 557 year orbit which has a huge eccentricity of 0.44. Let us know what you see!


  1. Griiker says:

    Pluto is a planet, cause millions of fifth graders can’t be wrong. Besides, somewhere in the byzantine laws of nature, there must be a grandfather clause or two.

  2. David Dickinson says:

    The ultimate irony would be if New Horizons finds a big smiley face on Pluto in 2015.

  3. Both Pluto and Eris are planets because in spite of the controversial ruling by four percent of the IAU, most of whom are not planetary scientists, dwarf planets are planets too. Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, coined the term in 1991 to indicate a third class of planets in addition to terrestrials and jovians–objects large enough to be rounded by their own gravity, a state known as hydrostatic equilibrium, but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all. Significantly, over 300 professional astronomers signed a formal petition rejecting the IAU’s 2006 resolution that demoted Pluto. Since this is an ongoing debate, you should invite people not just to Brown’s blog, which represents one side of the controversy, but to blogs representing both sides. I therefore invite everyone to regularly follow my Pluto Blog at as I discuss this evolving tale from the other side. I also encourage everyone interested to read two books: “The Case for Pluto” by Alan Boyle and “Is Pluto A Planet?” by Dr. David Weintraub.

  4. David Dickinson says:

    Thanks for the reply, Laurel; anyone interested in Dr. Weintraub’s book can also check out our review from a few years back:


  1. [...] a planet controversy, and was recently observed to be slightly smaller than Pluto during a 2010 stellar occultation. Could the remote outer solar system get any [...]

Speak Your Mind