Planets

What this part of the map covers

A planet is a big, round world that orbits a star, and it can be made mostly of rock or mostly of gas.^[2] The word goes back to the ancient Greek planētēs, meaning “wanderer”.^{[3, 5]} Our solar system has eight planets and five officially recognised dwarf planets.^[4] Earth and Mars appear here only in brief.

How planets form

The leading explanation, sometimes called the nebular theory, is that the Sun and the planets formed from the same big cloud of gas and dust, called a nebula.^[2]

Gravity pulled parts of the cloud together, and it flattened into a spinning disk of dust and gas, sometimes called a protoplanetary disk.^[2]

Planets are thought to start as dust grains smaller than the width of a human hair.^[1] When grains collide gently enough they stick together, growing into pebbles and then into rocks about a mile across, the building blocks called planetesimals.^[1] The bigger a piece grows, the stronger its gravity and the more material it pulls in, a process called accretion.^[2]

Why the giants sit far out. Far from the young Sun, where it is cold enough for water to freeze, ice joined the dust and helped build giant planetary cores.^[1] The cold also let gas molecules slow down enough to be drawn onto those cores, which is how Jupiter, Saturn, Uranus and Neptune are thought to have formed.^[1] Jupiter and Saturn are thought to have formed first, within the first 10 million years, while rocky planets may have taken tens of millions of years.^[1] Uranus and Neptune likely formed closer to the Sun and moved outward about 4 billion years ago.^{[15, 16]} Exactly where in a disk planets prefer to form is still a mystery and an active area of research.^[1]

Rocky planets and giant planets

Mercury, Venus, Earth and Mars are the rocky, or terrestrial, planets.^{[10, 1]}

Jupiter and Saturn are gas giants, made mostly of hydrogen and helium.^[14] Uranus and Neptune are called ice giants: 80% or more of their mass is a hot, dense fluid of “icy” materials (water, methane and ammonia) above a small rocky core.^{[15, 16]} One NASA formation page calls all four outer planets gas giants, so the split into two kinds is about what they are made of.^{[1, 15]} None of the four giants has a true surface.^{[13, 14, 15, 16]} NASA’s planetary fact sheet, updated 18 March 2025, lists ring systems for all four giants and for none of the rocky planets.^[6]

A short tour

Earth and Mars, in brief. Earth is the third planet from the Sun and the fifth largest.^[3] Mars is the fourth planet and the seventh largest, about half the width of Earth.^{[3, 4]} The fact sheet gives Mars a year of 687.0 Earth days and a surface gravity of 3.7 m/s², against 9.8 m/s² for Earth.^[6]

Mercury is the smallest planet and the nearest to the Sun, only slightly larger than Earth’s Moon.^[9] It is also the fastest, circling the Sun every 88 Earth days.^[9] Its days can reach about 430 °C, and its nights can drop to about −180 °C.^[9]

Venus is about the same width as Earth, which is why it is sometimes called Earth’s twin.^[4] Its thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet, at about 467 °C on the surface.^[11] Venus spins slowly backwards, so its day of about 243 Earth days is longer than its year of 225 Earth days.^{[12, 11]}

Jupiter is the largest planet, about 11 times wider than Earth.^[4] It has the shortest day in the solar system, about 9.9 hours.^[13] Its Great Red Spot, a storm twice as wide as Earth, has been seen for more than 300 years.^[13]

Saturn, the second largest planet, is about nine times wider than Earth.^[4] It is the only planet with an average density less than water.^[14] Its rings are billions of chunks of ice and rock, reaching up to 282,000 km from the planet yet typically only about 10 m thick in the main rings.^[14]

Uranus is tilted by 97.77 degrees, so it seems to roll around the Sun on its side, and it has the most extreme seasons in the solar system.^[15] That tilt may come from a collision with an Earth-sized object long ago.^[15]

Neptune is the most distant planet, more than 30 times as far from the Sun as Earth, and the only one that cannot be seen with the naked eye.^[16] It is the windiest world, with clouds of frozen methane blown along at more than 2,000 km/h.^[16]

Sizes and distances

NASA’s planet-size page, dated 22 April 2025, gives each planet’s width at the equator and its average distance from the Sun:^[4]

Planet	Equatorial diameter	Average distance from the Sun
Mercury	about 4,880 km	58 million km
Venus	about 12,104 km	108 million km
Earth	about 12,756 km	149.7 million km
Mars	about 6,792 km	227.9 million km
Jupiter	about 142,984 km	778 million km
Saturn	about 120,536 km	1.4 billion km
Uranus	about 51,118 km	2.9 billion km
Neptune	about 49,528 km	4.5 billion km

What counts as a planet

The International Astronomical Union (IAU), a world organisation of astronomers that names objects in our solar system, agreed a definition of a planet in 2006.^{[3, 7]} As NASA’s dwarf-planet page summarises it, a planet must orbit the Sun, be mostly round, and be big enough that its gravity has cleared away other objects of similar size near its orbit.^[7] A dwarf planet orbits the Sun and is nearly round but has not cleared its orbit.^[7] So far the IAU recognises five, in order of distance from the Sun: Ceres, Pluto, Haumea, Makemake and Eris.^[7]

Clyde Tombaugh discovered Pluto in 1930, and it was long counted as the ninth planet until the IAU reclassified it as a dwarf planet in 2006, leaving eight planets.^{[7, 3]} Pluto lies in the Kuiper Belt beyond Neptune, where other objects might cross its orbit.^[8] It is about 2,377 km across, roughly one-fifth the width of Earth.^[4] Ceres, the smallest of the five, is the only dwarf planet in the inner solar system, in the asteroid belt between Mars and Jupiter.^{[4, 3]}

Common misconceptions

“Mercury is the hottest planet because it is closest to the Sun.” The hottest is Venus, thanks to its dense atmosphere.^{[9, 10]}

“Pluto is always the farthest world.” Its oval orbit brought it closer to the Sun than Neptune from 1979 to 1999.^[16] The two can never collide, because Pluto makes two laps of the Sun for every three that Neptune makes.^[16]

“Ice giants are balls of solid ice.” Their “icy” materials are mostly a hot, dense fluid.^[15]

What this page does not cover

Earth and Mars get only their rank, size, distance and a few fact-sheet values here. The page does not cover moons, planets around other stars, asteroids and comets, or detailed models of how planets form.

Going further

See Solar System and The Sun for the wider family, Moons for the worlds that orbit planets, Orbits, Kepler’s Laws and Gravity for how planets move, Planet Earth for our own world and Space Exploration for the missions.