What Are Planets?

Planets are large, roughly spherical bodies that orbit a star, possess sufficient self-gravity to have achieved hydrostatic equilibrium, and have gravitationally cleared the region around their orbit of comparable-sized debris. This three-part definition was formally adopted by the International Astronomical Union (IAU) in 2006 and applies to the eight recognized planets of the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The definition distinguishes planets from dwarf planets, which satisfy the first two criteria but have not cleared their orbital neighborhoods, and from minor bodies such as asteroids and comets, which satisfy neither mass criterion. Beyond the solar system, thousands of confirmed exoplanets orbit other stars, detected by transit photometry, radial velocity measurements, and direct imaging, with masses spanning from sub-Earth to several Jupiter masses.

Planets are studied by planetary science, a discipline that draws on geophysics, atmospheric dynamics, chemistry, and remote sensing. The eight solar system planets span a factor of roughly 300,000 in mass and differ fundamentally in composition, internal structure, and atmospheric character, making the solar system itself a laboratory for understanding the range of outcomes that planetary formation can produce.

Planetary Classification and Internal Structure

Solar system planets fall into two broad structural categories. The four terrestrial planets, Mercury, Venus, Earth, and Mars, have solid silicate crusts over iron-rich cores and relatively thin or absent atmospheres compared with their size. Earth's iron-nickel core generates a global magnetic field through convective dynamo action; Mars has only a remnant crustal magnetism from an ancient dynamo that ceased more than three billion years ago.

The four outer planets divide further into gas giants and ice giants. Jupiter and Saturn are dominated by hydrogen and helium extending to metallic hydrogen in their interiors, where pressures exceed 1.4 million atmospheres and electrical conductivity drives powerful magnetic fields. NASA Science's planetary overview describes Uranus and Neptune as ice giants, bodies containing substantial proportions of water, ammonia, and methane ices beneath relatively thin hydrogen envelopes, with internal structures that differ from the hydrogen-dominated gas giants despite comparable overall size.

The IAU's 2006 planet definition established that hydrostatic equilibrium, the condition where internal pressure balances self-gravity, is the threshold that distinguishes planets and dwarf planets from irregular small bodies. Pluto, previously classified as a planet, was reclassified as a dwarf planet because it shares the Kuiper Belt with thousands of similar trans-Neptunian objects and has not dynamically cleared that region.

Planetary Atmospheres and Surface Environments

Planetary atmospheres range from Mercury's negligible exosphere to the dense CO2 envelope of Venus, where surface pressures reach 92 bars and temperatures exceed 460 degrees Celsius. The presence and character of an atmosphere depends on a planet's gravity, its distance from the Sun governing volatile retention rates, and internal volcanic outgassing that replenishes lost species. Earth's nitrogen-oxygen atmosphere is unique in the solar system in maintaining liquid water on the surface and supporting the biosphere, itself a major contributor to atmospheric composition through photosynthetic oxygen production and biological methane cycling.

Extraterrestrial phenomena such as lightning, atmospheric circulation bands, and ring systems are distributed unevenly across planetary types. Jupiter's Great Red Spot, a persistent anticyclonic storm wider than Earth, has been observed continuously since the seventeenth century. Saturn's ring system, composed of water-ice particles ranging from grains to boulders, is the most extensive in the solar system.

Applications

Planets have applications in a range of fields, including:

• Planet exploration using orbiters, landers, and rovers to characterize surface composition and search for past or present habitability
• Exoplanet research to understand planet formation and the frequency of Earth-like worlds around other stars
• Comparative climatology: using Venus, Mars, and the giant planets to test atmospheric circulation models applicable to Earth's climate
• Spacecraft navigation and trajectory design, which requires precise planetary ephemerides
• Search for biosignatures in planetary atmospheres using spectroscopic techniques developed for solar system targets