Everything in the solar system moves: planets orbit the Sun, moons orbit planets, and every body spins on its axis. Understanding these motions was one of the first great successes of science, and it is still an active field. The laws of motion Precise orbit calculations led to the discovery of Neptune in 1846, predicted … Read more
A planet’s magnetic field is invisible, but it reveals what is happening deep inside and protects the planet from the solar wind. Astronomers study these fields mostly with spacecraft, and also through auroras and radio waves. Where do planetary magnetic fields come from? Most come from a dynamo: a flowing, electrically conducting fluid inside a … Read more
The difference comes down to one criterion: whether the object has cleared its orbital neighborhood. This definition was adopted by the International Astronomical Union (IAU) in 2006, and it is why Pluto is no longer counted among the planets. The IAU definition of a planet A planet in our solar system is a body that: … Read more
