Astronomy
Planets in Space
Quick fact
Jupiter's Great Red Spot is a storm larger than Earth that has been raging for at least 400 years.
Why this is interesting
When you look up at the night sky, some points of light appear to wander differently from the stars—they are planets. But what exactly makes a world a planet?
Read the full explanation
Understanding Planets in Space
A planet is a large, round object that orbits a star. Unlike stars, planets don't produce their own light—they shine by reflecting sunlight. In our Solar System, planets come in two main types: small, rocky ones like Earth and Mars (terrestrial) and huge, gas-dominated ones like Jupiter and Saturn (gas giants). The International Astronomical Union defines a planet as a body that (1) orbits the Sun, (2) is massive enough to be pulled into a spherical shape by its own gravity, and (3) has cleared its orbital path of other debris. This definition famously reclassified Pluto as a dwarf planet. Planets form from a disk of dust and gas around a young star; particles collide and stick together, eventually growing into large bodies.
A deeper explanation
A planet's spherical shape is a direct result of gravity. For a body larger than about 500 km across, its own gravity overcomes the mechanical strength of rock and pulls it into a sphere—the most energy-efficient shape. The 'cleared its orbit' criterion means the planet has enough mass to gravitationally dominate its neighborhood, either absorbing or ejecting smaller objects over time. This distinction separates planets from dwarf planets like Pluto. Understanding planets matters because they are the only known places where life can exist. By studying planets in our Solar System and exoplanets in other systems, we learn about the processes that shaped Earth, the diversity of worlds, and the conditions that might support biology elsewhere. The discovery of thousands of exoplanets has shown that planets are common in the galaxy and come in an incredible variety.