A planet is a massive astronomical body that is in an orbit around a star. The criteria for an object being a planet falls within the limits of its mass. The mass of the object for it to be called as a planet should be larger than that of an asteroid but not large enough to have its own thermonuclear fusion in the core.

Planets are categorized as terrestrial and giants. For example, in our solar system, the planets Mercury, Venus, Earth, and Mars are the terrestrial planets as they are mostly made up of silicate rocks and other metals. The other four planets, Jupiter, Saturn, Uranus, and Neptune are the gas-giants as they mostly contain gas.

Frequently Asked Questionsedit

What are the formal requirements for an object to be a planet?edit

As per the International Astronomical Union, for an astronomical body to be classified as a planet, it has to pass the following three criteria.[1]

  • The said astronomical object should be orbiting a star.
  • The mass of the object should be massive enough to have its own gravity shape itself into a spheroid, assuming a hydrostatic equilibrium.
  • The object has cleared its neighbours around the orbit, with its gravity.

Why do planets rotate about an axis?edit

When a planetary system forms from a nebula, the angular momentum is conserved in the accretion disk. This accretion disk is where the primordial planetesimals are believed to evolve. As the protostar begins to evolve, the planetesimals evolve into protoplanets along with a rotation to conserve the angular momentum of the system. And due to inertia, they continue to rotate on its axis unless any external forces like an asteroid impact or a satellite affect it.

Why planets orbit in the same plane?edit

When the nebula collapses, the dense gases at the centre spins fast to conserve angular momentum of the total system. This creates an accretion disk around it in a plane. The planets orbit in the same plane as that of its primordial accretion disc where it evolved.


  1. International Astronomical Union | IAU. (2017). Retrieved 9 August 2017, from