Where does space start? If space starts right beyond the solid part of a planet, then wouldn't Jupiter not be a planet? And if it starts at the end of the atmosphere, at what point does one escape the atmosphere? Most of all, if it is where the gravitational influence ends, at what point of effect does it "end?"

  • $\begingroup$ In the context of the last part of your question - Gravitational influence is a function of the mass of all bodies involved in the interaction. For most humans Earth's gravitational pull becomes negligible no more than a few hundred kilometres above it's surface... yet the Moon is gravitationally bound to Earth over 250K Km away! $\endgroup$
    – Everyone
    Dec 2 '13 at 18:55
  • $\begingroup$ I've marked this as duplicate of How far do you have to be from Earth to be "in space". Answers there should demonstrate that these are arbitrarily selected borders and in nature they are not clearly defined. Well, they might be, but there are also many different ways to go about it. For gas giants, we usually speak of "a planet" at a point where atmospheric pressure is equal or greater than 1 bar pressure, tho that could vary depending on your needs, e.g. some papers used 0.1 bar. $\endgroup$
    – TildalWave
    Dec 2 '13 at 18:59

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