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You could stuff 1,300 Earths into Jupiter


MSolar systemuch like its neighbor Jupiter, the sixth planet from the sun has a rocky core and a gaseous surface. But Saturn is chiefly known for its intricate series of rings that encircle it. The mile-thick rings are made of countless orbiting ice particles, from less than an inch to several feet in size.

Up close, it\'s clear that Saturn has more rings than we can count. But though you can\'t see all of them from Earth, you can spot three of them with a good telescope,.

The two outermost rings are separated by a dark band called the Cassini Division, named for the astronomer who discovered it in 1675. The Cassini division isn\'t empty, but it has less material in it. The middle ring is the brightest, and just inside it is a fuzzy one that can be difficult to spot.

Saturn has 18 known satellites, made mostly of ice and rock. The largest, Titan, orbits Saturn every 16 days and is visible through a good-sized amateur telescope. Titan, which is larger than the planet Mercury, has a thick atmosphere that obscures its surface. Though researchers aren\'t sure how many moons Saturn has, the total is likely at least 20, and may be much higher.

Historical notes

When Galileo Galilei first studied Saturn in the early 1600s, he thought it was an object with three parts. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing -- a symbol with one large circle and two smaller ones -- in his notebook, as a noun in a sentence describing his discovery. Debate raged for more than 40 years about these \"ears,\" until Christiaan Huygens proposed that they were rings. Giovanni Domenico Cassini later discovered a gap between the rings, which gained his name, and he also proposed that the rings were not solid objects, but rather made of small particles.


TSolar systemhe seventh planet from the sun is much like its gaseous neighbors, with a cloudy surface, rapid winds, and a small rocky core.


Perhaps because of a collision with a large object long ago, Uranus orbits at an extreme tilt of 98 degrees -- sort of on its side. This causes one pole to point toward the sun for decades, giving the planet strange seasons.

Uranus has numerous satellites and a faint set of rings. If all the possible satellites being studied are confirmed, Uranus would have 16 regular and five irregular moons, making it the most populated planetary satellite system known. Saturn is known to have 18 satellites (there may be more, but they have not been well-documented).

Historical notes

Uranus was thought to be a star until William Herschel discovered in 1781 that it orbited the Sun.


TSolar systemhe eighth planet from the Sun -- well, some of the time it\'s eighth, but more on that later -- has a rocky core surrounded by ice, hydrogen, helium and methane.Like the other gas planets, Neptune has rapidly swirling winds, but it is thought to contain a deep ocean of water. Its quick rotation fuels fierce winds and myriad storm systems. The planet has a faint set of rings and 8 known moons.

Because of Pluto\'s strange orbit, Neptune is sometimes the most distant planet from the Sun. Since 1979, Neptune was the ninth planet from the Sun. On February 11, 1999, it crossed Pluto\'s path and once again become the eighth planet from the Sun, where will remain for 228 years.


Historical notes

Neptune was discovered in 1846 after mathematical calculations of Uranus\' movements predicted the existence of another large body.


PSolar systemluto, which is only about two-thirds the size of our moon, is a cold, dark and frozen place. Relatively little is known about this tiny planet with the strange orbit. Its composition is presumed to be rock and ice, with a thin atmosphere of nitrogen, carbon monoxide and methane. The Hubble Space Telescope has produced only fuzzy images (above) of the distant object.

Pluto\'s orbit

Pluto\'s 248-year orbit is off-center in relation to the sun, which causes the planet to cross the orbital path of Neptune. From 1979 until early 1999, Pluto had been the eighth planet from the sun. Then, on February 11, 1999, it crossed Neptune\'s path and once again became the solar system\'s most distant planet. It will remain the ninth planet for 228 years.

Pluto\'s orbit is inclined, or tilted, 17.1 degrees from the ecliptic -- the plane that Earth orbits in. Except for Mercury\'s inclination of 7 degrees, all the other planets orbit more closely to the ecliptic.

Interestingly, a similar thing happens with Jupiter\'s moons: Many orbit on the ecliptic, but some are inclined from that plane.

Did you wonder: Will Pluto and Neptune ever collide? They won\'t, because their orbits are so different. Pluto intersects the solar system\'s ecliptic, or orbital plane, twice as its orbit brings it \"above,\" then \"below\" that plane where most of the other planets\' revolve -- including Neptune. And, though they are neighbors Pluto and Neptune are always more than a billion miles apart.

Is it a planet at all?

Some astronomers think Pluto may have wandered into the system of planets from a more distant region known as the Kuiper belt -- a region beyond the orbit of Pluto thought to contain Pluto-like objects and comets that orbit the sun in a plane similar to the planets of the solar system.

If that\'s the case, Pluto is not a planet at all, but is probably more like a large asteroid or comet. Some have also suggested that it may have once been a moon of Neptune and escaped.

The International Astronomical Union, the organization responsible for classifying planets, gives these reasons for questioning Pluto\'s status as a planet:

  • All the other planets in the outer solar system are gaseous, giant planets whereas Pluto is a small solid object

  • Pluto is smaller than any other planet by more than a factor of 2.

  • Pluto\'s orbit is by far the most inclined with respect to the plane of the solar system, and also the most eccentric, with only the eccentricity of Mercury\'s orbit even coming close

  • Pluto\'s orbit is the only planetary orbit which crosses that of another planet (during 1999 Pluto will again cross Neptune\'s orbit, thus regaining its status as the most distant planet)

  • Pluto\'s satellite, Charon, is larger in proportion to its planet than any other satellite in the solar system.