The object in the solar system that is always at one focus of a planet’s orbit is the Sun.

Quick Scoop: The Core Idea

  • Planets travel around the Sun in elliptical orbits, not perfect circles.
  • In each of these ellipses, the Sun sits at one of the two foci; the other focus is just empty space.
  • This is a statement of Kepler’s First Law of planetary motion: each planet’s orbit is an ellipse with the Sun at one focus.

So, if the question is “What object in the solar system is always one foci (focus) of a planet’s orbit?”, the answer is the Sun.

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