July 29 – Cold, Cruel Fate

Today’s factismal: Eris “the Pluto killer” was discovered eleven years ago.

One of the most contentious questions in science today is “What is a planet?” Scientists can (and do) argue over the question for hours at a time, mainly because how you answer it all depends on what characteristics you think are important for planets. Most notably, the International Astronomical Union (a body that was set up to keep us from arguing over names) famously decided that not only was Pluto not a planet, it wasn’t even related to “real” planets. Interestingly, they did this even though their own subcommittee said that Pluto should remain a planet and even though many planetologists consider Pluto to be one.

The Eris system (Image courtesy NASA)

The Eris system
(Image courtesy NASA)

But why was Pluto “killed”? Why did they decide that Pluto wasn’t a planet? It all starts with Pluto itself. When it was found, Pluto was so far away that it was just a dim light in the telescope. As a result, everything that we thought about Pluto was based on the other planets that were out there: Jupiter, Saturn, Uranus, and Neptune. All of those planets are big and gaseous (what planetologists call “jovian planets”), so the natural assumption was that Pluto was more of the same. But as telescopes got better, Pluto got smaller. It shrank from being the size of Jupiter to being the size of the Earth to being smaller than the Moon (for what it is worth, the planet Mercury is only a little larger than the Moon).

Pluto and its largest moon, Charon (Image courtesy NASA)

Pluto and its largest moon, Charon
(Image courtesy NASA)

And, as if that weren’t bad enough, we started seeing other objects out near Pluto that were about the same size as the planet itself. The most famous of these new planets was Eris, aptly named for the Greek goddess of discord, which was actually larger than Pluto. The astronomers then panicked over the prospect of having more than ten planets in the Solar System and led the IAU to redefine the word so that they wouldn’t have to quit using their fingers to count. (To be fair, this wasn’t the first time that the problem had arisen; it happened with the Galilean planets and with the minor planets.) As a result, Pluto and the other planets out there became known as “dwarf planets” (an appellation that was originally intended to apply to Earth).

A plot of planetary size versus density. Notice how Pluto ends up with the junk.

A plot of planetary size versus density. Notice how Pluto ends up with the junk.

But call it a dwarf planet, a KBO, or a giant cookie, Pluto is still out there.Even better, there are thousands upon thousands upon thousands of other planets out there, just waiting to be found! If you’d like to try your hand at discovering the next planet of discord, then why not head over to Planet Hunters? They’ve got tons of Kepler data just waiting for you!

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