A planet is a celestial body that orbits a star (or stellar remnant), is massive enough for its own gravity to shape it into a sphere, and has cleared—or dominates—its orbital neighborhood. This definition is set by the IAU and excludes dwarf planets like Pluto.
Within our Solar System, there are eight recognized planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Additionally, there are at least five dwarf planets, including Pluto, Eris, Haumea, Makemake, and Ceres.
Beyond our Solar System, astronomers have confirmed over 6,000 exoplanets—ranging from gas giants to rocky super-Earths—offering rich insights into the diversity of planetary systems.
Planets fall into several categories: terrestrial (rocky), gas giants, ice giants, super‑Earths, and rogue planets, which wander free without orbiting a star.
How Planets Form: This process begins in a rotating disk of gas and dust—called a protoplanetary disk—surrounding a young star. Dust grains stick together through collisions, growing into planetesimals.
Through accretion, these planetesimals collide and merge, forming planetary embryos. In inner disk regions—where temperatures are high—rocky planets form. Beyond the frost line, ice and gas can rapidly build up to form gas and ice giants.
Planets may shift their orbits due to gravitational interactions either with other forming planets or the disk itself. This helps explain why some exoplanets, known as hot Jupiters, orbit very close to their stars.
The planet formation process spans millions of years. Observatories like ALMA, JWST, and Hubble have captured disks featuring gaps and rings—strong indicators of forming planets interacting with their birth environments.
Studying planets—including their atmospheres, potential for water, and internal composition—can help answer questions about their habitability. Missions such as ESA’s PLATO and NASA’s upcoming Habitable Worlds Observatory will further explore these themes.