We know that planets revolve around their parent star (or specifically Barycenter), they maintain a specific speed of revolution so that they don't end up smashing into their planet star. Which force or energy do that, so that they maintain their speed? If we have infinite amount of time, would planets smash into their parent star ?

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    $\begingroup$ It is a (very) basic feature of physics that no force is required to maintain the speed of an object unless it is acted on by other external forces. i.e. Why do you think they should slow down? $\endgroup$
    – ProfRob
    Aug 21 '15 at 13:13
  • $\begingroup$ @RobJeffries As we see in some cases that two stars or black hole spinning around each other merge into each other. What cause them to merge? $\endgroup$
    – Sirius
    Aug 21 '15 at 17:54
  • $\begingroup$ Gravitational waves. And, by the way, they speed-up. $\endgroup$
    – ProfRob
    Aug 21 '15 at 18:17
  • $\begingroup$ I might be misunderstanding the question, but constant "speed or revolution" (angular momentum) does require a force, namely gravity. If there was no force at all, the planets would continue in straight lines, not ellipses. $\endgroup$
    – user21
    Aug 21 '15 at 23:56
  • $\begingroup$ In addition to some other good comments: planets do not keep constant speed unless they are in circular orbit. $\endgroup$
    – Brick
    Aug 25 '15 at 5:57

Newton's First Law of Motion states that "an object in motion stays in motion with the same speed ... unless acted upon by an unbalanced force". On earth, an object in motion will slow down and stop due to air resistance (the unbalanced force), but in space, there is no atmosphere to cause air resistance, so the object will maintain its speed.

  • $\begingroup$ Why they start revolving around their parent star at the time of the formation of a solar system; why don't all the matter just merge into a nascent star? $\endgroup$
    – Sirius
    Aug 21 '15 at 17:57
  • $\begingroup$ @Sirius This question has been asked before in many forms. e.g. astronomy.stackexchange.com/questions/387/… , but many others here and on Physics SE. $\endgroup$
    – ProfRob
    Aug 21 '15 at 18:18
  • $\begingroup$ This answer is wrong. Planets generally DO change speed. The exception is if their orbit is circular. The real point is that they there are not losing energy, and that is because they are only exposed to conservative forces. $\endgroup$
    – Brick
    Aug 25 '15 at 6:00

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