Our Planetary Neighborhood
Our solar system contains objects ranging in size from the sun, the largest item, to tiny grains of rock in the asteroid belt.
<p>Take a tour of our cosmic neighborhood in pictures. Come on, let's go!
The sun, the star at the center of our solar system, controls everything within its mighty gravity field, commanding planets to orbit or pulling comets straight into it. It holds 99.8 percent of the solar system's mass, and measures roughly 109 times the diameter of the Earth. The visible part of the sun reaches a temperature of roughly 10,000 degrees F (5,500 degrees C), while temperatures in the core exceed 27 million degrees F (15 million degrees C), driven by nuclear reactions. It's plenty dangerous!
Sun Plasma Flickers
A close-up, profile view of an active region in extreme ultraviolet light showcased several small spurts of plasma as they flickered out and retreated back into the sun over about 13 hours (June 16, 2011). This wavelength captures ionized helium at about 60,000 degrees not far above the sun's surface. Flashes of small solar flares can be seen triggering most of these spurts.
Mercury, the closest planet to the sun at 35,983,095 miles (57,909,175 km), is a hot world that whizzes around at the crazy speed of nearly 112,000 mph (180,000 kph), fastest in the solar system. Its radius is about 1,516.0 miles (2,439.7 km). Also, it possesses an extremely high temperature, a scorching 840 degrees F (450 degrees C), what with being so close to the sun and all.
Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image taken by NASA's Messenger probe on Jan. 14, 2008, which is a mosaic of three different images. Messenger is now orbiting Mercury.
Venus, our mysterious hot neighbor 67,237,910 miles (108,208,930 km) from the sun, possesses a gaseous atmosphere composed of deadly carbon dioxide with clouds of sulfuric acid. Its radius measures about 3,760.4 miles (6,051.8 km). Venus achieves a temperature of 870 degrees F (465 degrees C). Whew, that's hot! It's even hotter than that Mercury! Venus is the hottest planet in the solar system.
Planet Size Comparison: Venus, Earth and Mars
We've nearly reached our home planet, Earth. So here's a quick look at how our blue marble stacks up to its nearest neighbors. A comparison of the sizes of planets Venus (left), Earth and Mars.
Earth, our home planet, 3rd from the sun, exists at the so-called "Goldilocks distance" from the sun (92,955,820 miles or 149,597,890 km), where it experiences neither too hot nor too cold temperatures, thus allowing liquid water to exist on its surface. It is the only known planet in the solar system to possess an atmosphere containing free oxygen, oceans of liquid water on its surface, and life. Its radius is about 3,958.8 miles (6,371.00 km). Earth is definitely the place!
Our moon is the only other place in the solar system that that humans have visited. It's a cold round rock possessing caches of frozen water. Our moon orbits the Earth about once every 27 days at a distance of 238,855 miles (384,400 km). The moon's gravitation pull creates the tides in Earth's bodies of water.
<p>How did it form? The leading theory suggests that a body smashed into Earth approximately 4.5 billion years ago, and the debris from both Earth and the impactor accumulated to form our natural satellite. Let's go back and visit it again!
The Earth and the moon as seen on Jan. 15, 2011, from the International Space Station by astronaut Paolo Nespoli.
Mars, the mysterious red ball 141,637,725 miles (227,943,824 km) from the sun, is a desolate rock that may have once held rivers of flowing water. The Red Planet possesses stark landscapes featuring both the highest mountain and the deepest, longest valley in the solar system. Its radius is about 2,106.1 miles (3,389.5 km).
<p>Mars is much colder than Earth, largely due to its greater distance from the sun. The average temperature hovers around a frosty minus 80 degrees F (minus 60 degrees C). Channels, valleys, and gullies cut into the entire surface of Mars, and suggest that liquid water might have flowed across the planet's surface. Let's go there next!
Mars Viking Lander
NASA's Viking probes were the first ever to successfully set footpad on Mars in a powered landing. The Viking 1 lander set down in July 1976 and didn't go silent until November 1982. Viking 2 landed in September 1976 and kept working until April 1980.
After Mars comes the asteroid belt, a girdle of rocky objects between the orbits of Mars and Jupiter, about 186 million to 370 million miles (300 million to 600 million kilometers) from the sun.
<p>Asteroids are small, airless rocky objects revolving around the sun, but are too small to be planets. They are also known as planetoids or minor planets. Most asteroids lie in the asteroid belts, but there are others across the solar system.
<p>Asteroids represent the leftovers from the formation of our solar system about 4.6 billion years ago. They are dirty bastards that could rip your spacecraft to shreds in an instant. If you had a spacecraft. You wish you had a spacecraft.
Asteroids Visited by Spacecraft
Here's a look at the handful of asteroids that have been visited by robot probes from Earth. Only a few near-Earth objects would fit NASA's proposed guidelines for a manned mission to an asteroid.
Jupiter is the most massive planet in our solar system, more than twice as massive as all the other planets combined. Strong east-west winds in the planet's upper atmosphere create the dark belts and light zones of Jupiter's variegated appearance. The enormous planet lies 483,638,564 miles (778,340,821 km) from the sun, and has a radius of about 43,440.7 miles (69,911 km).
<p>Jupiter also possesses four large moons and many smaller moons. Jupiter has a gaseous composition, referred to as a "gas giant." Some astronomers believe Jupiter may have once possessed the capability to coalesce into a binary star companion of our sun. If only Jupiter had any real ambition. Lazy gas giant.
Jupiter's Moons: Family Portrait
This montage shows the best views of Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter.
Saturn, another gas giant like Jupiter, spins faster than any other planet, and is the furthest planet that can be seen by the naked human eye, 886,489,415 miles (1,426,666,422 km) from the sun. Saturn, with a radius of about 36,183.7 miles (58,232 km), possesses moons but it is better known for its rings, composed of tiny bits of ice and rock, believed to be debris left over from comets, asteroids or shattered moons. Does having rings make you special, Saturn? No, because Jupiter, Uranus and Neptune also possess rings. But Saturn, yours are definitely the most stunning.
Fabulous Saturn: Rings, Baubles and Spiky Spokes
Speaking of Saturn's rings: From trillions of icy ring dust particles, finer than baker's flour, to more than 60 moons, each a unique world, the kingdom of Saturn is rich with mystery.
Uranus consists of a bunch of frozen water, methane, and ammonia, with a blue-green color caused by hydrogen and helium in its atmosphere. Its crazy axis of rotation tilts so far over that it is almost pointing at the sun. Just remember that Uranus is spinning on its side, like a top that's fallen over. This planet is 1,783,744,300 miles (2,870,658,186 km) from the sun, and has a radius of about 15,759.2 miles (25,362 km).
<p>Uranus, though visible to the naked eye, was long considered a star, until British astronomer William Herschel discovered Uranus accidentally on March 13, 1781. And let's have no more terrible puns on the name of "Uranus."
The Weird Tilt of Uranus
Uranus' tilt essentially has the planet orbiting the Sun on its side, the axis of its spin is nearly pointing at the Sun.
Neptune shines a deep blue color. Its color comes from an unidentified compound in its atmosphere composed of mainly hydrogen and helium. This planet orbits 2,795,173,960 miles (4,498,396,441 km) from the sun, and has a radius of about 15,299.4 miles (24,622 km). Among the planets, Neptune is known for its crazy-fast winds, which can blow up to 1,500 miles per hour (2,400 kilometers per hour), the fastest detected yet in the solar system.
<p>Also, Neptune was discovered only after astronomers noticed irregular behavior in the orbit of Uranus. And no, you should never use the words "irregular" and "Uranus" in the same sentence.
The Winds of Neptune
Neptune's winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system.
Comets are cold, dirty aggregations of rock and ice that release gas or dust. The solid core of a comet consists mostly of ice and dust coated with dark organic material, with the ice composed mainly of frozen water. When heated by the sun, the comet's surface ice turns into a gas, forming a cloud called a coma which stretches into a dust tail and an ion tail. Comets zoom in crazily elliptical orbits around the sun. They can take decades to make one orbit, such as Halley's Comet, which returns once every 76 years. Many have much longer orbital durations, over 200 years. When comets travel close enough to the Earth, their long tails can trigger meteor showers. Sometimes comets crash into the sun or into planets like Jupiter, and sometimes they only appear once and never return.
John Gleason captured this amazing shot of the comet Hale-Bopp in March 1997.
Kuiper Belt — Trans-Neptunian Region
Out past the orbit of Neptune and the other planets, the solar system gets mighty cold and dark. Really. The area known as the Kuiper belt holds hundreds of thousands of icy bodies larger than 60 miles (100 km) wide, as well as an estimated trillion or more comets.
Smallest Object in Outer Solar System Spotted
This is an artist's impression of a small Kuiper Belt Object (KBO) occulting a star. NASA's Hubble Space Telescope recorded this brief event and allowed astronomers to determine that the KBO was only one-half of a mile across, setting a new record for the smallest object ever seen in the Kuiper Belt.
Pluto, the dwarf planet, has gone through some changes. Clyde Tombaugh discovered Pluto in 1930, based on predictions from Percival Lowell and other astronomers. Pluto is only 1,400 miles (2,300 kilometers) wide and is about 3.6 billion miles from Earth. It has four moons, the latest of which was announced in July 2011.
<p>Pluto used to be classified as a full-fledged planet until the International Astronomical Union (IAU) downgraded it a dwarf planet in 2006 because its small size. Oh, no, they di'int! Oh, yes, they did! Pluto has serious planet envy now. Also, Pluto's orbit tilts 17 degrees from the ecliptic plane.
This is the most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003, before the telescope’s latest overhaul. The center disk (180 degrees) has a mysterious bright spot that is unusually rich in carbon monoxide frost. The image was released in February 2010.
Dwarf planets belong to a newer classification in the solar system naming scheme. The International Astronomical Union defines a dwarf planet as "an object in orbit around the Sun that is large enough (massive) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects." Got that? Other objects are okay!
The Weirdest Object in the Solar System?
Haumea is one of the strangest known objects in the solar system. It is as big across as Pluto but shaped like a cigar or perhaps an American football. These two images show the two extremes of its appearance as it spins.
But wait: there's more! Beyond the Kuiper belt lies the Oort cloud, which theoretically extends from 5,000 to 100,000 times the distance of Earth to the sun, and is home to up to two trillion icy bodies. An object called Sedna, about three-fourths the size of Pluto, might be the first dwarf planet discovered in the Oort cloud.
At last we arrive at the very edge of the solar system where the heliosphere lies, a teardrop-shaped region of space containing electrically charged particles given off by the sun. Many astronomers think that the limit of the heliosphere, known as the heliopause, lies about 9 billion miles (15 billion kilometers) from the sun. And that completes our tour of the solar system. Welcome to interstellar space!