Mercury

The smallest planet in our solar system and nearest to the Sun, Mercury is only slightly larger than Earth's Moon. From the surface of Mercury, the Sun would appear more than three times as large as it does when viewed from Earth, and the sunlight would be as much as seven times brighter. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system – that title belongs to nearby Venus, thanks to its dense atmosphere. But Mercury is the fastest planet, zipping around the Sun every 88 Earth days. Mercury is appropriately named for the swiftest of the ancient Roman gods.With a radius of 1,516 miles (2,440 kilometers), Mercury is a little more than 1/3 the width of Earth. If Earth were the size of a nickel, Mercury would be about as big as a blueberry. From an average distance of 36 million miles (58 million kilometers), Mercury is 0.4 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes sunlight 3.2 minutes to travel from the Sun to MercuryMercury is the second densest planet, after Earth. It has a large metallic core with a radius of about 1,289 miles (2,074 kilometers), about 85 percent of the planet's radius. There is evidence that it is partly molten, or liquid. Mercury's outer shell, comparable to Earth's outer shell (called the mantle and crust), is only about 400 kilometers (250 miles) thick. Formation Mercury formed about 4.5 billion years ago when gravity pulled swirling gas and dust together to form this small planet nearest the Sun. Like its fellow terrestrial planets, Mercury has a central core, a rocky mantle and a solid crust..

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Venus

Venus is the second planet from the Sun and is Earth’s closest planetary neighbor. It’s one of the four inner, terrestrial (or rocky) planets, and it’s often called Earth’s twin because it’s similar in size and density. These are not identical twins, however – there are radical differences between the two worlds. The Latest NASA Balloon Detects California Earthquake – Next Stop, Venus? Venus Resources Venus has a thick, toxic atmosphere filled with carbon dioxide and it’s perpetually shrouded in thick, yellowish clouds of sulfuric acid that trap heat, causing a runaway greenhouse effect. It’s the hottest planet in our solar system, even though Mercury is closer to the Sun. Surface temperatures on Venus are about 900 degrees Fahrenheit (475 degrees Celsius) – hot enough to melt lead. The surface is a rusty color and it’s peppered with intensely crunched mountains and thousands of large volcanoes. Scientists think it’s possible some volcanoes are still active. Venus has crushing air pressure at its surface – more than 90 times that of Earth – similar to the pressure you'd encounter a mile below the ocean on Earth. Another big difference from Earth – Venus rotates on its axis backward, compared to most of the other planets in the solar system. This means that, on Venus, the Sun rises in the west and sets in the east, opposite to what we experience on Earth. (It’s not the only planet in our solar system with such an oddball rotation – Uranus spins on its side.) Venus was the first planet to be explored by a spacecraft – NASA’s Mariner 2 successfully flew by and scanned the cloud-covered world on Dec. 14, 1962. Since then, numerous spacecraft from the U.S. and other space agencies have explored Venus, including NASA’s Magellan, which mapped the planet's surface with radar. Soviet spacecraft made the most successful landings on the surface of Venus to date, but they didn’t survive long due to the extreme heat and crushing pressure. An American probe, one of NASA's Pioneer Venus Multiprobes, survived for about an hour after impacting the surface in 1978.

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Earth

Orbit and Rotation As Earth orbits the Sun, it completes one rotation every 23.9 hours. It takes 365.25 days to complete one trip around the Sun. That extra quarter of a day presents a challenge to our calendar system, which counts one year as 365 days. To keep our yearly calendars consistent with our orbit around the Sun, every four years we add one day. That day is called a leap day, and the year it's added to is called a leap year. Earth's axis of rotation is tilted 23.4 degrees with respect to the plane of Earth's orbit around the Sun. This tilt causes our yearly cycle of seasons. During part of the year, the northern hemisphere is tilted toward the Sun and the southern hemisphere is tilted away. With the Sun higher in the sky, solar heating is greater in the north producing summer there. Less direct solar heating produces winter in the south. Six months later, the situation is reversed. When spring and fall begin, both hemispheres receive roughly equal amounts of heat from the Sun. Structure Earth is composed of four main layers, starting with an inner core at the planet's center, enveloped by the outer core, mantle and crust. The inner core is a solid sphere made of iron and nickel metals about 759 miles (1,221 kilometers) in radius. There the temperature is as high as 9,800 degrees Fahrenheit (5,400 degrees Celsius). Surrounding the inner core is the outer core. This layer is about 1,400 miles (2,300 kilometers) thick, made of iron and nickel fluids. In between the outer core and crust is the mantle, the thickest layer. This hot, viscous mixture of molten rock is about 1,800 miles (2,900 kilometers) thick and has the consistency of caramel. The outermost layer, Earth's crust, goes about 19 miles (30 kilometers) deep on average on land. At the bottom of the ocean, the crust is thinner and extends about 3 miles (5 kilometers) from the sea floor to the top of the mantle. Formation When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun. Like its fellow terrestrial planets, Earth has a central core, a rocky mantle and a solid crust..

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Mars

Namesake Mars was named by the ancient Romans for their god of war because its reddish color was reminiscent of blood. Other civilizations also named the planet for this attribute – for example, the Egyptians called it "Her Desher," meaning "the red one." Even today, it is frequently called the "Red Planet" because iron minerals in the Martian dirt oxidize, or rust, causing the surface to look red. Potential for Life Scientists don't expect to find living things currently thriving on Mars. Instead, they're looking for signs of life that existed long ago, when Mars was warmer and covered with water. Size and Distance With a radius of 2,106 miles (3,390 kilometers), Mars is about half the size of Earth. If Earth were the size of a nickel, Mars would be about as big as a raspberry. From an average distance of 142 million miles (228 million kilometers), Mars is 1.5 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes sunlight 13 minutes to travel from the Sun to Mars. Orbit and Rotation As Mars orbits the Sun, it completes one rotation every 24.6 hours, which is very similar to one day on Earth (23.9 hours). Martian days are called sols – short for "solar day." A year on Mars lasts 669.6 sols, which is the same as 687 Earth days. Mars' axis of rotation is tilted 25 degrees with respect to the plane of its orbit around the Sun. This is another similarity with Earth, which has an axial tilt of 23.4 degrees. Like Earth, Mars has distinct seasons, but they last longer than seasons here on Earth since Mars takes longer to orbit the Sun (because it's farther away). And while here on Earth the seasons are evenly spread over the year, lasting 3 months (or one quarter of a year), on Mars the seasons vary in length because of Mars' elliptical, egg-shaped orbit around the Sun. Spring in the northern hemisphere (autumn in the southern) is the longest season at 194 sols. Autumn in the northern hemisphere (spring in the southern) is the shortest at 142 days. Northern winter/southern.

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Jupiter

Nine spacecraft have studied Jupiter up close. NASA's Juno spacecraft is currently studying the gas giant planet from orbit. The spacecraft, which arrived at Jupiter in July 2016, is the first to study the planet's mysterious, cloud-shrouded interior. Scientists also use the Earth-orbiting Hubble Space Telescope and ground-based telescopes to regularly check in on Jupiter. Pioneer 10 was the first spacecraft to fly past Jupiter. It was followed by Pioneer 11, Voyager 1 and Voyager 2 flybys. NASA's Galileo mission was first to orbit Jupiter and to send an atmospheric probe into the stormy clouds. The international Ulysses mission used Jupiter's powerful gravity to hurl itself into orbital passes of the Sun's northern and southern poles. Both Cassini and New Horizons studied Jupiter as they hurtled on to their main science targets — Saturn for Cassini and Pluto and the Kuiper Belt for New Horizons. Two new missions are in the works to make close studies of Jupiter's moons NASA's Europa Clipper and ESA's JUpiter ICy Moons Explorer (JUICE). Jupiter holds a unique place in the history of space exploration. In 1610, astronomer Galileo Galilei used a new invention called the telescope to look at Jupiter and discovered the first moons known to exist beyond Earth. The discovery ended incorrect, ancient belief that everything, including the Sun and other planets, orbited the Earth..

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Saturn

Saturn has the second-shortest day in the solar system. One day on Saturn takes only 10.7 hours (the time it takes for Saturn to rotate or spin around once), and Saturn makes a complete orbit around the Sun (a year in Saturnian time) in about 29.4 Earth years (10,756 Earth days). Its axis is tilted by 26.73 degrees with respect to its orbit around the Sun, which is similar to Earth's 23.5-degree tilt. This means that, like Earth, Saturn experiences seasons. It's hard to imagine, but Saturn is the only planet in our solar system whose average density is less than water. The giant gas planet could float in a bathtub if such a colossal thing existed. Surface As a gas giant, Saturn doesn’t have a true surface. The planet is mostly swirling gases and liquids deeper down. While a spacecraft would have nowhere to land on Saturn, it wouldn’t be able to fly through unscathed either. The extreme pressures and temperatures deep inside the planet crush, melt and vaporize spacecraft trying to fly into the planet. Magnetosphere Saturn's magnetic field is smaller than Jupiter's but still 578 times as powerful as Earth's. Saturn, the rings, and many of the satellites lie totally within Saturn's enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind. Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines. On Earth, these charged particles come from the solar wind. Cassini showed that at least some of Saturn's aurorae are like Jupiter's and are largely unaffected by the solar wind. Instead, these aurorae are caused by a combination of particles ejected from Saturn's moons and Saturn's magnetic field's rapid rotation rate. But these "non-solar-originating" aurorae are not completely understood yet.

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Uranus

One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once). And Uranus makes a complete orbit around the Sun (a year in Uranian time) in about 84 Earth years (30,687 Earth days). Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees—possibly the result of a collision with an Earth-sized object long ago. This unique tilt causes the most extreme seasons in the solar system. For nearly a quarter of each Uranian year, the Sun shines directly over each pole, plunging the other half of the planet into a 21-year-long, dark winter. Uranus is also one of just two planets that rotate in the opposite direction than most of the planets (Venus is the other one), from east to west. Structure Uranus is one of two ice giants in the outer solar system (the other is Neptune). Most (80 percent or more) of the planet's mass is made up of a hot dense fluid of "icy" materials—water, methane and ammonia—above a small rocky core. Near the core, it heats up to 9,000 degrees Fahrenheit (4,982 degrees Celsius). Uranus is slightly larger in diameter than its neighbor Neptune, yet smaller in mass. It is the second least dense planet; Saturn is the least dense of all. Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus' cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color. Formation Uranus took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this ice giant. Like its neighbor Neptune, Uranus likely formed closer to the Sun and moved to the outer solar system about 4 billion years ago, where it is the seventh planet from the Sun..

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Neptune

One day on Neptune takes about 16 hours (the time it takes for Neptune to rotate or spin once). And Neptune makes a complete orbit around the Sun (a year in Neptunian time) in about 165 Earth years (60,190 Earth days). Sometimes Neptune is even farther from the Sun than dwarf planet Pluto. Pluto's highly eccentric, oval-shaped orbit brings it inside Neptune's orbit for a 20-year period every 248 Earth years. This switch, in which Pluto is closer to the Sun than Neptune, happened most recently from 1979 to 1999. Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the Sun, Pluto makes two. This repeating pattern prevents close approaches of the two bodies. Neptune’s axis of rotation is tilted 28 degrees with respect to the plane of its orbit around the Sun, which is similar to the axial tilts of Mars and Earth. This means that Neptune experiences seasons just like we do on Earth; however, since its year is so long, each of the four seasons lasts for over 40 years. Structure Neptune is one of two ice giants in the outer solar system (the other is Uranus). Most (80 percent or more) of the planet's mass is made up of a hot dense fluid of "icy" materials—water, methane and ammonia—above a small, rocky core. Of the giant planets, Neptune is the densest. Scientists think there might be an ocean of super hot water under Neptune's cold clouds. It does not boil away because incredibly high pressure keeps it locked inside. Formation Neptune took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this ice giant. Like its neighbor Uranus, Neptune likely formed closer to the Sun and moved to the outer solar system about 4 billion years ago. Surface Neptune does not have a solid surface. Its atmosphere (made up mostly of hydrogen, helium and methane) extends to great depths, gradually merging into water and other melted ices over a heavier, solid core with about the same mass as Earth..

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