But part of the problem with the definition - I mean, as an example - is that if you look at Mars - if Mars - if we discovered a Mars-sized object out beyond the orbit of Pluto, it couldn't be a planet, because over the age of the solar system it wouldn't have time to clear its orbit.

NPR: Astronomers Prepare to Fight Pluto Demotion

To date, of the roughly 800 planets catalogued beyond our Solar System, only four are known to orbit stars in a cluster.

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The fact that different bodies in the solar system change their relative positions as they orbit the sun will, Dr Hooke concedes, complicate routing a bit: when the earth and Mars are on opposite sides of the sun, for example, it might be best to send e-mail via Venus.

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Some galaxies contain black holes with billions of times the mass of the sun, occupying a volume no bigger across than the orbit of Neptune around our solar system but extending their influence through a cosmic bubble 30 million light years across.

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Designed to be flexible for launching spacecraft for crew and cargo missions, SLS and Orion will expand human presence beyond low-Earth orbit and enable new missions of exploration in the solar system.

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The largest planet in the solar system, Jupiter, has these cloud of Trojan asteroids that straddle its orbit, so that would seem to violate a plain reading of the definition.

NPR: Astronomers Prepare to Fight Pluto Demotion