Solar SystemIdentifying the Known Planets

The SunThe Sun► Our solar system's star, the Sun, has Our solar system's star, the Sun, has

inspired mythological stories in cultures inspired mythological stories in cultures around the world, including those of the around the world, including those of the ancient Egyptians, the Aztecs of Mexico, ancient Egyptians, the Aztecs of Mexico, Native American tribes of North America, the Native American tribes of North America, the Chinese, and many others. A number of Chinese, and many others. A number of ancient cultures built stone structures or ancient cultures built stone structures or modified natural rock formations to observe modified natural rock formations to observe the Sun and Moon, they charted the seasons, the Sun and Moon, they charted the seasons, created calendars, and monitored solar and created calendars, and monitored solar and lunar eclipses. lunar eclipses.

► These architectural sites show evidence of These architectural sites show evidence of deliberate alignments to astronomical deliberate alignments to astronomical phenomena: sunrises, moonrises, moonsets, phenomena: sunrises, moonrises, moonsets, even stars or planets. The Sun is the even stars or planets. The Sun is the closest star to Earth, at a mean distance closest star to Earth, at a mean distance from our planet of 149.60 million kilometers from our planet of 149.60 million kilometers (92.96 million miles). This distance is (92.96 million miles). This distance is known as an astronomical unit (abbreviated known as an astronomical unit (abbreviated AU), and sets the scale for measuring AU), and sets the scale for measuring distances all across the solar system. distances all across the solar system.

► The Sun, a huge sphere of mostly ionized The Sun, a huge sphere of mostly ionized gas, supports life on Earth. It powers gas, supports life on Earth. It powers photosynthesis in green plants, and is photosynthesis in green plants, and is ultimately the source of all food and fossil ultimately the source of all food and fossil fuel. The connection and interactions fuel. The connection and interactions between the Sun and Earth drive the seasons, between the Sun and Earth drive the seasons, ocean currents, weather, and climate.ocean currents, weather, and climate.

MercuryMercury► Mercury's elliptical orbit takes the small Mercury's elliptical orbit takes the small

planet as close as 47 million kilometers (29 planet as close as 47 million kilometers (29 million miles) and as far as 70 million million miles) and as far as 70 million kilometers (43 million miles) from the kilometers (43 million miles) from the SunSun. .

► If one could stand on the scorching surface If one could stand on the scorching surface of Mercury when it is at its closest of Mercury when it is at its closest approach, the Sun would appear almost three approach, the Sun would appear almost three times as large as it does when viewed from times as large as it does when viewed from Earth. Temperatures on Mercury's surface can Earth. Temperatures on Mercury's surface can reach 430 degrees Celsius (800 degrees reach 430 degrees Celsius (800 degrees Fahrenheit). Because the planet has no Fahrenheit). Because the planet has no atmosphere to retain that heat, nighttime atmosphere to retain that heat, nighttime temperatures on the surface can drop to -170 temperatures on the surface can drop to -170 degrees Celsius (-280 degrees Fahrenheit). degrees Celsius (-280 degrees Fahrenheit).

► Because Mercury is so close to the Sun, it Because Mercury is so close to the Sun, it is hard to directly observe from is hard to directly observe from EarthEarth, , except during twilight. Mercury makes an except during twilight. Mercury makes an appearance indirectly, however 13 times each appearance indirectly, however 13 times each century, Earth observers can watch Mercury century, Earth observers can watch Mercury pass across the face of the Sun, an event pass across the face of the Sun, an event called a transit. These rare transits fall called a transit. These rare transits fall within several days of May 8 and November within several days of May 8 and November 10. The first two transits of Mercury in the 10. The first two transits of Mercury in the 21st century occurred in May 2003 and 21st century occurred in May 2003 and November 2006. The next transit will occur November 2006. The next transit will occur on May 9, 2016.on May 9, 2016.

VenusVenus► At first glance, if At first glance, if EarthEarth had a twin, it had a twin, it

would be Venus. The two planets are would be Venus. The two planets are similar in size, mass, composition, and similar in size, mass, composition, and distance from the distance from the SunSun. But there the . But there the similarities end. Venus has no ocean. similarities end. Venus has no ocean.

► Venus is covered by thick, rapidly Venus is covered by thick, rapidly spinning clouds that trap surface heat, spinning clouds that trap surface heat, creating a scorched greenhouse-like creating a scorched greenhouse-like world with temperatures hot enough to world with temperatures hot enough to melt lead and pressure so intense that melt lead and pressure so intense that standing on Venus would feel like the standing on Venus would feel like the pressure felt 900 meters deep in Earth's pressure felt 900 meters deep in Earth's oceans. These clouds reflect sunlight in oceans. These clouds reflect sunlight in addition to trapping heat. Because Venus addition to trapping heat. Because Venus reflects so much sunlight, it is usually reflects so much sunlight, it is usually the brightest planet in the night sky.the brightest planet in the night sky.

EarthEarth► Earth, our home planet, is the only Earth, our home planet, is the only

planet in our solar system known to planet in our solar system known to harbor life - life that is incredibly harbor life - life that is incredibly diverse. diverse.

► All of the things we need to survive are All of the things we need to survive are provided under a thin layer of provided under a thin layer of atmosphere that separates us from the atmosphere that separates us from the uninhabitable void of space. uninhabitable void of space.

► Earth is made up of complex, interactive Earth is made up of complex, interactive systems that are often unpredictable. systems that are often unpredictable. Air, water, land, and life - including Air, water, land, and life - including humans - combine forces to create a humans - combine forces to create a constantly changing world that we are constantly changing world that we are striving to understand. striving to understand.

► Viewing Earth from the unique Viewing Earth from the unique perspective of space provides the perspective of space provides the opportunity to see Earth as a whole. opportunity to see Earth as a whole. Scientists around the world have Scientists around the world have discovered many things about our planet discovered many things about our planet by working together and sharing their by working together and sharing their findings.findings.

► Some facts are well known. For instance, Some facts are well known. For instance, Earth is the third planet from the Sun Earth is the third planet from the Sun and the fifth largest in the solar and the fifth largest in the solar system. Earth's diameter is just a few system. Earth's diameter is just a few hundred kilometers larger than that of hundred kilometers larger than that of VenusVenus. The four seasons are a result of . The four seasons are a result of Earth's axis of rotation being tilted Earth's axis of rotation being tilted more than 23 degreesmore than 23 degrees

Earth’s MoonEarth’s Moon► The regular daily and monthly rhythms of The regular daily and monthly rhythms of

Earth'sEarth's only natural satellite, the only natural satellite, the Moon, have guided timekeepers for Moon, have guided timekeepers for thousands of years. thousands of years.

► Its influence on Earth's cycles, notably Its influence on Earth's cycles, notably tides, has also been charted by many tides, has also been charted by many cultures in many ages. More than 70 cultures in many ages. More than 70 spacecraft have been sent to the Moon; spacecraft have been sent to the Moon; 12 astronauts have walked upon its 12 astronauts have walked upon its surface and brought back 382 kg (842 surface and brought back 382 kg (842 pounds) of lunar rock and soil to Earth. pounds) of lunar rock and soil to Earth.

► The presence of the Moon stabilizes The presence of the Moon stabilizes Earth's wobble. This has led to a much Earth's wobble. This has led to a much more stable climate over billions of more stable climate over billions of years, which may have affected the years, which may have affected the course of the development and growth of course of the development and growth of life on Earth.life on Earth.

► On the side picture the false-On the side picture the false-colors show the Moon's soil and colors show the Moon's soil and mineral composition. Mare mineral composition. Mare Tranquillitatis - the 'Sea of Tranquillitatis - the 'Sea of Tranquility' - is the dark blue Tranquility' - is the dark blue region on the rightregion on the right

MarsMars► The red planet Mars has inspired wild flights The red planet Mars has inspired wild flights

of imagination over the centuries, as well as of imagination over the centuries, as well as intense scientific interest. Whether fancied intense scientific interest. Whether fancied to be the source of hostile invaders of to be the source of hostile invaders of Earth, the home of a dying civilization, or a Earth, the home of a dying civilization, or a rough-and-tumble mining colony of the future, rough-and-tumble mining colony of the future, Mars provides fertile ground for science Mars provides fertile ground for science fiction writers, based on seeds planted by fiction writers, based on seeds planted by centuries of scientific observations. centuries of scientific observations.

► We know that Mars is a small rocky body once We know that Mars is a small rocky body once thought to be very Earth-like. Like the other thought to be very Earth-like. Like the other "terrestrial" planets - "terrestrial" planets - MercuryMercury, , VenusVenus, and , and EarthEarth - its surface has been changed by - its surface has been changed by volcanism, impacts from other bodies, volcanism, impacts from other bodies, movements of its crust, and atmospheric movements of its crust, and atmospheric effects such as dust storms. It has polar ice effects such as dust storms. It has polar ice caps that grow and recede with the change of caps that grow and recede with the change of seasons; areas of layered soils near the seasons; areas of layered soils near the Martian poles suggest that the planet's Martian poles suggest that the planet's climate has changed more than once, perhaps climate has changed more than once, perhaps caused by a regular change in the planet's caused by a regular change in the planet's orbit. orbit.

► Martian tectonism - the formation and change Martian tectonism - the formation and change of a planet's crust - differs from Earth's. of a planet's crust - differs from Earth's. Where Earth tectonics involve sliding plates Where Earth tectonics involve sliding plates that grind against each other or spread apart that grind against each other or spread apart in the seafloors, Martian tectonics seem to in the seafloors, Martian tectonics seem to be vertical, with hot lava pushing upwards be vertical, with hot lava pushing upwards through the crust to the surface. through the crust to the surface. Periodically, great dust storms engulf the Periodically, great dust storms engulf the entire planet. The effects of these storms entire planet. The effects of these storms are dramatic, including giant dunes, wind are dramatic, including giant dunes, wind streaks, and wind-carved features.streaks, and wind-carved features.

JupiterJupiter► The most massive planet in our solar system, The most massive planet in our solar system,

with four planet-sized moons and many with four planet-sized moons and many smaller moons, Jupiter forms a kind of smaller moons, Jupiter forms a kind of miniature solar system. Jupiter resembles a miniature solar system. Jupiter resembles a star in composition. star in composition.

► In fact, if it had been about eighty times In fact, if it had been about eighty times more massive, it would have become a star more massive, it would have become a star rather than a planet. On January 7, 1610, rather than a planet. On January 7, 1610, using his primitive telescope, astronomer using his primitive telescope, astronomer Galileo Galilei saw four small 'stars' near Galileo Galilei saw four small 'stars' near Jupiter. He had discovered Jupiter's four Jupiter. He had discovered Jupiter's four largest moons, now called largest moons, now called IoIo, , EuropaEuropa, , GanymedeGanymede, and , and CallistoCallisto. Collectively, these . Collectively, these four moons are known today as the Galilean four moons are known today as the Galilean satellites.satellites.

► Galileo would be astonished at what we have Galileo would be astonished at what we have learned about Jupiter and its moons in the learned about Jupiter and its moons in the past 30 years. Io is the most volcanically past 30 years. Io is the most volcanically active body in our solar system. active body in our solar system.

► Ganymede is the largest planetary moon and Ganymede is the largest planetary moon and is the only moon in the solar system known is the only moon in the solar system known to have its own magnetic field. A liquid to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of ocean may lie beneath the frozen crust of Europa. Icy oceans may also lie deep beneath Europa. Icy oceans may also lie deep beneath the crusts of Callisto and Ganymede. the crusts of Callisto and Ganymede.

► In 2003 alone, astronomers discovered In 2003 alone, astronomers discovered 23 new moons23 new moons orbiting the giant planet, orbiting the giant planet, giving Jupiter a total moon count of 49 giving Jupiter a total moon count of 49 officially namedofficially named -- the most in the solar -- the most in the solar system. The numerous small outer moons may system. The numerous small outer moons may be be asteroidsasteroids captured by the giant planet's captured by the giant planet's gravity.gravity.

SaturnSaturn► Saturn was the most distant of the five Saturn was the most distant of the five

planets known to the ancients. In 1610, planets known to the ancients. In 1610, Italian astronomer Galileo Galilei was the Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope. first to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on To his surprise, he saw a pair of objects on either side of the planet. He sketched them either side of the planet. He sketched them as separate spheres and wrote that Saturn as separate spheres and wrote that Saturn appeared to be triple-bodied. Continuing his appeared to be triple-bodied. Continuing his observations over the next few years, Galileo observations over the next few years, Galileo drew the lateral bodies as arms or handles drew the lateral bodies as arms or handles attached to Saturn. attached to Saturn.

► In 1659, Dutch astronomer Christiaan Huygens, In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than using a more powerful telescope than Galileo's, proposed that Saturn was Galileo's, proposed that Saturn was surrounded by a thin, flat ring. In 1675, surrounded by a thin, flat ring. In 1675, Italian-born astronomer Jean-Dominique Italian-born astronomer Jean-Dominique Cassini discovered a 'division' between what Cassini discovered a 'division' between what are now called the A and B rings. are now called the A and B rings.

► It is now known that the gravitational It is now known that the gravitational influence of Saturn's moon Mimas is influence of Saturn's moon Mimas is responsible for the Cassini Division, which responsible for the Cassini Division, which is 4,800 kilometers (3,000 miles) wide. Like is 4,800 kilometers (3,000 miles) wide. Like JupiterJupiter, Saturn is made mostly of hydrogen , Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater and helium. Its volume is 755 times greater than that of Earth. than that of Earth.

► Winds in the upper atmosphere reach 500 Winds in the upper atmosphere reach 500 meters (1,600 feet) per second in the meters (1,600 feet) per second in the equatorial region. (In contrast, the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top strongest hurricane-force winds on Earth top out at about 110 meters, or 360 feet, per out at about 110 meters, or 360 feet, per second.) These super-fast winds, combined second.) These super-fast winds, combined with heat rising from within the planet's with heat rising from within the planet's interior, cause the yellow and gold bands interior, cause the yellow and gold bands visible in the atmosphere.visible in the atmosphere.

UranusUranus► Once considered one of the blander-looking Once considered one of the blander-looking

planets, Uranus (pronounced YOOR un nus) has planets, Uranus (pronounced YOOR un nus) has been revealed as a dynamic world with some been revealed as a dynamic world with some of the brightest clouds in the outer solar of the brightest clouds in the outer solar system and 11 rings. system and 11 rings.

► The first planet found with the aid of a The first planet found with the aid of a telescope, Uranus was discovered in 1781 by telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh astronomer William Herschel. The seventh planet from the Sun is so distant that it planet from the Sun is so distant that it takes 84 years to complete one orbit. takes 84 years to complete one orbit. Uranus, with no solid surface, is one of the Uranus, with no solid surface, is one of the gas giant planets (the others are Jupiter, gas giant planets (the others are Jupiter, Saturn, and Neptune). Saturn, and Neptune).

► The atmosphere of Uranus is composed The atmosphere of Uranus is composed primarily of hydrogen and helium, with a primarily of hydrogen and helium, with a small amount of methane and traces of water small amount of methane and traces of water and ammonia. Uranus gets its blue-green and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is color from methane gas. Sunlight is reflected from Uranus' cloud tops, which lie reflected from Uranus' cloud tops, which lie beneath a layer of methane gas. beneath a layer of methane gas.

► As the reflected sunlight passes back As the reflected sunlight passes back through this layer, the methane gas absorbs through this layer, the methane gas absorbs the red portion of the light, allowing the the red portion of the light, allowing the blue portion to pass through, resulting in blue portion to pass through, resulting in the blue-green color that we see. The the blue-green color that we see. The planet's atmospheric details are very planet's atmospheric details are very difficult to see in visible light. The bulk difficult to see in visible light. The bulk (80 per-cent or more) of the mass of Uranus (80 per-cent or more) of the mass of Uranus is contained in an extended liquid core is contained in an extended liquid core consisting primarily of 'icy' materials consisting primarily of 'icy' materials (water, methane, and ammonia), with higher-(water, methane, and ammonia), with higher-density material at depth.density material at depth.

NeptuneNeptune► The eighth planet from the Sun, Neptune The eighth planet from the Sun, Neptune

was the first planet located through was the first planet located through mathematical predictions rather than mathematical predictions rather than through regular observations of the sky. through regular observations of the sky. (Galileo had recorded it as a fixed star (Galileo had recorded it as a fixed star during observations with his small during observations with his small telescope in 1612 and 1613.) telescope in 1612 and 1613.)

► When Uranus didn't travel exactly as When Uranus didn't travel exactly as astronomers expected it to, a French astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, mathematician, Urbain Joseph Le Verrier, proposed the position and mass of proposed the position and mass of another as yet unknown planet that could another as yet unknown planet that could cause the observed changes to Uranus' cause the observed changes to Uranus' orbit. After being ignored by French orbit. After being ignored by French astronomers, Le Verrier sent his astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at predictions to Johann Gottfried Galle at the Berlin Observatory, who found the Berlin Observatory, who found Neptune on his first night of searching Neptune on his first night of searching in 1846.in 1846.

► Seventeen days later, its largest moon, Seventeen days later, its largest moon, Triton, was also discovered. Nearly 4.5 Triton, was also discovered. Nearly 4.5 billion kilometers (2.8 billion miles) billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun from the Sun, Neptune orbits the Sun once every 165 years. once every 165 years.

► It is invisible to the naked eye because It is invisible to the naked eye because of its extreme distance from Earth. of its extreme distance from Earth. Interestingly, due to Pluto's unusual Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually elliptical orbit, Neptune is actually the farthest planet (including dwarf the farthest planet (including dwarf planets) from the Sun for a 20-year planets) from the Sun for a 20-year period out of every 248 Earth years.period out of every 248 Earth years.

PlutoPluto► Once known as the smallest, coldest, and most Once known as the smallest, coldest, and most

distant planet from the Sun, Pluto has a dual distant planet from the Sun, Pluto has a dual identity, not to mention being enshrouded in identity, not to mention being enshrouded in controversy since its discovery in 1930. On August controversy since its discovery in 1930. On August 24, 2006, the International Astronomical Union 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official (IAU) formally downgraded Pluto from an official planet to a dwarf planet. According to the new planet to a dwarf planet. According to the new rules a planet meets three criteria: it must orbit rules a planet meets three criteria: it must orbit the Sun, it must be big enough for gravity to the Sun, it must be big enough for gravity to squash it into a round ball, and it must have squash it into a round ball, and it must have cleared other things out of the way in its orbital cleared other things out of the way in its orbital neighborhood. The latter measure knocks out Pluto neighborhood. The latter measure knocks out Pluto and 2003UB313 (Eris), which orbit among the icy and 2003UB313 (Eris), which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt. the asteroid belt.

► (1) A "planet" is a celestial body that (a) is in (1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood round) shape, and (c) has cleared the neighborhood around its orbit.around its orbit.

► (2) A "dwarf planet" is a celestial body that (a) (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a neighborhood around its orbit, and (d) is not a satellite.satellite.

► (3) All other objects except satellites orbiting (3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small the Sun shall be referred to collectively as "Small Solar-System Bodies".Solar-System Bodies".

► Discovered by American astronomer Clyde Tombaugh in Discovered by American astronomer Clyde Tombaugh in 1930, Pluto takes 248 years to orbit the Sun. 1930, Pluto takes 248 years to orbit the Sun. Pluto's most recent close approach to the Sun was Pluto's most recent close approach to the Sun was in 1989. Between 1979 and 1999, Pluto's highly in 1989. Between 1979 and 1999, Pluto's highly elliptical orbit brought it closer to the Sun than elliptical orbit brought it closer to the Sun than Neptune, providing rare opportunities to study this Neptune, providing rare opportunities to study this small, cold, distant world and its companion moon, small, cold, distant world and its companion moon, Charon.Charon.

Kuiper BeltKuiper Belt► The Kuiper (pronounced Ki-Per) Belt is The Kuiper (pronounced Ki-Per) Belt is

often called our Solar System's 'final often called our Solar System's 'final frontier.' This disk-shaped region of frontier.' This disk-shaped region of icy debris is about 4.5 to 7.5 billion icy debris is about 4.5 to 7.5 billion km (2.8 billion to 4.6 billion miles), km (2.8 billion to 4.6 billion miles), 30 to 50 Astronomical Units (AU). from 30 to 50 Astronomical Units (AU). from our Sun. our Sun.

► Its existence confirmed only a decade Its existence confirmed only a decade ago, the Kuiper Belt and its collection ago, the Kuiper Belt and its collection of icy objects - KBOs - are an emerging of icy objects - KBOs - are an emerging area of research in planetary science. area of research in planetary science. No spacecraft has ever traveled to the No spacecraft has ever traveled to the Kuiper Belt, but NASA's New Horizons Kuiper Belt, but NASA's New Horizons mission, planned to arrive at Pluto in mission, planned to arrive at Pluto in 2015, might be able to penetrate farther 2015, might be able to penetrate farther into the Kuiper Belt to study one of into the Kuiper Belt to study one of these mysterious objects.these mysterious objects.

Dwarf PlanetsDwarf Planets► A "dwarf planet" is a A "dwarf planet" is a

celestial body that (a) is in celestial body that (a) is in orbit around the Sun, (b) has orbit around the Sun, (b) has sufficient mass for its self-sufficient mass for its self-gravity to overcome rigid body gravity to overcome rigid body forces so that it assumes a forces so that it assumes a hydrostatic equilibrium hydrostatic equilibrium (nearly round) shape, (c) has (nearly round) shape, (c) has not cleared the neighborhood not cleared the neighborhood around its orbit, and (d) is around its orbit, and (d) is not a satellite. not a satellite.