What are planets? Definition, types and how many are in the solar system

What Are Planets? Types, Characteristics, and the IAU Definition

For most of human history, "planet" meant simply a wandering star — a point of light that moved against the fixed background of the night sky. Today, the term has a precise scientific definition, a contested history (Pluto was reclassified in 2006), and implications that extend to thousands of worlds orbiting distant stars. This article explains what planets are, how they form, and why the distinction between planet types matters for understanding any solar system — including our own.

The IAU Definition of a Planet (2006)

The International Astronomical Union (IAU) formally defined "planet" in Resolution B5 (August 2006). To be classified as a planet in our Solar System, a body must:

1. Orbit the Sun (not another planet — that would make it a moon).
2. Have sufficient mass for hydrostatic equilibrium — gravity must pull it into a roughly spherical shape.
3. Have cleared the neighborhood around its orbit — through gravity, it has either absorbed, ejected, or captured into resonance the vast majority of nearby objects.

This third criterion is why Pluto lost its planetary status — it shares its orbital neighborhood with thousands of Kuiper Belt Objects and hasn't gravitationally dominated its zone. Pluto is now classified as a dwarf planet, along with Eris, Makemake, Haumea, and Ceres.

The Eight Planets of Our Solar System

By the IAU definition, our Solar System has eight planets, traditionally divided into two groups:

Terrestrial (Rocky) Planets — Inner Solar System

• Mercury: Closest to the Sun. No atmosphere (stripped by solar wind). Surface temperatures range from −180 °C (night) to 430 °C (day) — the widest temperature swing of any planet.
• Venus: Dense CO₂ atmosphere creates a runaway greenhouse effect. Surface temperature: 465 °C — hotter than Mercury despite being farther from the Sun.
• Earth: The only confirmed biosphere in the known universe. Plate tectonics, liquid water oceans, and an oxygen-nitrogen atmosphere make it uniquely habitable.
• Mars: Thin CO₂ atmosphere, ancient riverbeds, and polar ice caps of water and CO₂. Active target for human exploration (NASA Artemis precursor, SpaceX Starship).

Giant Planets — Outer Solar System

• Jupiter: The largest planet — 2.5× more massive than all other planets combined. 95 confirmed moons (2024), including Europa with its subsurface ocean.
• Saturn: The ringed giant. Its ring system extends 282,000 km from center with only 10–100 m thickness. Density so low it would float on water (0.69 g/cm³).
• Uranus: An ice giant tilted 98° on its axis — it essentially rolls along its orbit. Methane gives it its distinctive blue-green color.
• Neptune: The windiest planet: 2,100 km/h winds. Its moon Triton orbits retrograde — almost certainly a captured Kuiper Belt Object.

How Planets Form: Core Accretion and Disk Instability

The dominant model for planet formation is core accretion:

1. Dust grains in the protoplanetary disk collide and stick, growing into pebbles, then planetesimals (km-scale bodies).
2. Planetesimals collide and merge into protoplanets over millions of years.
3. Beyond the ice line (where water freezes), protoplanets rapidly acquire thick gas envelopes from the disk — forming gas and ice giants.
4. Closer to the star, the disk is drier and hotter; rocky planets form more slowly and retain less gas.

The entire process completes in roughly 100 million years — fast by geological standards. The oldest rocks found on Earth date to ~4.4 billion years ago; the Solar System itself formed ~4.57 billion years ago.

Exoplanets: Planets Beyond Our Solar System

As of 2024, the NASA Exoplanet Archive lists over 5,700 confirmed exoplanets. The distribution of types reveals something surprising: the most common type of planet in the galaxy appears to be the super-Earth / mini-Neptune (1.5–4 Earth radii) — a category entirely absent from our own Solar System.

Key exoplanet classes:

• Hot Jupiters: Gas giants orbiting within 0.1 AU of their star. Not predicted by early models; their existence forced a revision of planet migration theory.
• Super-Earths: Rocky planets larger than Earth but smaller than Neptune. TRAPPIST-1 (39 light-years away) hosts seven Earth-sized planets, three in the habitable zone.
• Ocean worlds: Theoretical planets covered entirely by water oceans hundreds of km deep — suggested by some Kepler data for certain super-Earths.

The James Webb Space Telescope (JWST) has begun characterizing exoplanet atmospheres via transmission spectroscopy — detecting CO₂, methane, and water vapor signatures that could eventually point to biosignatures.

Observe Planets from the Atacama Desert

Five of the Solar System's eight planets are visible to the naked eye from the Atacama: Mercury, Venus, Mars, Jupiter, and Saturn. Under Bortle Class 1 skies, they stand out brilliantly against the Milky Way band. With our Celestron AVX 11" telescopes, the details become extraordinary:

• Jupiter's cloud bands and four Galilean moons change position nightly
• Saturn's rings tilted open, the Cassini Division clearly resolved
• Mars shows polar ice caps and albedo features during opposition

At Atacama Stargazing, our guided tours include a live tour of whatever planets are visible that night, explained by professional astronomers in the context of current space missions and planetary science.

Book your planetary observation tour in Atacama — and see the Solar System live, not in a textbook.

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Jupiter, Saturn, and Mars visible from the Atacama

The planets you just learned about are visible to the naked eye from the Atacama Desert on clear nights. With professional telescopes you can see Saturn's rings and Jupiter's moons in stunning detail. Read our stargazing guide to plan your visit.

Observe planets from the Atacama Desert →