Moons are Planets too

Moons of the solar system compared to Earth. Credit: NASA

What makes a planet a planet? The answer turns out to be rather contentious. The official definition of a planet, as defined by the International Astronomical Union (IAU) is that a planet must satisfy three conditions:

  1. It must orbit the Sun.
  2. It must be in hydrostatic equilibrium.
  3. It must have cleared its orbital neighborhood.

By this definition there are just eight planets in our solar system, most notably excluding Pluto. This has stirred all manner of controversy, even among astronomers. Several alternative definitions have been proposed, but a new study argues we should look to history for the solution.

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Rogue Planets Could be Habitable

An artist's illustration of a rogue planet, dark and mysterious. Image Credit: NASA

The search for potentially habitable planets is focused on exoplanets—planets orbiting other stars—for good reason. The only planet we know of with life is Earth and sunlight fuels life here. But some estimates say there are many more rogue planets roaming through space, not bound to or warmed by any star.

Could some of them support life?

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