Is Pluto still a planet?

Much to my surprise, the International Astronomical Union has indeed voted on a resolution that defines exactly what a planet is and Pluto no longer qualifies. This means that Pluto is indeed not a planet any more and that our solar system no longer has the nine planets I learned about as a kid, but only eight!

Don't worry, though, Pluto is now to be known as a dwarf planet, so it's not been completely ejected from our solar system, just relabeled. The eight planets in our solar system are now, in order from the Sun outwards, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

As The Planetary Society explains, "a “planet” is now defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit."

"A dwarf planet, according to the new definition, is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite."

Got it?

There are now three dwarf planets in our solar system, so Pluto isn't alone. It's joined by the asteroids Ceres and Xena (aka UB313), though more than a dozen are on the IAU's dwarf planet watch list, if you can believe it.

One key reason that Pluto has been given the boot is because its orbit is not in the same ecliptic plane as the rest of the planets and isn't circular as are the planets, but is rather "eccentric". Indeed, at certain points in its orbit, Pluto is actually closer to the sun than Neptune. You can see some of this shown here:

A wee bit of data about Pluto too: Discovered in 1930 by Clyde Tombaugh, Pluto has an orbital period of 90,613 days (what we on Earth would consider 248 years) and is only 18% the size of Earth with a surface area of far less than 1% of our own planet. Gravity is far less than our planet's gravity too, at 0.58 m/s² (about 0.06 Earth Gee). Sound like somewhere worth visiting? Maybe not. The average surface temperature is a chilly 44K which converts to -380F. Pluto also has three moons, Charon, about 35% the size of Earth's moon, and two much smaller moons provisionally called Nix and Hydra.

Additional reading:

• The International Astronomical Union
• NASA's Pluto fact sheet
• Windows to the Universe (NCAR)
• Orbital simulator of Pluto
• Kids guide to Pluto
• Wikipedia Entry on Pluto