Credit: NASA/Hubble Heritage

Saturn's Rings: As Saturn travels around the Sun, we see its majestic ring system from different angles. These images were taken with the Hubble Space Telescope from the year 1996 (bottom left) to 2000 (top right).

Notice how thin the rings are! They are 300,000 km across, and only a kilometer thick at most. They are made up of chunks of ice, most of which are about the size of softballs, though there are larger bodies, some a few kilometers across, and smaller pieces, all the way down to very fine dust. Each ring particle orbits Saturn like a tiny moon.

Planets

(Links | Questions)

Ancient people noticed that while most of the stars did not move, certain bright "stars" wandered through the constellations of the zodiac. These were the planets. Mercury, Venus, Mars, Jupiter, and Saturn are visible to the unaided eye. The planets Uranus and Neptune and the dwarf planets Ceres, Pluto and Eris were discovered with telescopes.

Many space missions have been sent to the planets, but they have all been unmanned probes. No human has yet traveled beyond our Moon to another planet, but in the coming decades, the first person may set foot on Mars.

The inner planets, Mercury, Venus, Earth and Mars, are called the terrestrial or earthlike planets. They are rocky planets with metal cores which have solid surfaces. The outer planets, Jupiter, Saturn, Uranus and Neptune, are jovian or jupiterlike. They have no solid surfaces, and are made mostly of hydrogen and helium, which is why they are also known as the gas giants.

The dwarf planet Ceres is a cratered ball of rock and is the largest object in the asteroid belt. The dwarf planets Pluto and Eris are frigid, icy worlds, and are part of the Kuiper Belt.

The Ask an Astronomer team's favorite links about Planets:

Definition of a Planet - The history and current meaning of the term "planet".
The ~~Nine~~ Eight Planets: An excellent resource for information about each planet.
NASA's Planetary Photojournal has all the best images of planets.
Weekly Planet Roundup: Find out what planets are visible in the night sky, as part of Sky and Telescope Magazine's Sky at a Glance feature.
Celestia: A free, easy to use, downloadable 3D simulation of the solar system and nearby stars. Point and click to travel to any planet, moon, satellite or star. Awesome!
If you are interested in detailed calculations of the positions of the planets, see NASA's Solar System Dynamics site and How to Compute Planetary Positions.

Previously asked questions about Planets:

General questions:

Why are stars and planets round? (Beginner)
Is there really a 10th planet? (Beginner)
What are the requirements for being a planet? (Beginner)
What are the names of the earth, moon, sun, and solar system? (Beginner)
Who named the planets and who decides what to name them? (Beginner)
How can I find my age on another planet? (Beginner)
Can we colonize other planets? (Beginner)
Will we discover an 11th planet? What would it be called? (Beginner)
Why are craters round? (Beginner)
What is between the Oort cloud and the closest star? (Beginner)
Do planets radiate noise? (Beginner)
Who discovered each planet? (Intermediate)
Why is our solar system so different from all the others we've found? (Intermediate)
Can the theory of formation of the solar system explain all the observables? (Intermediate)
What is the size of the Solar System? (Intermediate)
Why do planets rotate? (Intermediate)
How do I make a scale model of the Solar System? (Intermediate)
If clocks run slow in a gravitational field, how can we know the true age of the Solar System and Universe? (Intermediate)
What is the physical difference between a star and a planet? (Intermediate)
Where is the supernova remnant that led to our solar system? (Intermediate)
How far is each planet from Earth? (Intermediate)
What is the evidence supporting the nebula theory of Solar System formation? (Intermediate)
How does melting a material reset its radioactive clock? (Intermediate)
If the sun went out and our atmosphere froze solid, what color would it be? (Intermediate)
How do we know what other planets (and stars, galaxies, etc.) are made of? (Intermediate)
Could a planetary system survive if its star merged with another, or if its star went supernova? (Intermediate)
Can we prove that the Moon isn't hollow? (Intermediate)
How are galaxies and solar systems similar? (Advanced)
How can we tell what the interiors of planets are like? (Advanced)
What color would Mars be if you were standing on it? (Advanced)

Orbits:

What direction do planets rotate? (Beginner)
When was the last time all 9 planets were aligned? (Beginner)
How do you measure a planet's mass? (Beginner)
Why do planets have elliptical orbits? (Beginner)
Why do the planets orbit the sun? (Beginner)
Can the sun rise from the west on Mars? (Beginner)
Why do all the planets orbit in the same plane? (Intermediate)
How can two moons of Saturn share the same orbit? (Intermediate)
Why do most moons orbit their planets at the equator? (Intermediate)
Can double stars have planets? (Intermediate)
What is the significance of the second focus of elliptical orbits of planets in our Solar System? (Intermediate)
What is the pattern to the distances between each planet and the sun? (Intermediate)
Are the planes of solar systems aligned with the plane of the Galaxy? (Intermediate)
Could a planetary system survive if its star merged with another, or if its star went supernova? (Intermediate)
How close do you have to come to the Earth to be influenced by its gravity? (Advanced)
What is the three-body problem? (Advanced)
When the Sun converts mass to energy, do the orbits of the planets change? (Advanced)

Planet watching:

How do I tell which planet is which? (Beginner)
What is the observational difference between a star and a planet? (Beginner)
What was that eclipse I saw in the sky? (Beginner)
What color is Venus? (Beginner)
Can I see details on Venus? (Intermediate)
Why do planets not twinkle? (Intermediate)
Why can't we see Venus at night when it is an inner planet? (Intermediate)
What color is each planet? (Intermediate)
Why do pictures of distant galaxies have higher resolution than those of nearby planets? (Intermediate)

Moons:

Where can I find information on the moons of other planets? (Intermediate)
Does the Moon rotate? Are there other moons that always keep one face toward their planet? (Intermediate)
How can two moons of Saturn share the same orbit? (Intermediate)
Why do most moons orbit their planets at the equator? (Intermediate)
Why does the Earth have only one moon? (Intermediate)
Is the gravitational force exerted by the Earth on the Moon equal to the centripetal force acting on the Moon? (Intermediate)
Why are there active volcanos on Io? (Intermediate)
Will astronauts ever be able to go ice-skating on Europa? (Intermediate)
Have astronomers discovered Earth's second moon? (Intermediate)
Can we prove that the Moon isn't hollow? (Intermediate)
How do planets capture satellites? (Advanced)

Ring systems:

Which planets have rings? (Beginner)
Where do planetary rings come from, and what are they made of? (Beginner)
Why are Saturn's rings flat? (Intermediate)
Can terrestrial planets have rings? (Intermediate)

Venus:

What was that eclipse I saw in the sky? (Beginner)
What color is Venus? (Beginner)
Can I see details on Venus? (Intermediate)
Why can't we see Venus at night when it is an inner planet? (Intermediate)

Comparisons with Earth:

Can the sun rise from the west on Mars? (Beginner)
Which planet is most similar to Earth? (Intermediate)
Is there oxygen in the atmospheres of other planets? (Intermediate)
What color would Mars be if you were standing on it? (Advanced)

Mars:

How did Mars get so dusty? (Beginner)
During close approaches, can Mars appear in our sky as large as the Moon? (Beginner)
Can the sun rise from the west on Mars? (Beginner)
Is it sensible to consider terraforming Mars? (Beginner)
Why does one side of Mars look more catastrophic than the other? (Intermediate)
What are the atmospheres of Mars and Pluto like? (Intermediate)
If Mars is only about 35-60 million miles away at close approach, why does it take 6-8 months to get there? (Intermediate)
What was different on Mars in the past to allow liquid water to be present? (Intermediate)
What color would Mars be if you were standing on it? (Advanced)

Jupiter:

Do planets radiate noise? (Beginner)
Why doesn't the hydrogen on Jupiter explode? (Intermediate)
Why doesn't the ammonia and methane in Jupiter's atmosphere sink into the core? (Advanced)

Saturn:

How can two moons of Saturn share the same orbit? (Intermediate)
Why are Saturn's rings flat? (Intermediate)

Uranus:

Why is Uranus tipped on its side? (Beginner)

Pluto:

What are the atmospheres of Mars and Pluto like? (Intermediate)
Is Pluto a planet? (Intermediate)
What happened to the girl who named Pluto? (Intermediate)

How to ask a question:

If you have a question about Planets which isn't answered above, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

URL: http://curious.astro.cornell.edu/planets.php
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Last modified: October 18, 2007 12:14:32 PM

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