Planets

Overview

A planet, under the definition adopted by the International Astronomical Union in 2006, orbits the Sun, is massive enough for its own gravity to have pulled it into a nearly round shape, and has cleared the neighbourhood around its orbit. The Solar System has eight.

• The 2006 definition has three clauses; it was the third — clearing the orbital neighbourhood — that reclassified Pluto.
• The four inner planets are rock and metal; the four outer ones are dominated by hydrogen, helium, and ices.
• Uranus and Neptune are 'ice giants', a compositionally distinct class from Jupiter and Saturn.
• Every planet's mass, radius and orbit on this platform is a catalogued measurement with a recorded source.

The definition and why it was contested

Before 2006 'planet' had no formal definition — it was a list maintained by convention. Discoveries in the Kuiper Belt forced the issue: Eris was found to be comparably sized to Pluto, and further similar bodies were expected, so either the list would grow indefinitely or a criterion was needed.

The IAU's resolution requires a body to orbit the Sun, to be in hydrostatic equilibrium, and to have cleared its orbital neighbourhood. Pluto satisfies the first two but shares its orbital zone with many Kuiper Belt objects, so it was reclassified as a dwarf planet. The third clause remains debated — some planetary scientists argue that a definition based on the body's own properties would be more useful than one based on its surroundings — and it is worth presenting that as a live disagreement rather than a settled matter.

The terrestrial planets

Mercury, Venus, Earth and Mars are small, dense worlds of rock and metal with solid surfaces and few or no moons. They formed in the inner Solar System where temperatures were too high for water ice and other volatiles to condense, leaving only refractory materials to accrete.

Their differences are instructive. Mercury has an unusually large iron core and effectively no atmosphere. Venus, nearly Earth's twin in size and mass, has a dense carbon-dioxide atmosphere and a surface near 460 °C under roughly 90 bar of pressure. Mars retains a thin atmosphere and abundant geological evidence of past liquid water. Comparing them is how planetary science tests ideas about atmospheric evolution and habitability.

Gas giants and ice giants

Jupiter and Saturn are overwhelmingly hydrogen and helium, with no solid surface — pressure rises smoothly until hydrogen becomes a metallic fluid deep inside. Both radiate more energy than they receive from the Sun, and both have extensive satellite systems and ring systems, Saturn's being far the most prominent.

Uranus and Neptune are compositionally different enough to warrant a separate class. They contain proportionally much more water, ammonia and methane — collectively termed 'ices' in planetary science regardless of physical state — and much less hydrogen and helium. Both have been visited only once, by Voyager 2 in 1986 and 1989 respectively, which is why they remain the least characterised planets in the Solar System.

How planetary properties are measured

• Mass comes from gravitational effects: on moons, on spacecraft trajectories, or on other planets.
• Radius comes from direct imaging, from occultations of background stars, or from radar ranging.
• Density is derived from mass and radius, and is the primary constraint on bulk composition.
• Atmospheric composition comes from spectroscopy, both remote and in situ from probes.
• Interior structure is inferred from gravitational field measurements, magnetic fields, and seismology where available.

In-depth, individual pages in this category.

• Mercury

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  The smallest planet, closest to the Sun.

• Venus

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  Earth's size, but a scorching greenhouse world.

• Earth

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  The third planet — and the only known home of life.

• Mars

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  The Red Planet, a cold desert world.

• Jupiter

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  The largest planet — a banded gas giant.

• Saturn

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  The ringed gas giant.

• Uranus

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  The sideways-spinning ice giant.

• Neptune

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  The deep-blue, windy ice giant at the edge.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Solar SystemEvery planet, moon, and small body with measured data.
• Comparative planetologyWhat comparing worlds reveals about each of them.
• Planetary geologySurfaces, features, and geological processes.
• ExoplanetsPlanets around other stars, for comparison.

Frequently asked

Why is Pluto not a planet?

Under the 2006 IAU definition a planet must have cleared the neighbourhood around its orbit. Pluto orbits within the Kuiper Belt alongside many other bodies and does not dominate its orbital zone, so it was reclassified as a dwarf planet. It meets the other two criteria — it orbits the Sun and is round — and some planetary scientists continue to argue that the clearing criterion is the wrong test.

What is the difference between a gas giant and an ice giant?

Composition. Jupiter and Saturn are dominated by hydrogen and helium. Uranus and Neptune contain a much larger proportion of heavier volatile compounds — water, ammonia and methane, which planetary science calls 'ices' regardless of their actual physical state at depth — and correspondingly less hydrogen and helium.

Which planet is hottest?

Venus, despite Mercury being closer to the Sun. Venus's dense carbon-dioxide atmosphere traps heat so effectively that its surface temperature is around 460 °C, hotter and far more uniform than Mercury's, which swings enormously between day and night because it has essentially no atmosphere to redistribute heat.

Are there planets beyond Neptune?

No ninth planet meeting the IAU definition has been found. There is an active hypothesis, based on the clustered orbits of some distant trans-Neptunian objects, that an undiscovered massive planet may exist far beyond Neptune, but it has not been detected and alternative explanations for the clustering — including observational selection effects — remain viable.