The Solar SystemThe solar system is made up of the Sun, the8planetsand5dwarf planetsand their176known moons,asteroids,comets, dust and gas. The planets, asteroids, and comets travel around theSun, the center of our solar system.Most of the bodies in the solar system travel around the Sun along nearly circular paths ororbits, and all the planets travel about the Sun in the anticlockwise direction (when viewed from above).Solar systemformationbegan billions of years ago, when gases and dust began to come together to form the Sun, planets, and other bodies of thesolar system.The SunThe Sun is the closeststartoEarthand is the center of oursolar system. A giant, spinning ball of very hot gas, the Sun is fueled bynuclear fusion reactions. The light from the Sun heats our planet and makeslifepossible. The Sun is also anactivestar that displayssunspots,solar flares, eruptingprominences, andcoronal mass ejections. These phenomena, which are all related to the Sun'smagnetic field, impact ournear-Earth space environmentand determine our "space weather". In aboutfive billion years, the Sun will evolve into a Red Giant, and eventually, aWhite Dwarf star. Many cultures have had interestingmythsabout the Sun, in recognition of its importance to life on Earth.

PlanetsBy the current count of astronomers, our solar system includes 8planetsand 5dwarf planets. The planets were formed during the process ofsolar system formation, when clumps began to form in the disk of gas and dusk rotating about our young Sun. Eventually, only the planets and other small bodies in the solar system remained. The four rocky planets at the center of the solar systemMercury,Venus,Earth,Mars, are known as the inner planets.Jupiter,Saturn,Uranus, andNeptuneare all composed primarily of gas and are known as the outer planets. Find out more about the planets through the links below.Dwarf PlanetsIn 2006 the International Astronomical Union (IAU) approved a new classification scheme forplanetsand smaller objects in ourSolar System. Their scheme includes three classes of objects: "small solar system bodies" (including mostasteroidsandcomets), the much larger planets (includingEarth,Jupiter, and so on), and the new category of in-between sized "dwarf planets".There are currently five official dwarf planets.Pluto, formerly the smallest of the nine "traditional" planets, was demoted to dwarf planet status.Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The three other (for now!) dwarf planets areEris,Makemake, andHaumea. Pluto, Makemake, and Haumea orbit the Sun on the frozen fringes of our Solar System in theKuiper Belt. Eris, also aTrans-Neptunian Object, is even further from the Sun.What's the difference betweenregular planetsand dwarf planets? As you might guess, it's partly an issue of size, with dwarf planets being smaller. But just how big does a planet need to be to become a full-fledged planet instead of a dwarf? You might think the minimum size requirement is arbitrary, but the size cutoff is actually based on other properties of the object and its history in the Solar System.Both planets and dwarf planetsorbittheSun,notother planets (in which case we call them moons). Both must be large enough that their own gravity pulls them into the shapes of spheres; this rules out numerous smaller bodies like most asteroids, many of which haveirregular shapes. Planets clear smaller objects out of theirorbitsby sucking the small bodies into themselves or flinging them out of orbit. Dwarf planets, with their weaker gravities, areunableto clear out their orbits.Though there are just five dwarf planets now, their number is expected to grow. Scientists estimate there may be 70 dwarf planets amongst outer solar system objects that have been discovered already. Since we don't know the actual sizes or shapes of many of the objects we've found (because they are so far away), we can't yet determine whether they are actually dwarf planets or not. More observations and better telescopes will help us determine which other objects are dwarf planets. Astronomers speculate that there may be 200 or so dwarf planets out through the distance of theKuiper Belt, an icy band of frozen planetoids on the edge of our Solar System.AsteroidsAsteroids are small bodies that are believed to be left over from the beginning ofthe solar system4.6 billion years ago. They are rocky objects with round or irregular shapes up to several hundred km across, but most are much smaller.More than 100,000 asteroids lie in a belt betweenMarsandJupiter.These asteroids lie in a location in the solar system where there seems to be a jump in the spacing between the planets. Scientists think that this debris may be the remains of an early planet, which broke up early in the solar system. Several thousand of the largest asteroids in this belt have been given names.The chances of an asteroid colliding withEarthare very small! But some do come close to Earth, like Hermes (closest approach of 777,000 km).MeteorsMeteors are streaks of light, usually lasting just a few seconds, which people occasionally see in the night sky. They are sometimes called "shooting stars" or "falling stars", though they are notstarsat all. Meteors are caused by the entry of small pieces of rock, dust, or metal from space into theatmosphereat extremely high speeds. These particles, called "meteoroids" when they are floating around in space (think of very smallasteroids), are traveling at incredible speeds of tens of kilometers per second (tens of thousands of miles per hour) when they streak into the atmosphere. The incredible pressure meteoroids experience when they collide with Earth's atmosphere shatters them, transferring energy to atoms and molecules in the atmosphere, which then release the energy by glowing. This glow produces the bright trails oflightin the sky we see as meteors.Most meteoroid particles are quite small, ranging in size from a grain of sand to a pea-sized pebble. Almost all of them disintegrate in theatmospherelong before reaching the ground. Very rarely, a larger meteoroid actually survives to strike the ground, creating ameteor craterin a huge explosion. This explosion often vaporizes whatever solid material is left of the meteoroid after its fiery flight through the atmosphere. Sometimes, however, pieces of the meteoroid survive and are found in the crater or nearby. These chunks of rock or metal are called meteorites.Meteors are not the same thing ascomets. Meteors appear briefly as they streak through the sky. Comets are much larger objects that are actually still out in space. Comets can formtails, and though they do change position from night to night, they don't move fast enough for the eye to notice; they seem to hang in place in the sky. There is a connection, though, between some comets and some meteors. Several times each year Earth passes across theorbit of a comet, wheredust and small bits of rockfrom the comet have been left behind. When this happens we can see many meteors in a single night; sometimes as many as 100 or more per hour! These events are calledmeteor showers.Especially bright meteors are called fireballs. Some fireballs are so bright that they can be seen in the daytime. It would be possible to see meteors above any planet that has an atmosphere. A camera on theMars Exploration RoverSpirit captured a picture of meteor in the sky above Mars in 2004!How can you remember whether something is ameteor, ameteoroid, or ameteorite? Here's how I do it! When they are out in space, like asteroids, they are called meteoroids. When they are streaking through theatmosphereas bright flashes of light, we call them meteors - which reminds me ofmeteorology, which is the science concerned with weather and theatmosphere. [Meteorology isnotthe science of meteors!] When they reach theground, we call them meteorites- which reminds me of the stalactitesand stalagmitesthat are found under thegroundin caves. I hope that helps you remember too!CometsNot long ago, many people thought that comets were a sign that something bad was about to happen to them. People didn't understand how objects in the sky moved, so the sight of a comet must have been very disturbing. There are manyhistorical records and works of artwhich record the appearance of comets and link them with terrible events such as wars or plagues.Now we know that comets arelumpsof ice and dust that periodically come into the center of the solar system from somewhere in itsouter reaches, and that some comets makerepeated trips. When comets get close enough to the Sun, heat makes them start toevaporate. Jets of gas and dust form longtailsthat we can see from Earth. These tails can sometimes be millions of miles long.In 1985-1986, a spacecraft calledGiottovisited the most famous comet,Halley, on Halley's most recent visit to the inner solar system. In 1994, cometShoemaker-Levybecame trapped by the gravity of Jupiter and plunged into Jupiter's atmosphere!In 1996 and 1997 we saw cometHyakutake,and cometHale-Bopp.Hale-Bopp was one of the brightest comets ever seen from Earth.Comet Linearwas discovered in 1999 and made its closest approach of the Sun in July 2000. TheStardust spacecraftflew byComet Wild 2in January 2004, collecting samples of the comet to return to Earth. The newest comet mission isRosetta-- it will land on a comet namedChuryumov-Gerasimenko!Now scientists have identified a class of comets known assmall comets(though they originally were just called snowballs from space!)Would you like build your own custom comet? If you would, check out ourinteractive comet animation!