Topics

Planets

Worlds bound to a star.

Overview

A planet is a body massive enough to be rounded by its own gravity, orbiting a star, and clearing the neighbourhood around its orbit. The third clause — added by the International Astronomical Union in 2006 — is what reclassified Pluto. By that definition our Solar System has eight planets; by older definitions it had more.

Two flavours, with variations

Our Solar System has four small rocky "terrestrial" planets close to the Sun — Mercury, Venus, Earth, Mars — and four large gaseous outer ones: Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants, with proportionally more water, ammonia and methane). Beyond Neptune lies the Kuiper Belt and a population of icy bodies, including five recognised dwarf planets.

Exoplanets — worlds around other stars

Since the first confirmed exoplanet around a Sun-like star in 1995, more than 5,800 have been catalogued. Many do not resemble anything in our system: "hot Jupiters" that orbit closer than Mercury, "super-Earths" between Earth and Neptune in size with no equivalent here, "rogue planets" wandering free without a star at all. Most stars in the galaxy probably have planets.

Where planet ends and star begins

The line between large planet and small star is blurry. Brown dwarfs — failed stars massive enough to fuse deuterium but not hydrogen — occupy that grey zone. Jupiter would need to grow about 80 times more massive to ignite hydrogen fusion and become a true star. Our Solar System came close to being a binary system.

Conditions for life

A planet capable of liquid water at its surface needs a stable orbit in the "habitable zone" around its star, a substantial atmosphere, a magnetic field to deflect stellar radiation, and probably plate tectonics to recycle nutrients. Earth has all four. We have so far found no other world that demonstrably does — though several look intriguing.