What is A Planet? Read about planets..

• Our solar system’s eight planets come in two flavors: smaller rocky planets with solid ground (Mercury, Venus, Earth and Mars) and larger gas giants (Jupiter, Saturn, Uranus, and Neptune).
• You can spot six of the solar system’s planets with the naked eye from Earth—and all eight if you have binoculars or a telescope.
• Astronomers have discovered hundreds of worlds beyond our solar system and are spotting more every day. Scientists believe some of these “exoplanets” may support alien life. Who knows. Maybe an alien kid elsewhere in the galaxy is wondering if you exist.

1.. First of all we can discuss about our planet earth.

Earth is a special spot in the solar system for so many reasons—its sprawling continents, its blue seas, its New York-style pizza. But one Earthly thing stands out above the rest: its earthlings. Ours is the only planet known to harbor life. In fact, Earth’s unique combination of air, water, and land nurtures life of every sort, from microscopic amoebas to submarine-size blue whales to you, an intelligent life form contemplating the clockwork of the solar system.

Earth’s veil of atmosphere not only provides the right mix of breathable air for animals and plants, it also acts as a sort of force field against solar radiation and deadly space debris. As much as 22,000,000 pounds (10,000,000 kilograms) of meteors burn harmlessly in the Earth’s atmosphere each day. The solar system’s other planets are typically too hot or too cold to support liquid water, but Earth is just right. Life began more than 3.5 billion years ago in our oceans, which cover nearly 70 percent of the planet’s surface and are a source of the water vapor responsible for our weather. And before you get eager to blast off and explore the rest of the solar system, consider this: Scientists know more about the surface of the moon than they do the depths of the oceans. Our world still holds plenty of mysteries for the solar-system explorer..
LOCATION: Third rock from the sun.
DISTANCE FROM THE SUN: 91,402,640 to 94,509,460 miles (147,098,291 to 152,098,233 kilometers).
AVERAGE SURFACE TEMPERATURE: 59° F (15° C).
LENGTH OF SPACE JOURNEY FROM EARTH: You're already here!.
2>> Mercury.

Don’t panic when you peer from the porthole of your spaceship as it plummets to the surface of Mercury: You haven’t taken a wrong turn and touched down on the moon. Cratered, contoured with hills, and covered with dark dust, Mercury’s landscape certainly looks a lot like the moon’s. This itty-bitty planet—the smallest in the solar system—is only slightly larger than our moon, too. But one look up from Mercury’s surface at high noon will tell you you’re far from home. Seen from here, the sun appears three times larger in the sky than it does when viewed from Earth. You’re standing on the closest planet to the sun. Forget your sunglasses? Okay, now you can panic.

Mercury is a planet of extremes. By day (which actually lasts roughly 30 Earth days), it’s one of the solar system’s most scorching spots—more than four times hotter than boiling water. By night, temperatures plummet hundreds of degrees below freezing. (The planet’s weak gravity can’t keep a grip on a heat-trapping atmosphere.) But despite its harsh environment, this hot-and-cold rock offers perks for the spacefarer. A weak magnetic field protects visitors from the sun’s deadly solar radiation (Earth has a similar but much stronger field), and the crater-pocked surface may contain valuable minerals. The speediest of all the planets, Mercury orbits the sun every 88 days—which means any Earthlings born here could celebrate four birthdays for your every one.

LOCATION: First rock from the sun
DISTANCE FROM THE SUN: 28,583,702 to 35,983,125 miles (46,001,200 to 69,816,900 kilometers)
AVERAGE SURFACE TEMPERATURE: 243° F (117° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO MERCURY: 4 years
GRAVITY: If you weigh 100 pounds on Earth, you’d weigh 38 pounds here.
3>> Venus.

You expected to find seas, mountains—maybe even a few trees—on a planet considered “Earth’s twin.” Instead, Venus is like Earth in reverse. A day here lasts longer than a year, the sun rises in the west and sets in the east, and—whew!—is it ever hot outside! Venus’ average temperature is more than six times hotter than the hottest spot on Earth. That’s hot enough to turn a slab of lead into a molten puddle. Sunset won’t bring relief from the scorching heat. Day or night, from its north pole to its south pole, every day of the year, Venus is locked in a perpetual heat wave. Blame the blanketing atmosphere, more than 90 times thicker than Earth’s, of carbon dioxide. Good thing your spacesuit has air conditioning!

How did such a harsh planet earn the title of Earth’s twin? Venus is roughly the same size and density as Earth, which means the gravity is similar between here and home. The view from the scalding surface isn’t all that alien, either. Like Earth, Venus has clouds and wind—although the clouds are made of sulfuric acid and the wind exceeds tornado strength. The sunlight filtering from above bathes the landscape in a dim red glow. You’ll never spot Earth through those heavy clouds, but rest assured that your friends back home can see you. Venus is the brightest object in Earth’s sky after the moon.

LOCATION: Second rock from the sun.
DISTANCE FROM THE SUN: 66,782,596 to 67,693,905 miles (107,477,000 to 108,939,000 kilometers)
AVERAGE SURFACE TEMPERATURE: 867° F (462° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO VENUS: 6 months
GRAVITY: If you weigh 100 pounds on Earth, you’d weigh 91 pounds here.
4>>> Mars.

Astronomers peering at Mars in the 17th and 18th centuries saw signs of life everywhere. Seas! Continents! Canals that carried water to Martian farms! You spot none of these features as your spaceship deploys its parachutes for touchdown on this cold desert world—but then you never expected to. Mars has been the subject of many myths; you intend to separate science fact from science fiction as you step on the surface of the red planet.

For starters, the red planet isn’t even totally red! Browns, tans, golds, and flecks of green pop out as you scan the rocky, dusty, dune-laden landscape. Mars only looks red from far away because of rusting iron minerals in the rocks and soil. The soil blows into the air (occasionally in planetwide dust storms) to give the atmosphere a bloody tint. Water may have flowed on Mars long ago, in ancient seas and riverbeds that early astronomers confused for canals. That was back when the atmosphere was thicker. Now, the air here (mostly carbon dioxide) is too wispy to support water or hold much heat. If you took off your spacesuit and stood on the equator at high noon, your toes would feel toasty but your face would be freezing! The good news is summer here lasts for six months. The bad news: So does winter!
LOCATION: Fourth rock from the sun
DISTANCE FROM THE SUN: 128,409,598 to 154,865,853 miles (206,669,000 to 249,209,300 kilometers)
AVERAGE SURFACE TEMPERATURE: -81° F (-63° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO MARS: 7 months
GRAVITY: If you weigh 100 pounds (45 kilograms) on Earth, you’d weigh 38 pounds (17 kilograms) here.
5>>> Jupiter..

You’re not even close to leaving the solar system when you pull into orbit around Jupiter, but you’d swear you’ve just entered a new one. Your ship’s scanners flash with readings of nearby worlds and faint rings. Jupiter, the largest planet in our solar system, rules a system of its own. Some astronomers consider it a failed star. Welcome to the realm of giants.

Jupiter is a gas giant, a ball of mostly hydrogen and helium large enough to hold more than 1,300 Earths. You won’t find any solid surface to explore here, and skydiving from your ship’s airlock would be a bad idea. You’d sink deeper into clouds of ammonia and water vapor until the intense atmospheric pressure and heat compressed the hydrogen around you into a molten liquid. Science fiction writers have proposed exploring Jupiter in hot-air balloons high above the crushing depths below, but you’re happy sipping cocoa aboard your ship in orbit. It’s a safer place to watch Jupiter’s spectacular cloud bands whiz by at more than 300 miles an hour (530 kilometers an hour).

Several of Jupiter’s nearly 70 moons grab your attention. Mega-moon Ganymede is larger than Mercury and has its own magnetic field. Volcanoes on Io, the most volcanic body in the solar system, spew clouds of yellow sulfur 300 miles (500 kilometers) high. Frozen Europa might hide a liquid ocean beneath its icy crust. Scientists believe Europa might hide something else inside that ocean: life…

LOCATION: Fifth gas ball from the sun
DISTANCE FROM THE SUN: 460,237,112 to 507,040,015 miles (740,679,835 to 816,001,807 kilometers)
AVERAGE TEMPERATURE: -234° F (-148° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO JUPITER: 13 months
GRAVITY: If you weigh 100 pounds (45 kilograms) on Earth, you’d weigh 253 pounds (115 kilograms) here…
6>>> Saturn,.

You’ve probably used the word “awesome” a hundred times to describe anything from the graphics in a new video game to the jelly filling in your favorite donut, but you never knew the word’s true meaning until you pull into the shade of Saturn’s rings. More than 155,000 miles (250,000 kilometers) wide, dappled with spokes that rotate at different rates, Saturn’s awe-inspiring ring system is the most complex of all the planets. If Earth is the jewel in the solar system, then Saturn wears the crown.

The rings lose none of their luster when seen up close. What appears as a solid disc from a distance is actually a glittering shower of ice and rock. And although the rings stretch almost as far into space as the distance between the Earth and the moon, they’re incredibly thin—typically about 30 feet (10 meters) wide. Astronomers believe Saturn’s rings formed from bits of asteroids and comets that shattered before they reached the planet. Swoop low over Saturn’s atmosphere and you might watch two of the planet’s moons rise within gaps in the rings.
• Saturn is the only planet in the solar system that’s less dense than water—which means it would float if you dunked it in a large enough ocean.
• Titan, one of Saturn’s 53 moons and its largest, has an atmosphere of mostly nitrogen similar to ancient Earth’s.
• A small moon named Mimas sports an impact crater that makes it look just like the Death Star from Star Wars.
LOCATION: Sixth gas ball from the sun.
DISTANCE FROM THE SUN: 838,741,509 to 934,237,322 miles (1,349,823,615 to 1,503,509,229 kilometers)
AVERAGE TEMPERATURE: -288° F (-178° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO SATURN: 3 years
GRAVITY: If you weigh 100 pounds (45 kilograms) on Earth, you’d weigh 107 pounds (49 kilograms) here.
7>>> Uranus.

You can’t help but tilt your head as you watch Uranus creep closer on your spaceship’s view screen. Something about this big ball of bluish-green gas just seems...off. A closer look at the planet’s cloud bands and 13 faint rings gives you your answer: Uranus is off its rocker! Scientists suspect that a planet-size object knocked Uranus sideways in the early days of its formation. It has spun like a top toppling over ever since, which makes for some oddball seasons and decades devoid of even a glint of the sun’s faint light. The north pole is locked in more than 20 years of darkness in the winter and just as much sunlight in the summer, yet the temperature varies little here on the solar system’s coldest planet. Crank up the ship’s heater, young astronaut. You have wandered far from the sun.
Like Jupiter and Saturn, Uranus is a gas giant—a ball of gas surrounding an Earth-size core of hot liquids. More specifically, Uranus is considered an “ice giant” because its atmosphere is composed mostly of “icy” water, ammonia, and methane. Researchers have found that Uranus’ crushing atmosphere can compress methane into precious rocks. Those methane clouds drifting far below your ship might be raining diamonds.
LOCATION: Seventh gas ball from the sun
DISTANCE FROM THE SUN: 1,699,449,110 to 1,868,039,489 miles (2,734,998,229 to 3,006,318,143 kilometers)
AVERAGE TEMPERATURE: -357° F (-216° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO URANUS: 9 years
GRAVITY: If you weigh 100 pounds (45 kilograms) on Earth, you’d weigh 91 pounds (41 kilograms) here.
8<>> Neptune..

You better pack for a long trip when you trek to Neptune, the most distant of our solar system’s planets. It’s so far away, in fact, that it’s the only planet you can’t see from Earth with the naked eye. How did astronomers discover a planet they couldn’t even see? Through math! They noticed that Uranus—Neptune’s nearest planetary neighbor—traveled in a way that suggested the gravitational pull of an eighth planet. Crunching the numbers revealed Neptune, first confirmed through a telescope in 1846.

You won’t need sunglasses for this voyage. High noon on Neptune is no brighter than the last few moments of sunset on Earth. Not that you could stand on Neptune at noon. Like Jupiter and Saturn, Neptune is a gas giant—a big ball of gas surrounding an Earth-size core of hot liquids rather than rocks or other solid matter. And like fellow “ice giant” Uranus, Neptune’s atmosphere is composed mostly of water, ammonia, and methane. It’s the methane that gives Neptune its striking blue hue. You’ll want to keep your ship soaring high in the planet’s atmosphere, which extends down to crushing depths and is home to the windiest weather in the solar system. Clouds of frozen methane whoosh as fast as a fighter jet through storms the size of Earth. Please wear your seatbelt for the duration of your flight above Neptune.
LOCATION: Last gas ball from the sun.
DISTANCE FROM THE SUN: 2,771,162,074 to 2,819,185,846 miles (4,459,753,056 to 4,537,039,826 kilometers)
AVERAGE TEMPERATURE: -353° F (-214° C)
LENGTH OF SPACE JOURNEY FROM EARTH TO NEPTUNE: 12 years
GRAVITY: If you weigh 100 pounds (45 kilograms) on Earth, you’d weigh 114 pounds (52 kilograms) here.

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