Planet

Planet
CactiStaccingCrane · CC BY-SA 4.0 · source
Name	Planet
Caption	Generic depiction of a planetary body
Mass	Varies widely
Radius	Varies widely
Period	Varies widely
Discovered	Antiquity to modern era
Discovered by	Multiple cultures and observatories

Planet

A planet is a major category of astronomical body defined by criteria used in modern astronomy and codified by the International Astronomical Union; planets orbit stars such as Sun and can be found around stars cataloged in surveys like the Kepler mission and the Transiting Exoplanet Survey Satellite. Historically, thinkers from Aristotle through Copernicus and Galileo Galilei reshaped ideas about planetary motion that were formalized by Johannes Kepler and Isaac Newton; contemporary studies employ instruments associated with Hubble Space Telescope, James Webb Space Telescope, and ground facilities such as Very Large Telescope.

Definition and Classification

Modern classification of a planet originates in deliberations by the International Astronomical Union in 2006 and builds on earlier work by astronomers at institutions like the Royal Astronomical Society and the American Astronomical Society. Definitions distinguish planets from dwarf planets, natural satellites, and small Solar System bodies cataloged by surveys including Minor Planet Center records and missions like New Horizons. Taxonomies divide planets into categories exemplified by Solar System labels—terrestrial planets and gas giants—or by exoplanet classifications used in papers from European Southern Observatory teams and the Kepler catalog: hot Jupiters, super-Earths, and mini-Neptunes.

Formation and Evolution

Planet formation theory centers on models of protoplanetary disks observed around young stars in regions such as the Orion Nebula and Taurus Molecular Cloud, with processes described by the nebular hypothesis and augmented by theories of core accretion and disk instability. Terrestrial accretion involves collisions and differentiation as seen in studies of Earth and samples from Apollo program missions and Lunar Reconnaissance Orbiter analyses; giant-planet formation is constrained by studies of Jupiter and the Galileo probe and modeled with hydrodynamic simulations developed at institutions like Caltech and MIT. Late-stage evolution includes migration mechanisms tied to resonances observed in systems such as TRAPPIST-1 and the Upsilon Andromedae system, and atmospheric loss processes inferred from observations by Chandra X-ray Observatory and Spitzer Space Telescope.

Physical and Orbital Characteristics

Planetary physical properties span composition classes exemplified by Mercury, Venus, Earth, and Mars for rocky bodies, and Jupiter, Saturn, Uranus, and Neptune for giant planets. Measured parameters include mass, radius, density, and moment of inertia determined via techniques used by missions such as Cassini–Huygens and Juno. Orbital characteristics—semi-major axis, eccentricity, inclination, and resonances—are analyzed using frameworks from Kepler's laws and Newtonian mechanics and refined with post-Newtonian corrections relevant to studies involving Solar System dynamics and perturbations from objects cataloged by the Jet Propulsion Laboratory.

Atmospheres, Climates, and Habitability

Planetary atmospheres range from the dense carbon dioxide envelope of Venus to the hydrogen–helium envelopes of Jupiter and Saturn, with compositional analyses performed by instruments aboard Voyager probes and spectrometers on the Kepler and James Webb Space Telescope. Climate systems on planets are studied through models produced by groups at NASA and the European Space Agency, employing radiative transfer and general circulation models that interpret data from missions like Mars Reconnaissance Orbiter and the Pioneer Venus project. Habitability assessments reference the concept of a star's habitable zone as formalized in exoplanet literature and involve biomarkers sought in atmospheres by collaborations such as those at the SETI Institute and the European Southern Observatory; targets include planets orbiting stars cataloged by Gaia and systems like Proxima Centauri and Kepler-186.

Types and Examples

Planetary types are exemplified by Solar System cases: Mercury (airless, iron-rich), Venus (runaway greenhouse), Earth (liquid water, biosphere), Mars (cold desert), Jupiter (gas giant), Saturn (ringed giant), Uranus and Neptune (ice giants). Exoplanet archetypes include 51 Pegasi b as the prototype hot Jupiter, Kepler-22b and Kepler-452b as super-Earth/mini-Neptune candidates, and multi-planet systems such as TRAPPIST-1 and Kepler-11 that inform population studies led by teams at Harvard–Smithsonian Center for Astrophysics and Max Planck Institute for Astronomy.

Observation and Exploration

Observation techniques span direct imaging performed with instruments at European Southern Observatory facilities and coronagraphic campaigns using Hubble Space Telescope, to indirect methods like transit photometry pioneered by the Kepler mission and radial velocity surveys from observatories such as Lick Observatory and W. M. Keck Observatory. In situ exploration has included sample-return and lander missions: Apollo program lunar samples, Viking program landers on Mars, the Hayabusa and Hayabusa2 asteroid missions, and the ongoing analyses of returned material by teams at Smithsonian Institution and Johnson Space Center. Future exploration goals are articulated by agencies like NASA and Roscosmos and planned missions such as proposals to return samples from Mars and to probe icy moons cataloged by studies of Europa and Enceladus.

Category:Planets