22 April 2011

N is for Ninth Planet

My Very Eager Monkey Jumped Straight Under Nine Planets.

That’s how we were taught as children to remember the order of the nine planets of our solar system.  But I was a geek, so I could rattle them off in order by name:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.

But on 24 August 2006, that all changed as the scientific community decided our solar system had only eight planets.  Pluto, the smallest and furthest of the nine solar bodies, had been demoted from planetary status.

The road to this decision started in 1978 when astronomers discovered a moon orbiting Pluto called Charon.  This discovery allowed the mathematicians to accurately calculate the mass of Pluto, and they found it to be considerably smaller than expected.  So small, in fact, that it was only 4% the mass of Mercury.  Even the Earth’s moon was larger. 

Then during the 1990’s further astronomical bodies were found beyond the orbit of Pluto that had masses larger than the tiny planet.  If Pluto was to remain a planet, then they too would need to be classified as planets.  In the end, the simplest solution was to create a new definition for a planet and to do so such that Pluto did not fit that criteria, but instead fell into a new category called a dwarf planet

So as of today, 22 April 2011, we officially have eight planets and five dwarf planets (called Ceres, Pluto, Haumea, Makemake, and Eris).

However, the story is not yet complete. Earlier this year, two astronomers postulated the existence of a new massive ninth planet which they named Tyche (pronounced “Ty-kee”).  If this planet does exist, it will be huge – 5 times the size of our largest planet Jupiter!

Tyche_proposed_nith planet
Credit: Ben McGee - Astrowright

The evidence for its existence comes from a detailed analysis of comets entering the solar system in orbits that are inconsistent with the current theory of the operation of the solar system.  The existence of a large planet at the outer-limits of our solar system could explain the strange orbits of these comets, since its gravitational pull would yank them into their current non-standard trajectories.

NASA has a satellite called WISE (Wide-field Infrared Survey Explorer) that over the next two years should be able to prove or disprove the existence of this new planetary neighbour.  But for now, we’re stuck with eight planets.

My Very Eager Monkey Jumped Straight Under Nothing.

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