Our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All eight orbit the Sun in the same general direction, but they are far from being variations of the same world. The four inner planets are small, rocky bodies with solid surfaces, while the four outer planets are much larger worlds composed mainly of gases, liquids and materials astronomers call ices.
The planets also differ enormously in size, temperature and speed. Mercury completes a year in only 88 Earth days, while Neptune takes almost 165 Earth years. Jupiter is more than 29 times wider than Mercury, and a day on fast-spinning Jupiter lasts less than ten hours. Comparing these differences helps reveal the basic structure of our planetary system.
The Planets in Order From the Sun
The eight planets in order of increasing average distance from the Sun are:
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
One common mnemonic for remembering the order is: “My Very Educated Mother Just Served Us Noodles.” The first letter of each word matches the first letter of a planet.
The first four planets occupy the relatively compact inner solar system. Beyond Mars lies the main asteroid belt, followed by the four giant planets. Neptune orbits about 30 times farther from the Sun than Earth does, but even Neptune is not the edge of the solar system. Dwarf planets, comets and countless smaller icy bodies continue far beyond its orbit.
What Makes an Object a Planet?
Under the definition adopted by the International Astronomical Union in 2006, a planet in our solar system must orbit the Sun, have enough gravity to become nearly round, and have cleared the neighborhood around its orbit. The last requirement means that it must be gravitationally dominant along its orbital path.
Pluto satisfies the first two conditions but not the third because it shares its region with many objects in the Kuiper Belt. It is therefore classified as a dwarf planet rather than one of the eight planets. This is a scientific classification based on orbital behavior, not a suggestion that Pluto has become less interesting or important.
The Planets Form Three Main Groups
The planets fall into three useful groups. Mercury, Venus, Earth and Mars are terrestrial, or rocky, planets. They are relatively small and dense, with solid surfaces and metallic cores. Jupiter and Saturn are gas giants made predominantly of hydrogen and helium. Uranus and Neptune are ice giants: they contain a larger proportion of water, ammonia and methane-related materials beneath their atmospheres than the gas giants do.
These groups reflect the conditions under which the planets formed. Close to the young Sun, it was warm enough that mainly rock and metal could condense into planets. Farther out, colder conditions allowed more volatile material to accumulate, helping the outer planets grow far larger.
Planet Sizes and Orbits Compared
Distances in the table are average distances from the Sun. One astronomical unit (AU) is the average distance between Earth and the Sun, approximately 149.6 million kilometers (93 million miles).
| Planet | Type | Diameter | Average distance | Length of year |
|---|---|---|---|---|
| Mercury | Rocky | 4,879 km | 0.39 AU | 88 Earth days |
| Venus | Rocky | 12,104 km | 0.72 AU | 225 Earth days |
| Earth | Rocky | 12,756 km | 1.00 AU | 365.25 days |
| Mars | Rocky | 6,794 km | 1.52 AU | 687 Earth days |
| Jupiter | Gas giant | 142,984 km | 5.20 AU | 11.86 Earth years |
| Saturn | Gas giant | 120,536 km | 9.58 AU | 29.45 Earth years |
| Uranus | Ice giant | 51,118 km | 19.2 AU | 84 Earth years |
| Neptune | Ice giant | 49,528 km | 30.1 AU | 164.8 Earth years |
Planet Rotation and Temperatures Compared
The rotation period shows approximately how long each planet takes to turn once on its axis. It is not always the same as the time from one sunrise to the next, as Mercury and Venus demonstrate. The temperatures are rounded planetary means rather than forecasts of conditions everywhere on each world; Mercury’s mean, for example, conceals a vast difference between day and night. For Jupiter, Saturn, Uranus and Neptune, which have no solid surface, the figures refer to an atmospheric level where the pressure is comparable to sea-level pressure on Earth.
| Planet | Rotation period | Mean temperature |
|---|---|---|
| Mercury | 59 Earth days | 167°C |
| Venus | 243 Earth days, retrograde | 464°C |
| Earth | 23.9 hours | 15°C |
| Mars | 24.6 hours | −65°C |
| Jupiter | 9.9 hours | −110°C |
| Saturn | 10.7 hours | −140°C |
| Uranus | 17.2 hours, retrograde | −195°C |
| Neptune | 16.1 hours | −200°C |
The Four Rocky Planets
Mercury Is the Smallest and Closest Planet
Mercury is the smallest planet and the one nearest the Sun. Its battered, cratered surface resembles the Moon, but Mercury has a large metallic core that occupies much of its interior. With only an extremely thin exosphere to retain heat, surface temperatures vary from about 430°C (800°F) during the day to around −180°C (−290°F) at night.
Mercury takes about 59 Earth days to rotate once, yet its motion around the Sun makes the interval from one sunrise to the next much longer. One solar day on Mercury lasts 176 Earth days, exactly twice the length of its 88-day year. Mercury is visible without a telescope, although it can be difficult to spot because it always remains fairly close to the Sun in our sky.
Venus Is the Hottest Planet
Venus is almost the same size as Earth, but its surface conditions are radically different. Its dense carbon-dioxide atmosphere produces an extreme greenhouse effect, raising the average surface temperature to about 464°C (867°F). This makes Venus hotter than Mercury even though Mercury is closer to the Sun.
Venus rotates extremely slowly and in the opposite direction to most planets, so the Sun would appear to rise in the west and set in the east. One axial rotation takes about 243 Earth days, while the time from one sunrise to the next is about 117 Earth days. Scientists do not know precisely why Venus acquired this retrograde rotation. Early impacts and the long-term effects of solar and atmospheric tides may all have contributed, so it should not be attributed confidently to a single collision.
Earth Has Liquid Surface Water and Life
Earth is the largest of the rocky planets and the only world known to support life. Liquid water covers about 71 percent of its surface, while its atmosphere, global water cycle and magnetic field help maintain conditions in which complex life can survive. Earth’s active plate tectonics continually reshapes the crust and recycles material between the surface and interior.
Earth rotates once in approximately 23.9 hours and completes an orbit in about 365.25 days. Its axial tilt of roughly 23.4 degrees produces the annual cycle of seasons. Earth also has one large natural satellite, the Moon, whose gravity drives most ocean tides and helps stabilize the planet’s axial orientation.
Mars Preserves Evidence of a Wetter Past
Mars is a cold desert world colored red by iron minerals in its soil. It has the largest volcano in the solar system, Olympus Mons, and the enormous canyon network Valles Marineris. Although liquid water is not stable across most of its surface today, dried river valleys, lake beds and water-altered minerals show that ancient Mars was once wetter.
A Martian day lasts about 24.6 hours, making its daily cycle remarkably similar to Earth’s, but its year lasts 687 Earth days. Mars has a thin atmosphere composed mainly of carbon dioxide, two small moons and polar caps containing water ice and frozen carbon dioxide. It is readily visible to the unaided eye and appears especially bright around favorable oppositions.
The Four Giant Planets
Jupiter Is the Largest Planet
Jupiter is the largest planet in the solar system, with a diameter about 11 times that of Earth. It is composed mainly of hydrogen and helium and has no solid surface on which a spacecraft could land. Its rapidly rotating atmosphere forms alternating cloud bands, powerful storms and the Great Red Spot, a long-lived storm larger than Earth.
Despite its immense size, Jupiter has the shortest rotation period of any planet, completing a turn in about 9.9 hours. It also has a faint ring system and a large family of moons. The four largest—Io, Europa, Ganymede and Callisto—are visible through modest binoculars or a small telescope and can noticeably change position from night to night.
Saturn Has the Most Spectacular Rings
Saturn is the second-largest planet and is best known for its extensive rings. The rings appear continuous from Earth, but they consist of countless particles made mostly of water ice, with some rocky material and dust, ranging from tiny grains to much larger fragments. Other giant planets also have rings, but Saturn’s are wider, brighter and much easier to observe.
Saturn rotates in about 10.7 hours and takes roughly 29.5 Earth years to orbit the Sun. Its rapid rotation and fluid structure give the planet a visibly flattened shape. A small telescope can reveal its rings, making Saturn one of the most distinctive and rewarding objects for a beginning observer.
Uranus Rotates on Its Side
Uranus is an ice giant with a blue-green appearance produced partly by methane in its atmosphere. Its rotation axis is tilted by about 98 degrees, leaving the planet effectively rotating on its side. This extreme tilt may have resulted from a major collision or a series of gravitational interactions early in the solar system’s history.
Uranus completes one rotation in about 17.2 hours and, like Venus, rotates in the direction opposite to most planets. Its 84-year orbit and unusual tilt create seasons lasting roughly two decades. Uranus has a system of faint rings and can technically reach naked-eye visibility under exceptionally dark skies, but binoculars or a telescope and an accurate finder chart are normally needed to identify it.
Neptune Is the Most Distant Planet
Neptune is the eighth and most distant planet, orbiting the Sun at an average distance of about 30 AU. It is slightly smaller than Uranus but more massive. Methane contributes to its blue appearance, while its atmosphere contains rapidly moving clouds, large storms and the fastest measured planetary winds, which can exceed 1,200 miles (2,000 kilometers) per hour.
Neptune rotates once in about 16 hours, but one journey around the Sun takes nearly 165 Earth years. It was predicted mathematically from its gravitational effect on Uranus before Johann Galle observed it in 1846 using calculations sent by Urbain Le Verrier; John Couch Adams had independently made a similar prediction. Neptune is far too faint to see without optical aid and requires a telescope to appear as more than a star-like point.
Planet Sizes Are Easier to Compare Than Their Distances
The enormous difference between planetary sizes and interplanetary distances makes the complete solar system difficult to show at one consistent scale. If Earth were represented by a sphere only 1 centimeter wide, Jupiter would be about 11 centimeters wide—but it would need to be placed roughly 600 meters from the model Sun. Neptune would be several kilometers away.
For this reason, most solar system diagrams enlarge the planets and compress the spaces between their orbits. Such illustrations are useful for showing order and grouping, but they should clearly state when sizes or distances are not to scale. The second diagram compares the planets at a common size scale so that the dominance of Jupiter and Saturn, and the relative similarity of Earth and Venus, are immediately visible.
Five Planets Can Be Seen Without a Telescope
Mercury, Venus, Mars, Jupiter and Saturn are the five planets traditionally visible to the unaided eye. They usually look like bright stars, but their positions gradually change against the more distant background constellations. Venus is normally the brightest planet, while Jupiter can also become exceptionally prominent. Mars often has a noticeable orange-red color, and Saturn generally appears steadier and less brilliant.
The planets follow the ecliptic, the same broad path through the sky taken by the Sun and the zodiac constellations. Because Mercury and Venus orbit closer to the Sun than Earth does, they are seen only before sunrise or after sunset. Mars, Jupiter and Saturn can appear much farther from the Sun in our sky and may remain visible for much of the night. Binoculars enhance many planetary views, while even a modest telescope can reveal Venus’s phases, Jupiter’s major moons and cloud belts, Saturn’s rings, and surface markings on Mars under favorable conditions.
The Planets Are Only Part of the Solar System
The eight planets contain most of the mass orbiting the Sun, but they are only one part of a much richer system. Dwarf planets, moons, asteroids, comets, rings, dust and distant icy bodies preserve evidence of how the solar system formed and changed over approximately 4.6 billion years.
Taken together, the planets record that history in different ways. The rocky planets reveal what happened in the warmer inner region around the young Sun, while the giant planets preserve material gathered in the colder outer system. Their present sizes, compositions, rotations and orbits are therefore not just isolated facts: they are clues to the origin and structure of our solar system.