All of a sudden, our neighbourhood might be getting a little more crowded.
Next week the International Astronomical Union will meet at its annual convention. The hot topic?
What defines a “planet”!?
This is a very important question because thanks to more powerful telescopes… we have discovered a number of very large “rocks” (a.k.a. asteroids) in our Solar System. Some in the big belt between Mars and Jupiter, and some waaayyy out with Pluto.
These large rocks are, in one case at least, even larger than Pluto itself! (It’s known as “Xena”)
New Scientist explains it a little
To be a planet:
#1: Must be big large enough to be “rounded” by it’s own gravity (this could mean planets only 500-1000KM across… Pluto is 2000KM)
#2: It must primarily orbit a star (see below… about “double planets”)
#3: It must not be a moon of another planet
So apparently this definition would give our Solar System 3 new planets!!
It would go like this:
#5: Ceres (Formerly a large asteroid… but was once considered a planet in the 1910s
#6: Vesta (Possibly added depending on further observations)
#7: Pleilas (Possibly added depending on further observations)
#12: Pluto – Charon (now the twelth planet! is actually a “Double Planet” because Charon is so big compared to Pluto, they actually orbit each other while they both orbit the Sun
Here are some new definitions as thye have been proposed:
Planet: A round thing orbiting a star. More precisely, according to the draft definition: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”
Pluton: A planet orbiting beyond Neptune, taking more than 200 Earth years to circle the Sun. So far, it would include Pluto; Pluto’s former moon, Charon; and “Xena” (2003 UB313).
Satellite: Anything orbiting a planet, as long as the mutual centre of gravity does not fall outside the planet. Includes several bodies much larger than many planets, such as Jupiter’s moon Ganymede (diameter: 5262 kilometres).
Small solar system body: Anything orbiting the Sun that’s not a planet or a satellite. Most asteroids and comets would be SSSBs. Currently called minor planets.