What is the solar system?

The solar system consists of a central star, the Sun, and all of the smaller celestial bodies that continuously travel around it. The smaller bodies include eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, which are themselves orbited by more than 140 moons. (Only Mercury and Venus have no moons.) In addition, the solar system contains millions of rocky asteroids and billions of icy comets. All of these objects are held together in a group by the Sun's gravity.

How did the solar system form?

The planets, asteroids and comets in the solar system are loose objects left over from the formation of the Sun. Originally the gas and dust that would become the Sun was the core of a cloud much larger than the solar system, probably several light-years across. (One light-year is equal to approximately 6,000,000,000,000 miles.) The core was slowly rotating at first, but as it collapsed it spun faster, like a spinning ice-skater pulling in his arms. The rotation prevented the material at the core's equator from collapsing as fast as material at the poles, so the core became a spinning disk.

Gas and dust in the disc spiraled gradually in to the center, where it accumulated to form the Sun. But because dust is denser than gas, some of the dust settled to the mid-plane of the disc. These dust particles stuck together to make clumps, then clumps stuck together to make rocks, then rocks collided to make planets. In the case of the "gas giant" planets, Jupiter, Saturn, Uranus and Neptune, the rocky cores were massive enough to also attract some of the gas. The outer layers of these planets are made up of hydrogen and other gases.

So the Sun is the collapsed core of an interstellar gas cloud, and the planets, asteroids and comets are small lumps of dust or ice chunks which stayed in orbit instead of spiraling into the Sun. The planets all formed within a very short period, probably a few million years, about five billion years ago.

How old is the solar system?

The solar system is about 4.6 billion years old.

How big is the solar system?

There are no physical boundaries in space. The solar system consists of eight planets orbiting around one star: the Sun. Neptune, the farthest planet from the Sun, orbits approximately 30astronomical units from the Sun. An astronomical unit is a unit of length used by astronomers. One astronomical unit equals the distance from Earth to the Sun: 93 million miles (149 million km). Some of the comets associated with the solar system travel on orbits that take them much farther from the Sun than Neptune.

How many planets are there in our solar system?

Our solar system officially has eight planets and one star: the Sun. In order from the Sun out, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The discovery of an object larger than Pluto in 2005 rekindled the debate over whether such objects, belonging to the "Kuiper Belt" – a collection of icy bodies located beyond Neptune – should be called planets. Pluto and other large members of the Kuiper Belt are now considered "dwarf planets."

Can you see any planets with the "naked eye?"

Mercury, Venus, Mars, Jupiter and Saturn can be seen with the unaided eye. That is how they were discovered by ancient civilizations.

Uranus and Neptune were discovered using telescopes.

Are there differences among the planets in our solar system?

Planets come in different sizes, compositions and colors. The four planets closer to the Sun are called "rocky" planets. They are small in size and similar to Earth in composition. They have no rings and only two of them (Earth and Mars) have moons.

The four outer planets, also called "gas giants," are much larger than the rocky planets. They all have rings and have many moons. The gas giants are made up mostly of hydrogen, helium, frozen water, ammonia, methane and carbon monoxide.

Which planets are called the "rocky" or "terrestrial" planets?

Mercury, Venus, Earth and Mars are called "rocky" or "terrestrial" planets. They are similar to Earth in composition. Heat from the Sun evaporated lightweight elements like hydrogen and helium into interplanetary space. Mostly rock and metal was left in this zone and clumped together to form the inner rocky planets.

Which planets are called the "gaseous" planets?

Jupiter, Saturn, Uranus and Neptune are called the "gaseous" planets. Jupiter and Saturn contain the largest percentages of hydrogen and helium, while Uranus and Neptune contain the largest shares of ices – frozen water, ammonia, methane and carbon monoxide.

Which planets have rings?

The four gas giants, Jupiter, Saturn, Uranus and Neptune, have rings.

How thick are Saturn's rings?

The Hubble telescope has captured snapshots of Saturn with its rings nearly edge-on to our view. Read more about it.

Saturn's rings are incredibly thin. The main rings are generally only about 30 feet (10 meters) thick, though parts of the main and other rings can be several kilometers thick. The rings are made of dusty ice, in the form of boulder-sized and smaller chunks that gently collide with each other as they orbit around Saturn. Saturn's gravitational field constantly disrupts these ice chunks, keeping them spread out and preventing them from combining to form a moon. The rings have a slight pale reddish color due to the presence of organic material mixed with the water ice.

Find out more from HubbleSite:

How close do Earth and Mars get?

Mars and Earth are like two cars on a racetrack as they journey around the Sun. Earth is on the inside track and travels faster than Mars, which is on the outside. When Earth laps Mars about every two years, it comes as close as 35 million miles (56 million km) or as far away as 63 million miles (101 million km) because of Mars’ highly elongated orbit.

What is a comet?

Comets are small, fragile, irregularly shaped bodies composed mostly of a mixture of water ice (ice composed of H20), dust, and carbon- and silicon-based compounds. They have highly elliptical orbits that repeatedly bring them very close to the Sun and then swing them into space. Comets have three distinct parts: a nucleus, a coma and a tail. The solid core is called the nucleus, which develops a coma with one or more tails when a comet sweeps close to the Sun. The coma is the dusty, fuzzy cloud around the nucleus of a comet, and the tail extends from the comet and points away from the Sun. The coma and tails of a comet appear only when the comet is near the Sun.

Find out more from HubbleSite:

How did comets form?

Comets are some of the material left over from the formation of the planets. Our entire solar system, including comets, was created by the collapse of a giant, diffuse cloud of gas and dust about 4.6 billion years ago. Much of the matter merged into planets, but some remained to form small lumps of frozen gas and dust in the outer region of the solar system, where temperatures were cold enough to produce ice.

Find out more from HubbleSite:

Why do comets have tails?

Comets are lumps of frozen water ice, gas and dust. As a comet approaches the Sun, it starts to heat up. The ice transforms directly from a solid to a vapor, releasing the dust particles embedded inside. Sunlight and the stream of charged particles flowing from the Sun – the solar wind – sweeps the evaporated material and dust back in a long tail. The comet's ingredients determine the types and number of tails.

What are the types of comet tails?

There are two types of comet tails: dust and gas ion. A dust tail contains small, solid particles that are about the same size found in cigarette smoke. This tail forms because sunlight pushes on these small particles, gently shoving them away from the comet’s nucleus. Because the pressure from sunlight is relatively weak, the dust particles end up forming a diffuse, curved tail. A gas ion tail forms when ultraviolet sunlight rips one or more electrons from gas atoms in the coma, making them into ions (a process called ionization). The solar wind then carries these ions straight outward away from the Sun. The resulting tail is straighter and narrower. Both types of tails may extend millions of kilometers into space. As a comet heads away from the Sun, its tail dissipates, its coma disappears, and the matter contained in its nucleus freezes into a rock-like material.

Where do comets come from?

Comets are found in two main regions of the solar system: the Kuiper Belt and the Oort Cloud. There are two types of comets: short-period comets and long-period comets.

Short-period comets – comets that frequently return to the inner solar system – probably come from the Kuiper Belt beyond the orbit of Neptune. Astronomers estimate that this belt contains at least 200 million objects, which are thought to have remained essentially unchanged since the birth of the solar system 4.6 billion years ago.

Long-period comets, which can take thousands of years to complete their orbits, are thought to emanate from the Oort Cloud, a vast group of frozen bodies in the outer part of the solar system. The Oort Cloud is thought to extend 50,000 times the distance from the Earth to the Sun.

Oort Cloud comets, like their Kuiper Belt brothers, probably originated in the region of the solar system between Jupiter and Neptune, but were ejected from to the Oort Cloud by close encounters with the gravity of the giant planets.

Comets are kicked out of the Oort Cloud and the Kuiper Belt by the pull of the gravity of another object – a planet, a star, or another small body. They then begin their journey toward the inner solar system and the Sun.

What path do comets follow through the solar system?

Planets have nearly circular orbits, but comets have elongated paths around the Sun. A comet is at "aphelion" when its orbit is farthest from the Sun. It is at "perihelion" when it is closest to the Sun. Due to angular momentum, a comet will travel fastest at perihelion and will slow down as it approaches aphelion.

Comets can be classified by their orbital period: that is, the time it takes them to make one complete trip around the Sun. Comets with short and intermediate orbital periods of less than 200 years – like Comet Halley, whose orbital period is 76 years – spend most of their time between Pluto and the Sun. These comets originally formed in the Kuiper Belt, but a gravitational "push" from the planets, especially Jupiter, swung them closer to the Sun.

A long-period comet will have an orbital period of more than 200 years. Comet Hale-Bopp, for example, completes an orbit about every 4,000 years. Scientists think that this type of comet spends most of its time way out in the Oort Cloud at the farthest edge of our solar system.

What is the difference between a meteor, a meteoroid, a meteorite, an asteroid and a comet?

Most of us probably have seen meteors or shooting stars. A meteor is the flash of light that we see in the night sky when a small chunk of interplanetary debris burns up as it passes through our atmosphere. "Meteor" refers to the flash of light caused by the debris, not the debris itself.

The debris is called a meteoroid. A meteoroid is a piece of interplanetary matter that is smaller than a kilometer and frequently only millimeters in size. Most meteoroids that enter the Earth's atmosphere are so small that they vaporize completely and never reach the planet's surface.

If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is called a meteorite. Although the vast majority of meteorites are very small, their size can range from about a fraction of a gram (the size of a pebble) to 100 kilograms (220 lbs) or more (the size of a huge, life-destroying boulder).

Asteroids are generally larger chunks of rock that come from the asteroid belt located between the orbits of Mars and Jupiter.

Comets are asteroid-like objects covered with ice, methane, ammonia, and other compounds that develop a fuzzy, cloud-like shell called a coma and sometimes a visible tail whenever they orbit close to the Sun.

Find out more from HubbleSite:

What is the asteroid belt?

The asteroid belt is a zone between the orbits of Mars and Jupiter. Scientists believe that the asteroids in the asteroid belt never formed a planet because the gravity of nearby Jupiter kept pulling them apart. Today, millions of asteroids probably inhabit the asteroid belt, with many more scattered throughout the solar system.

Find out more from HubbleSite:

What is Planet X?

When astronomer Percival Lowell decided to search for a planet beyond Neptune in the 1930s, he called the object of his search "Planet X." This search led to the discovery of Pluto, but for many years some astronomers believed that another world larger than Pluto must exist undiscovered beyond Neptune. They thought this because Neptune's orbit seemed to be influenced by the gravity of an unseen planet. More recent studies indicate that Neptune's orbit is not influenced by undiscovered bodies, and that a large "Planet X" most likely does not exist.

Some of the myths and misconceptions about Planet X are debunked on Phil Plait's Bad Astronomy website:

The prospects for new planet discoveries in our solar system are small. However, astronomers are now finding hundreds of planets orbiting other stars.

Has Hubble looked for Planet X?

Astronomers are always on the lookout for new members of the solar system. The most active programs involve searches for new comets, asteroids and satellites of the outer planets, but a few scientists have undertaken an on-again, off-again search for planets beyond Neptune.

Astronomers can rule out an object as big as Jupiter orbiting a little further out than Neptune, since its gravitational effect on Neptune’s orbit would be pretty obvious. It would also be easily visible because of its brightness. To go undetected, a planet similar in size and composition to the other outer planets would have to be very far from the Sun.

We do know of many other objects orbiting the Sun that are similar to Pluto, which belongs to the Kuiper Belt – a field of icy, comet-like bodies located beyond Neptune. In 2005, an object likely larger than Pluto, named Eris, was located in the Kuiper Belt.

Find out more on HubbleSite:

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