Why is Pluto no longer a planet?

In 2006 the International Astronomical Union redefined "planet" to require three criteria: orbiting the Sun, hydrostatic equilibrium (roughly spherical shape), and gravitational dominance — clearing the neighbourhood of its orbit. Pluto meets the first two but not the third: it shares the Kuiper Belt with thousands of other icy objects. It was reclassified as a dwarf planet, joining Ceres, Eris, Makemake, and Haumea in that category.

What separates rocky planets from gas giants?

The four inner (terrestrial) planets — Mercury, Venus, Earth, Mars — are rocky worlds with solid surfaces, thin or no rings, and relatively few moons. They formed from the denser dust and rock that condensed close to the young Sun. The four outer planets are much larger: Jupiter and Saturn are gas giants (mostly hydrogen and helium), while Uranus and Neptune are ice giants (more water, methane, and ammonia ices with a smaller hydrogen/helium envelope). The outer planets captured vast amounts of gas before the Sun's solar wind dispersed the protoplanetary disk.

Which planet is most like Earth?

Venus is closest in size and mass to Earth (diameter 95% of Earth's) and is sometimes called Earth's twin — but its surface is hellishly hot (462°C average), with crushing atmospheric pressure 92 times Earth's and an atmosphere of mostly carbon dioxide. In terms of habitability potential, Mars is more promising: evidence of ancient river valleys suggests liquid water once flowed there, and Mars has the thinnest atmosphere (~1% of Earth's) that future humans might work in with pressure suits. See our exoplanet guide for how astronomers compare other solar systems to ours.

How long would it take to travel to Neptune?

Neptune is about 30 AU from the Sun — roughly 4.5 billion km. Voyager 2 reached Neptune in about 12 years after launching in 1977 on a gravity-assist trajectory past Jupiter, Saturn, and Uranus. A direct trajectory using current chemical rockets would take longer. NASA's New Horizons spacecraft (launched 2006) passed Pluto in 9.5 years; it used Jupiter's gravity for a boost. Convert distances with our Distance Converter tool — Neptune's light-travel time is about 4 hours from the Sun.

Scale 02 · Solar System Beginner

The eight planets at a glance

Four rocky worlds close in, four giants far out. Each planet is a unique world — here are the key facts about all eight, with what makes each one remarkable.

See all planets Data & Charts →

All eight planets

Planet Distance Radius Year length Type

Mercury 0.39 AU 2,439 km 88 days Rocky

Venus 0.72 AU 6,052 km 225 days Rocky

Earth 1.00 AU 6,371 km 365 days Rocky

Mars 1.52 AU 3,390 km 687 days Rocky

Jupiter 5.20 AU 69,911 km 11.9 yr Gas giant

Saturn 9.58 AU 58,232 km 29.5 yr Gas giant

Uranus 19.2 AU 25,362 km 84 yr Ice giant

Neptune 30.1 AU 24,622 km 165 yr Ice giant

Rocky

Mercury

Smallest planet; extreme temperature swings from −180°C to 430°C.

Rocky

Venus

Hottest planet (462°C average) due to runaway greenhouse effect. Rotates backward.

Rocky

Earth

Only known planet with life. Surface liquid water. One large Moon.

Rocky

Mars

Tallest volcano (Olympus Mons, 22 km) and longest canyon in the Solar System.

Gas giant

Jupiter

Largest planet. Great Red Spot storm. 95+ moons including Europa.

Gas giant

Saturn

Iconic ring system of ice and rock. Least dense planet (floats on water).

Ice giant

Uranus

Tilted 98° — rolls around the Sun on its side. 27 known moons.

Ice giant

Neptune

Strongest winds in Solar System (~2,100 km/h). Farthest planet; 16-hr day.

For interactive comparison charts — diameter, mass, distance, and gravity — see the Data & Charts page. Use our Distance Converter to calculate how far any planet is from Earth today in light-travel time.

Frequently asked questions

: In 2006 the International Astronomical Union redefined "planet" to require three criteria: orbiting the Sun, hydrostatic equilibrium (roughly spherical shape), and gravitational dominance — clearing the neighbourhood of its orbit. Pluto meets the first two but not the third: it shares the Kuiper Belt with thousands of other icy objects. It was reclassified as a dwarf planet, joining Ceres, Eris, Makemake, and Haumea in that category.
: The four inner (terrestrial) planets — Mercury, Venus, Earth, Mars — are rocky worlds with solid surfaces, thin or no rings, and relatively few moons. They formed from the denser dust and rock that condensed close to the young Sun. The four outer planets are much larger: Jupiter and Saturn are gas giants (mostly hydrogen and helium), while Uranus and Neptune are ice giants (more water, methane, and ammonia ices with a smaller hydrogen/helium envelope). The outer planets captured vast amounts of gas before the Sun's solar wind dispersed the protoplanetary disk.
: Venus is closest in size and mass to Earth (diameter 95% of Earth's) and is sometimes called Earth's twin — but its surface is hellishly hot (462°C average), with crushing atmospheric pressure 92 times Earth's and an atmosphere of mostly carbon dioxide. In terms of habitability potential, Mars is more promising: evidence of ancient river valleys suggests liquid water once flowed there, and Mars has the thinnest atmosphere (~1% of Earth's) that future humans might work in with pressure suits. See our exoplanet guide for how astronomers compare other solar systems to ours.
: Neptune is about 30 AU from the Sun — roughly 4.5 billion km. Voyager 2 reached Neptune in about 12 years after launching in 1977 on a gravity-assist trajectory past Jupiter, Saturn, and Uranus. A direct trajectory using current chemical rockets would take longer. NASA's New Horizons spacecraft (launched 2006) passed Pluto in 9.5 years; it used Jupiter's gravity for a boost. Convert distances with our Distance Converter tool — Neptune's light-travel time is about 4 hours from the Sun.

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