Objects in space: definitions and locations of planets, comets, asteroids, meteroids

It can be hard keeping up with the differences between planets, comets, asteroids, and meteroids--especially when many people use the terms interchangeably. This list quickly distinguishes between these objects while providing a map (not to scale!) of the relative locations of the planets and other noteworthy cosmic features.

Remember the order of the planets using the mnemonic M-VEM-J-SUN (see image below). The Asteroid Belt comes after Mars, as does the dwarf planet Ceres; you can remember this by thinking ABC. The Kuiper belt falls after Neptune (spanning 30-55 AU [the distance from the sun to the earth] from the sun), followed by the Oort cloud (home of comets and stretching a whopping 55-50,000 AU from the sun); think KO (as in "knock-out") to recall the order. The dwarf planets Pluto and Eris inhabit the Kuiper belt.

Planet locations

1) luminosity
2) high surface temperature

1) orbits sun
2) nearly spherical
3) cleared its orbit -- this is what knocks Pluto off the list

Dwarf Planet:
1) orbits sun
2) nearly spherical
3) has NOT cleared its orbit
4) not a moon

1) orbits a planet or dwarf planet

1) contain volatiles such as water
2) also contains rock dust in ice, making it a "dirty snowball" of sorts
3) features two tails: a plasma tail that faces away from the sun, and a dust tail that faces opposite the direction of the comet's movement
4) often from Oort cloud

1) rocky
2) undifferentiated
3) between 0.1 - 600 km long
4) irregularly shaped

Meteoroid: comet/asteroid/planet fragment (less than 10 m long) in space

Meteor: when the fragment is burning in earth's atmosphere; most are dust-sized

Meteorite: after the fragment hits earth's surface

You can remember the meteoroid-meteor-meteorite order by recalling that asteroids are in space, so meteroids are also in space. The phrase "meteor showers" refers to the light show, indicating that a meteor is burning in the earth's atmosphere. Finally, many rock names end in -ite (diorite, for example), which can aid in recalling that meteorites have landed on the earth's surface.

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