The Basics

E1 # What is a Planet?

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Everyone knows that Earth, Mars and Jupiter are planets. But both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

  1. It must orbit a star (in our cosmic neighborhood, the Sun).
  2. It must be big enough to have enough gravity to force it into a spherical shape.
  3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

    4. How Scientists Use Earth as a Test Lab for Other Worlds

    The quest to understand our solar system begins close to home. Earthquake science paves the way for the study of quakes on icy ocean worlds like Jupiter’s moon Europa

    Credit: NASA

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