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Pluto ("dwarf") Planet

...and Yet it moves!

Pluto is a planet by just about everyone's definition, just about everyone except the International Astronomy Union, who have some rather unusual rules for defining what a "planet" is. See the Pluto Problem

One of the key things about Pluto these days is that there is a spacecraft on the way there, New Horizons. http://pluto.jhuapl.edu . It will arrive in July 2015 and will gather data on its 9 day flight through the Pluto system which comprises Pluto, Charon, Nix, Hydra, S/2011 P 1, and probably a few others. It will be a spectacular voyage of discovery.

New Horizons will not stop, and it will continue into interstellar space.

How bright is sunlight on Pluto? I've heard people say it's as if the sun is just a distant starlike point in the sky. This isn't exactly true, and at the distance of Pluto, sunlight is one thousandth that on Earth, or to put it another way, it's as bright as it is indoors at night in a domestic electrically-lit environment. Ie, bright enough to read a newspaper. It's brighter than the lighting in a subdued lighting restaurant, and very much brighter than moonlight. Looking at the sun from Pluto would be like staring at the filament of an incandescent lightbulb indoors, and it would leave spots on your eyes for a while.

Pluto was discovered in 1936 by Clyde Tombaugh, but early expectations were that it would be a large planet and likely to be able to perturb the orbit of Neptune. As time has gone on, the estimated size of Pluto has diminished. Part of the reason for that is the fact that the original notion of Pluto was "a large planet" and it was only later found to be at least two smaller objects (Pluto and Charon) separated by a distance, giving an impression of something larger. Imagine Jovian astronomers observing "The Planet Sol 3" and guesstimating it to be "a planet 239,000 miles across", and only later being disappointed to find it was two small objects known as Sol 3 A ("Earth") and Sol 3 B ("Luna") separated by 239,000 miles of empty space. They might downgrade Sol 3 to being just more of those little fragmentary rocky bits rather than a full-sized Jupiter style planet.

The current estimated size of Pluto is reckoned to be about a third the mass of the Moon.

On Pluto, a year lasts 248 earth years. A day lasts a week, but also a month lasts a week. That's because Pluto and Charon are both gravity-locked.

Helpful Pluto links include...

http://en.wikipedia.org/wiki/Pluto

http://nineplanets.org/pluto.html

http://csep10.phys.utk.edu/astr161/lect/pluto/pluto.html

http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level1/pluto.html

www.nasa.gov/mission_pages/newhorizons/main/index.html

and more.

Image of Pluto (pre New Horizons) by Nasa, created by careful observation of multiple eclipses of Pluto/Charon. Public domain Nasa image http://en.wikipedia.org/wiki/File:Pluto_animiert.gif

Resized by the Linux command: convert pluto1a.gif -coalesce pluto1b.gif convert pluto1b.gif -resize "192x192" pluto1c.gif