The Solar System

The Sun, the planets, moons, asteroids and comets: a guided tour of our cosmic neighborhood.

The Sun is a star at the center of our solar system.
There are eight recognized planets orbiting the Sun.
Mercury is the smallest planet and closest to the Sun.
Venus is the hottest planet due to its thick greenhouse atmosphere.
Earth is the only known planet to support life.
Mars is known as the Red Planet because of its iron-rich soil.
Jupiter is the largest planet in the solar system.
Saturn is famous for its prominent system of icy rings.
Uranus rotates on its side compared to the other planets.
Neptune is the farthest recognized planet from the Sun.
The Moon is Earth's only natural satellite.
Comets are icy bodies that form glowing tails near the Sun.
Asteroids are rocky bodies, many orbiting between Mars and Jupiter.
Pluto was reclassified as a dwarf planet in 2006.
The Earth takes about 365 days to orbit the Sun once.
The Sun's energy comes from nuclear fusion of hydrogen into helium.
The Sun contains more than 99 percent of the total mass of the solar system.
The Sun is composed mainly of hydrogen and helium by mass.
The Sun's core temperature reaches roughly 15 million degrees Celsius.
Light from the Sun takes about eight minutes to reach Earth.
The Sun's surface layer, the photosphere, has a temperature near 5,500 degrees Celsius.
The Sun's outer atmosphere is called the corona and is far hotter than its surface.
Sunspots are cooler, darker regions on the Sun caused by magnetic activity.
The solar wind is a stream of charged particles flowing outward from the Sun.
The Sun rotates faster at its equator than at its poles.
The four inner planets are rocky and are called the terrestrial planets.
The four outer planets are large and are called the gas and ice giants.
Jupiter and Saturn are classified as gas giants.
Uranus and Neptune are classified as ice giants.
Mercury has almost no atmosphere to retain heat at night.
Mercury experiences extreme temperature swings between its day and night sides.
Mercury orbits the Sun in about 88 Earth days.
A single day on Mercury lasts longer than its year.
Mercury is heavily cratered, resembling the surface of the Moon.
Venus rotates in the opposite direction to most planets, a motion called retrograde.
Venus has the longest rotation period of any planet, about 243 Earth days.
Venus is often called Earth's sister planet because of its similar size.
Venus is shrouded by thick clouds of sulfuric acid.
Surface temperatures on Venus exceed 460 degrees Celsius.
Venus has a crushing atmospheric pressure about 90 times that of Earth.
Earth orbits the Sun at an average distance of about 150 million kilometers.
This average Earth-Sun distance is defined as one astronomical unit.
Earth's atmosphere is composed mostly of nitrogen and oxygen.
Earth's axis is tilted about 23.5 degrees, which causes the seasons.
Earth's magnetic field protects the surface from harmful solar radiation.
Earth rotates once on its axis approximately every 24 hours.
Liquid water covers about 71 percent of Earth's surface.
Mars has the largest volcano in the solar system, Olympus Mons.
Olympus Mons stands roughly three times the height of Mount Everest.
Mars has a vast canyon system called Valles Marineris.
Mars has two small moons named Phobos and Deimos.
A day on Mars lasts about 24 hours and 37 minutes.
Mars has polar ice caps made of water and frozen carbon dioxide.
Mars has a thin atmosphere composed mainly of carbon dioxide.
Mars takes about 687 Earth days to complete one orbit.
Jupiter is more than twice as massive as all other planets combined.
Jupiter's Great Red Spot is a giant storm larger than Earth.
Jupiter has the shortest day of any planet, under 10 hours.
Jupiter has dozens of known moons orbiting it.
Jupiter's four largest moons are called the Galilean moons.
The Galilean moons are Io, Europa, Ganymede, and Callisto.
Ganymede is the largest moon in the solar system.
Io is the most volcanically active body in the solar system.
Europa is thought to have a liquid water ocean beneath its icy crust.
Jupiter has a faint ring system discovered by the Voyager 1 spacecraft.
Saturn is the least dense planet and would float in water.
Saturn's rings are made mostly of ice particles and rocky debris.
Titan is the only moon known to have a dense atmosphere.
Titan has lakes and seas of liquid methane and ethane.
Saturn has more confirmed moons than any other planet.
Enceladus is a moon of Saturn that ejects water vapor from its surface.
Saturn takes about 29 Earth years to orbit the Sun once.
Uranus orbits the Sun roughly every 84 Earth years.
Uranus appears blue-green because of methane in its atmosphere.
Uranus was the first planet discovered with a telescope.
Uranus has a faint set of dark rings.
The extreme tilt of Uranus causes decades-long seasons.
Neptune was the first planet found through mathematical prediction.
Neptune has the strongest winds recorded in the solar system.
Triton orbits Neptune in the opposite direction to the planet's rotation.
Neptune takes about 165 Earth years to orbit the Sun.
Neptune's blue color comes mainly from methane in its atmosphere.
The asteroid belt lies between the orbits of Mars and Jupiter.
Ceres is the largest object in the asteroid belt.
Vesta is one of the largest asteroids in the asteroid belt.
Most asteroids are irregularly shaped rather than spherical.
A meteoroid is a small rocky or metallic body traveling through space.
A meteor is the streak of light produced when a meteoroid burns in the atmosphere.
A meteorite is a meteoroid fragment that survives and lands on a planet's surface.
Meteor showers occur when Earth passes through debris left by comets.
The Kuiper Belt is a region of icy bodies beyond Neptune's orbit.
Pluto orbits the Sun within the Kuiper Belt.
Charon is about half the diameter of Pluto.
Pluto takes about 248 Earth years to orbit the Sun.
Eris is a distant dwarf planet roughly the same size as Pluto.
Makemake and Haumea are dwarf planets in the Kuiper Belt.
The Oort Cloud is a distant spherical shell of icy bodies surrounding the solar system.
Many long-period comets are thought to originate in the Oort Cloud.
A comet's tail always points away from the Sun.
Comets develop two tails, one of gas and one of dust.
The solid center of a comet is called the nucleus.
Halley's Comet returns to the inner solar system about every 76 years.
Halley's Comet was last visible from Earth in 1986.
The glowing cloud of gas around a comet's nucleus is called the coma.
Gravity is the force that holds planets in orbit around the Sun.
Planets orbit the Sun in elliptical, not perfectly circular, paths.
Johannes Kepler described the laws of planetary motion in the early 1600s.
Kepler's first law states that planets move in ellipses with the Sun at one focus.
Isaac Newton explained orbital motion through his law of universal gravitation.
The point in an orbit closest to the Sun is called perihelion.
The point in an orbit farthest from the Sun is called aphelion.
Nicolaus Copernicus proposed that the planets orbit the Sun, not Earth.
The Sun-centered model of the solar system is called the heliocentric model.
Galileo Galilei used a telescope to observe the moons of Jupiter in 1610.
Galileo's observations supported the heliocentric model of the solar system.
The plane in which most planets orbit the Sun is called the ecliptic.
All eight planets orbit the Sun in the same direction.
Viewed from above the Sun's north pole, the planets orbit counterclockwise.
The Moon orbits Earth about once every 27 days.
The same side of the Moon always faces Earth, a result of tidal locking.
The Moon's surface is covered with craters formed by impacts.
The dark plains on the Moon are called maria.
A lunar eclipse occurs when Earth passes between the Sun and the Moon.
A solar eclipse occurs when the Moon passes between the Sun and Earth.
The Moon's gravity is the main cause of ocean tides on Earth.
The Moon is about one quarter the diameter of Earth.
Astronauts first walked on the Moon during the Apollo 11 mission in 1969.
Neil Armstrong was the first person to set foot on the Moon.
The phases of the Moon result from its changing illumination by the Sun.
A full moon occurs when the Moon is opposite the Sun in the sky.
A new moon occurs when the Moon lies between Earth and the Sun.
The Sun's diameter is roughly 109 times that of Earth.
The Sun is classified as a yellow dwarf star.
Energy produced in the Sun's core takes thousands of years to reach the surface.
The Sun's gravity keeps the entire solar system bound together.
Mercury is named after the Roman messenger god.
Venus is named after the Roman goddess of love and beauty.
Mars is named after the Roman god of war.
Jupiter is named after the king of the Roman gods.
Saturn is named after the Roman god of agriculture.
Neptune is named after the Roman god of the sea.
The Voyager 1 spacecraft launched in 1977 to study the outer planets.
Voyager 1 has become the most distant human-made object from Earth.
The Voyager probes carry golden records with sounds and images of Earth.
The Cassini spacecraft studied Saturn and its moons for over a decade.
The New Horizons spacecraft flew past Pluto in 2015.
The Curiosity rover has been exploring the surface of Mars since 2012.
The Hubble Space Telescope has provided detailed images of solar system bodies.
Mariner 2 was the first spacecraft to fly past another planet, Venus, in 1962.
The first artificial satellite, Sputnik 1, was launched in 1957.
Mars rovers have found evidence that liquid water once flowed on the surface.
Jupiter's strong magnetic field is the largest of any planet.
Saturn's rings span hundreds of thousands of kilometers but are very thin.
The gaps in Saturn's rings are shaped by the gravity of its moons.
Phobos orbits Mars closer than any other moon orbits its planet.
The giant planets have no solid surface to stand on.
Hydrogen and helium make up most of Jupiter's and Saturn's composition.
The dwarf planet Ceres was once considered a planet after its discovery in 1801.
A planet must clear its orbital neighborhood to meet the modern definition.
Dwarf planets orbit the Sun but have not cleared their orbital paths.
The asteroid belt contains far less total mass than Earth's Moon.
Trojan asteroids share an orbit with a planet, often Jupiter.
Near-Earth objects are asteroids and comets that pass close to Earth's orbit.
A large asteroid impact is widely linked to the extinction of the dinosaurs.
The temperature of a planet generally decreases with distance from the Sun.
Mercury's surface holds water ice in permanently shadowed polar craters.
Venus is the brightest natural object in Earth's night sky after the Moon.
Venus is sometimes called the morning star or the evening star.
Earth is the densest planet in the solar system.
The Sun makes up the vast majority of the solar system's angular momentum's source mass.
Jupiter's moon Callisto is one of the most heavily cratered bodies known.
Saturn's rings are divided into named groups labeled with letters.
Uranus has 27 known moons, many named after literary characters.
Neptune was visited by only one spacecraft, Voyager 2, in 1989.
Voyager 2 is the only spacecraft to have flown past Uranus.
The giant planets all possess ring systems of varying brightness.
A planet's gravitational pull depends on its mass and size.
The escape velocity needed to leave a planet increases with the planet's mass.
The solar system moves through the Milky Way galaxy as it orbits the galactic center.
The Sun lies in one of the spiral arms of the Milky Way galaxy.
It takes the Sun roughly 225 million years to orbit the galactic center once.
An astronomical unit is the standard unit for measuring distances within the solar system.
Neptune lies about 30 astronomical units from the Sun.
The combined gravity of the planets slightly affects the Sun's position.
Tidal forces arise from differences in gravity across a body.
The Roche limit is the distance within which tidal forces can break a body apart.
Saturn's rings may have formed from a moon that came too close to the planet.
Most of the solar system's planets have at least one natural satellite.
Mercury and Venus are the only planets without natural moons.
A planet's orbital speed is fastest when it is closest to the Sun.
The plane of the solar system is relatively flat because it formed from a spinning disk.
This early flattened disk of gas and dust is called the protoplanetary disk.
Planets grew by the gradual accumulation of material in a process called accretion.
Rocky planets formed inside the frost line where ices could not condense.
Gas giants formed beyond the frost line where ices were abundant.
Comets are sometimes described as leftover icy debris from the solar system's formation.
A planet's albedo measures how much sunlight its surface reflects.
Venus has a very high albedo because of its reflective cloud cover.
The Sun's light and heat make life on Earth possible.
The habitable zone is the region around a star where liquid water can exist.

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The Solar System

The Sun, the planets, moons, asteroids and comets: a guided tour of our cosmic neighborhood.

The Sun is a star at the center of our solar system.
There are eight recognized planets orbiting the Sun.
Mercury is the smallest planet and closest to the Sun.
Venus is the hottest planet due to its thick greenhouse atmosphere.
Earth is the only known planet to support life.
Mars is known as the Red Planet because of its iron-rich soil.
Jupiter is the largest planet in the solar system.
Saturn is famous for its prominent system of icy rings.
Uranus rotates on its side compared to the other planets.
Neptune is the farthest recognized planet from the Sun.
The Moon is Earth's only natural satellite.
Comets are icy bodies that form glowing tails near the Sun.
Asteroids are rocky bodies, many orbiting between Mars and Jupiter.
Pluto was reclassified as a dwarf planet in 2006.
The Earth takes about 365 days to orbit the Sun once.
The Sun's energy comes from nuclear fusion of hydrogen into helium.
The Sun contains more than 99 percent of the total mass of the solar system.
The Sun is composed mainly of hydrogen and helium by mass.
The Sun's core temperature reaches roughly 15 million degrees Celsius.
Light from the Sun takes about eight minutes to reach Earth.
The Sun's surface layer, the photosphere, has a temperature near 5,500 degrees Celsius.
The Sun's outer atmosphere is called the corona and is far hotter than its surface.
Sunspots are cooler, darker regions on the Sun caused by magnetic activity.
The solar wind is a stream of charged particles flowing outward from the Sun.
The Sun rotates faster at its equator than at its poles.
The four inner planets are rocky and are called the terrestrial planets.
The four outer planets are large and are called the gas and ice giants.
Jupiter and Saturn are classified as gas giants.
Uranus and Neptune are classified as ice giants.
Mercury has almost no atmosphere to retain heat at night.
Mercury experiences extreme temperature swings between its day and night sides.
Mercury orbits the Sun in about 88 Earth days.
A single day on Mercury lasts longer than its year.
Mercury is heavily cratered, resembling the surface of the Moon.
Venus rotates in the opposite direction to most planets, a motion called retrograde.
Venus has the longest rotation period of any planet, about 243 Earth days.
Venus is often called Earth's sister planet because of its similar size.
Venus is shrouded by thick clouds of sulfuric acid.
Surface temperatures on Venus exceed 460 degrees Celsius.
Venus has a crushing atmospheric pressure about 90 times that of Earth.
Earth orbits the Sun at an average distance of about 150 million kilometers.
This average Earth-Sun distance is defined as one astronomical unit.
Earth's atmosphere is composed mostly of nitrogen and oxygen.
Earth's axis is tilted about 23.5 degrees, which causes the seasons.
Earth's magnetic field protects the surface from harmful solar radiation.
Earth rotates once on its axis approximately every 24 hours.
Liquid water covers about 71 percent of Earth's surface.
Mars has the largest volcano in the solar system, Olympus Mons.
Olympus Mons stands roughly three times the height of Mount Everest.
Mars has a vast canyon system called Valles Marineris.
Mars has two small moons named Phobos and Deimos.
A day on Mars lasts about 24 hours and 37 minutes.
Mars has polar ice caps made of water and frozen carbon dioxide.
Mars has a thin atmosphere composed mainly of carbon dioxide.
Mars takes about 687 Earth days to complete one orbit.
Jupiter is more than twice as massive as all other planets combined.
Jupiter's Great Red Spot is a giant storm larger than Earth.
Jupiter has the shortest day of any planet, under 10 hours.
Jupiter has dozens of known moons orbiting it.
Jupiter's four largest moons are called the Galilean moons.
The Galilean moons are Io, Europa, Ganymede, and Callisto.
Ganymede is the largest moon in the solar system.
Io is the most volcanically active body in the solar system.
Europa is thought to have a liquid water ocean beneath its icy crust.
Jupiter has a faint ring system discovered by the Voyager 1 spacecraft.
Saturn is the least dense planet and would float in water.
Saturn's rings are made mostly of ice particles and rocky debris.
Titan is the only moon known to have a dense atmosphere.
Titan has lakes and seas of liquid methane and ethane.
Saturn has more confirmed moons than any other planet.
Enceladus is a moon of Saturn that ejects water vapor from its surface.
Saturn takes about 29 Earth years to orbit the Sun once.
Uranus orbits the Sun roughly every 84 Earth years.
Uranus appears blue-green because of methane in its atmosphere.
Uranus was the first planet discovered with a telescope.
Uranus has a faint set of dark rings.
The extreme tilt of Uranus causes decades-long seasons.
Neptune was the first planet found through mathematical prediction.
Neptune has the strongest winds recorded in the solar system.
Triton orbits Neptune in the opposite direction to the planet's rotation.
Neptune takes about 165 Earth years to orbit the Sun.
Neptune's blue color comes mainly from methane in its atmosphere.
The asteroid belt lies between the orbits of Mars and Jupiter.
Ceres is the largest object in the asteroid belt.
Vesta is one of the largest asteroids in the asteroid belt.
Most asteroids are irregularly shaped rather than spherical.
A meteoroid is a small rocky or metallic body traveling through space.
A meteor is the streak of light produced when a meteoroid burns in the atmosphere.
A meteorite is a meteoroid fragment that survives and lands on a planet's surface.
Meteor showers occur when Earth passes through debris left by comets.
The Kuiper Belt is a region of icy bodies beyond Neptune's orbit.
Pluto orbits the Sun within the Kuiper Belt.
Charon is about half the diameter of Pluto.
Pluto takes about 248 Earth years to orbit the Sun.
Eris is a distant dwarf planet roughly the same size as Pluto.
Makemake and Haumea are dwarf planets in the Kuiper Belt.
The Oort Cloud is a distant spherical shell of icy bodies surrounding the solar system.
Many long-period comets are thought to originate in the Oort Cloud.
A comet's tail always points away from the Sun.
Comets develop two tails, one of gas and one of dust.
The solid center of a comet is called the nucleus.
Halley's Comet returns to the inner solar system about every 76 years.
Halley's Comet was last visible from Earth in 1986.
The glowing cloud of gas around a comet's nucleus is called the coma.
Gravity is the force that holds planets in orbit around the Sun.
Planets orbit the Sun in elliptical, not perfectly circular, paths.
Johannes Kepler described the laws of planetary motion in the early 1600s.
Kepler's first law states that planets move in ellipses with the Sun at one focus.
Isaac Newton explained orbital motion through his law of universal gravitation.
The point in an orbit closest to the Sun is called perihelion.
The point in an orbit farthest from the Sun is called aphelion.
Nicolaus Copernicus proposed that the planets orbit the Sun, not Earth.
The Sun-centered model of the solar system is called the heliocentric model.
Galileo Galilei used a telescope to observe the moons of Jupiter in 1610.
Galileo's observations supported the heliocentric model of the solar system.
The plane in which most planets orbit the Sun is called the ecliptic.
All eight planets orbit the Sun in the same direction.
Viewed from above the Sun's north pole, the planets orbit counterclockwise.
The Moon orbits Earth about once every 27 days.
The same side of the Moon always faces Earth, a result of tidal locking.
The Moon's surface is covered with craters formed by impacts.
The dark plains on the Moon are called maria.
A lunar eclipse occurs when Earth passes between the Sun and the Moon.
A solar eclipse occurs when the Moon passes between the Sun and Earth.
The Moon's gravity is the main cause of ocean tides on Earth.
The Moon is about one quarter the diameter of Earth.
Astronauts first walked on the Moon during the Apollo 11 mission in 1969.
Neil Armstrong was the first person to set foot on the Moon.
The phases of the Moon result from its changing illumination by the Sun.
A full moon occurs when the Moon is opposite the Sun in the sky.
A new moon occurs when the Moon lies between Earth and the Sun.
The Sun's diameter is roughly 109 times that of Earth.
The Sun is classified as a yellow dwarf star.
Energy produced in the Sun's core takes thousands of years to reach the surface.
The Sun's gravity keeps the entire solar system bound together.
Mercury is named after the Roman messenger god.
Venus is named after the Roman goddess of love and beauty.
Mars is named after the Roman god of war.
Jupiter is named after the king of the Roman gods.
Saturn is named after the Roman god of agriculture.
Neptune is named after the Roman god of the sea.
The Voyager 1 spacecraft launched in 1977 to study the outer planets.
Voyager 1 has become the most distant human-made object from Earth.
The Voyager probes carry golden records with sounds and images of Earth.
The Cassini spacecraft studied Saturn and its moons for over a decade.
The New Horizons spacecraft flew past Pluto in 2015.
The Curiosity rover has been exploring the surface of Mars since 2012.
The Hubble Space Telescope has provided detailed images of solar system bodies.
Mariner 2 was the first spacecraft to fly past another planet, Venus, in 1962.
The first artificial satellite, Sputnik 1, was launched in 1957.
Mars rovers have found evidence that liquid water once flowed on the surface.
Jupiter's strong magnetic field is the largest of any planet.
Saturn's rings span hundreds of thousands of kilometers but are very thin.
The gaps in Saturn's rings are shaped by the gravity of its moons.
Phobos orbits Mars closer than any other moon orbits its planet.
The giant planets have no solid surface to stand on.
Hydrogen and helium make up most of Jupiter's and Saturn's composition.
The dwarf planet Ceres was once considered a planet after its discovery in 1801.
A planet must clear its orbital neighborhood to meet the modern definition.
Dwarf planets orbit the Sun but have not cleared their orbital paths.
The asteroid belt contains far less total mass than Earth's Moon.
Trojan asteroids share an orbit with a planet, often Jupiter.
Near-Earth objects are asteroids and comets that pass close to Earth's orbit.
A large asteroid impact is widely linked to the extinction of the dinosaurs.
The temperature of a planet generally decreases with distance from the Sun.
Mercury's surface holds water ice in permanently shadowed polar craters.
Venus is the brightest natural object in Earth's night sky after the Moon.
Venus is sometimes called the morning star or the evening star.
Earth is the densest planet in the solar system.
The Sun makes up the vast majority of the solar system's angular momentum's source mass.
Jupiter's moon Callisto is one of the most heavily cratered bodies known.
Saturn's rings are divided into named groups labeled with letters.
Uranus has 27 known moons, many named after literary characters.
Neptune was visited by only one spacecraft, Voyager 2, in 1989.
Voyager 2 is the only spacecraft to have flown past Uranus.
The giant planets all possess ring systems of varying brightness.
A planet's gravitational pull depends on its mass and size.
The escape velocity needed to leave a planet increases with the planet's mass.
The solar system moves through the Milky Way galaxy as it orbits the galactic center.
The Sun lies in one of the spiral arms of the Milky Way galaxy.
It takes the Sun roughly 225 million years to orbit the galactic center once.
An astronomical unit is the standard unit for measuring distances within the solar system.
Neptune lies about 30 astronomical units from the Sun.
The combined gravity of the planets slightly affects the Sun's position.
Tidal forces arise from differences in gravity across a body.
The Roche limit is the distance within which tidal forces can break a body apart.
Saturn's rings may have formed from a moon that came too close to the planet.
Most of the solar system's planets have at least one natural satellite.
Mercury and Venus are the only planets without natural moons.
A planet's orbital speed is fastest when it is closest to the Sun.
The plane of the solar system is relatively flat because it formed from a spinning disk.
This early flattened disk of gas and dust is called the protoplanetary disk.
Planets grew by the gradual accumulation of material in a process called accretion.
Rocky planets formed inside the frost line where ices could not condense.
Gas giants formed beyond the frost line where ices were abundant.
Comets are sometimes described as leftover icy debris from the solar system's formation.
A planet's albedo measures how much sunlight its surface reflects.
Venus has a very high albedo because of its reflective cloud cover.
The Sun's light and heat make life on Earth possible.
The habitable zone is the region around a star where liquid water can exist.

Learn this lesson, not just read it

Load these facts straight into the free Class Coupon Speed Learning app, where they become fast, playful study activities with audio and translation in 100+ languages.

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