In 2003 astronomers discovered Eris, an object seemingly bigger than Pluto, in the frigid Trans-Neptunian wastes of the Solar System. Rather than becoming the Tenth Planet, Eris led directly to Pluto’s demotion from planetary status! But is this saga over? Martina Redpath has the details.

Image of Eris

Eris with its small moon Dysomnia are portrayed in this artist's impression which also includes Arcturus, Antares and the Zodiacal Light around the Sun. (Image Credit: NASA, ESA, and M. Brown (Caltech))

I’m sure most of you remember that fateful day back in 2006, when life as we know it changed. Some of you may remember what exactly you were doing when you heard the news. As a result textbooks were rewritten; no longer did energetic mothers serve nine pizzas, instead simply noodles or nachos. On August 24 2006, it happened: Pluto the ninth planet in our Solar System, was relegated, demoted, downgraded, basically was no longer classed as a planet.

After much discussion the International Astronomical Union (IAU) decided to reclassify Pluto, as it no longer fulfilled the criteria to be a regular planet. Pluto became a dwarf planet, not the only dwarf planet (just to add insult to injury) potentially one of hundreds.

Let’s refresh our memories, ever since Clyde Tombaugh (1906-97) discovered Pluto in 1930 it had steadily shrunk in size. Once thought to about eight times as massive as Earth, by the final decade of the last century, Pluto was known to be smaller than the Moon.   In 1992, a large number of icy lumps were discovered beyond the orbit of Neptune, ‘Trans-Neptunian Objects’, which have become known as the Kuiper Belt Objects (KBOs.) The Kuiper Belt is similar to the Asteroid Belt located between Mars and Jupiter, however it is much more massive, consisting of many small bodies that were left over from the formation of the Solar System. Discussion and debate regarding Pluto’s fate split opinion for years, after all, its orbit was different to the other planets being tilted by nearly 20⁰. The final straw in the decision to demote Pluto occurred in 2005 when Mike Brown, an astronomer from the California Institute of Technology and his team announced they discovered an object in 2003 beyond Pluto that was larger. The object (2003UB313) was suitably named after the Greek goddess of strife and discord, Eris.

However this is old news and no big deal, the world had started to get used to the idea of only eight planets and some dwarf planets. So imagine the surprise perhaps even excitement, when recently the startling news was released that actually Pluto may be bigger than Eris after all!

As Eris was passing a distant star, the length of the occultation, that is the length of time Eris blocked out the light from this star, showed Eris to be less than 2340km (about 1454 miles) wide. Based on its brightness on discovery, Eris had originally seemed much bigger than this. This makes Pluto slightly larger than Eris at 2342km (1455 miles). Does this mean that perhaps Pluto may be reinstated with planetary status once more?

Well, when the IAU gathered and decided Pluto’s fate, they redefined what the term planet really meant.

The criteria to be classed as a planet are as follows;

A planet must:

1.       Be in orbit around a star eg. the Sun.

2.       Have enough mass and gravity to be a sphere.

3.       Must have enough mass to dominate its orbit, ie. have cleared any objects out of its path.

It was this final requirement that caused Pluto to lose planetary status. Pluto is located in the Kuiper Belt so therefore has not cleared its orbit. So unfortunately not; despite a sense of nostalgia yearning for that time in our lives when we thought we knew there were nine planets, Pluto remains a dwarf planet, it was reclassified for a reason!

Image of NewHorizons at Pluto

New Horizons at Pluto: Artists Impression of the grand piano-sized probe arriving at Pluto with largest moon Charon in the background. (Image credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI))

This recent news may not change Pluto’s status, but what is really exciting is that even though Pluto may be bigger in size, Eris is still 25% more massive. It is a lot denser and may not have the same make up as neighbour Pluto. Eris is no longer Pluto’s twin, but a totally different world. These two planets seem to have similar surfaces yet different internal make-up. This has opened up many queries regarding these two world’s history and why they are so different. Perhaps when the spacecraft New Horizons arrives at Pluto in 2015 we’ll gain a much better understanding about the dwarf members of our Solar System.

Image of_Martina Redpath

Martina Redpath, ESO (Image credit: Armagh Planetarium)

Article by Martina Redpath.


Jimmy L. Bostic · December 4, 2013 at 22:07

Here’s Asteroid news!

    Jimmy L. Bostic · December 4, 2013 at 22:08

    Ida has a moon. Only asteroid with a moon, Ida.

Jimmy L. Bostic · December 4, 2013 at 21:54

Here are the 15 planets in order from Sun…. (Yes, you told us that before-ADMIN)

    Jimmy L. Bostic · December 4, 2013 at 21:56

    Sorry, I thought it was dleted!
    < >

    Jimmy L. Bostic · December 4, 2013 at 21:59

    admin, you forgot to say “That’s an interesting opinion.”

Jimmy L. Bostic · December 4, 2013 at 11:47

Here are the 15 planets in order from Sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Pluto, Haumea, Makemake, Eris, Quaoar, Sedna.

    admin · December 4, 2013 at 16:11

    That’s an interesting opinion.

Mike Tittensor · April 16, 2011 at 21:04

Hmph. They waited until the last day of the conference when half the delegates had either left or were worse the wear from “astronomy” and then had the vote!

Reinstate Pluto!

V774104: a Dark World Putting a New Light on the Solar System? | Astronotes · November 16, 2015 at 15:30

[…] Far, far beyond Pluto (which is currently about 33 au from the Sun) and more distant even than Eris (about 97 au from the Sun at the time of writing), V774104 almost certainly has a rocky core […]

New Horizons at Pluto | Astronotes · July 14, 2015 at 14:09

[…] we thought having a diameter of 2370km. This means Pluto is about 30km larger than Eris, making Pluto the largest known object in the Kuiper Belt. It does not mean that it will be reclassified from dwarf planet to […]

How Did New Horizons Get to Pluto So Quickly? | Astronotes · June 10, 2015 at 08:01

[…] around Pluto. New Horizons will scream past Pluto without stopping, actually passing between the dwarf planet and its largest moon Charon. After its rendezvous with Pluto New Horizons may be redirected to […]

Will 21 December 2012 be Doomsday? | Astronotes · October 15, 2013 at 02:59

[…] been discovered have in actual fact clipped up pieces of media coverage from 2003 when dwarf planet Eris was discovered, located in the Kuiper Belt of our Solar […]

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