Uranus is the seventh planet from the sun and the first to be discovered by scientists. Although Uranus is visible to the naked eye, it was long mistaken as a star because of the planet's dimness and slow orbit. The planet is also notable for its dramatic tilt, which causes its axis to point nearly directly at the sun. British astronomer William Herschel discovered Uranus on March 13, , with his telescope while surveying all stars down to those about 10 times dimmer than can be seen by the naked eye.
One star seemed different, and within a year Herschel realized the star followed a planetary orbit. Uranus as it was called commonly after or so was named after the Greek sky deity Ouranos, the earliest of the lords of the heavens. It is the only planet to be named after a Greek god rather than a Roman one. Before the name was settled on, many names had been proposed for the new planet, including Hypercronius "above Saturn" , Minerva the Roman goddess of wisdom and Herschel, after its discoverer.
German astronomer Johann Bode, who detailed Uranus' orbit, gave the planet its ultimate name. Bode argued that as Saturn was the father of Jupiter, the new planet should be named for the father of Saturn.
Uranus is blue-green in color, as a result of the methane in its mostly hydrogen-helium atmosphere. Unlike the other planets of the solar system, Uranus is tilted so far that it essentially orbits the sun on its side, with the axis of its spin nearly pointing at the star. This unusual orientation might be due to a collision with a planet-size body, or several small bodies, soon after it was formed.
A study suggested the colliding world could have been twice the size of Earth. This unusual tilt gives rise to extreme seasons that last for about 20 years. This means that for nearly a quarter of the Uranian year, which is equal to 84 Earth-years, the sun shines directly over each pole, leaving the other half of the planet to experience a long, dark and frigid winter.
Uranus has the coldest atmosphere of any of the planets in the solar system, even though it is not the most distant from the sun. That's because Uranus has little to no internal heat to supplement the heat from the sun. One astronomical unit abbreviated as AU , is the distance from the Sun to Earth. From this distance, it takes sunlight 2 hours and 40 minutes to travel from the Sun to Uranus.
One day on Uranus takes about 17 hours the time it takes for Uranus to rotate or spin once. And Uranus makes a complete orbit around the Sun a year in Uranian time in about 84 Earth years 30, Earth days. Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of This unique tilt causes the most extreme seasons in the solar system.
For nearly a quarter of each Uranian year, the Sun shines directly over each pole, plunging the other half of the planet into a year-long, dark winter. Uranus is also one of just two planets that rotate in the opposite direction than most of the planets Venus is the other one , from east to west. Uranus has 27 known moons. While most of the satellites orbiting other planets take their names from Greek or Roman mythology, Uranus' moons are unique in being named for characters from the works of William Shakespeare and Alexander Pope.
All of Uranus' inner moons appear to be roughly half water ice and half rock. The composition of the outer moons remains unknown, but they are likely captured asteroids. Uranus has two sets of rings. The inner system of nine rings consists mostly of narrow, dark grey rings. There are two outer rings: the innermost one is reddish like dusty rings elsewhere in the solar system, and the outer ring is blue like Saturn's E ring.
Some of the larger rings are surrounded by belts of fine dust. Uranus took shape when the rest of the solar system formed about 4. Like its neighbor Neptune, Uranus likely formed closer to the Sun and moved to the outer solar system about 4 billion years ago, where it is the seventh planet from the Sun.
Uranus is one of two ice giants in the outer solar system the other is Neptune. Near the core, it heats up to 9, degrees Fahrenheit 4, degrees Celsius. Uranus is slightly larger in diameter than its neighbor Neptune, yet smaller in mass. It is the second least dense planet; Saturn is the least dense of all. Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus' cloud tops.
Methane gas absorbs the red portion of the light, resulting in a blue-green color. The planet is mostly swirling fluids. The four planets farthest from the Sun are the outer planets. Figure below shows the relative sizes of the outer planets and the Sun.
These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants. This image shows the four outer planets and the Sun, with sizes to scale. From left to right, the outer planets are Jupiter, Saturn, Uranus, and Neptune. The gas giants are made up primarily of hydrogen and helium, the same elements that make up most of the Sun.
Astronomers think that hydrogen and helium gases comprised much of the solar system when it first formed. The Sun and the massive outer planets had enough gravity to keep hydrogen and helium from drifting away.
All of the outer planets have numerous moons. They all also have planetary rings , composed of dust and other small particles that encircle the planet in a thin plane. Because Jupiter is so large, it reflects a lot of sunlight.
Jupiter is extremely bright in the night sky; only the Moon and Venus are brighter Figure below. This brightness is all the more impressive because Jupiter is quite far from the Earth — 5.
It takes Jupiter about 12 Earth years to orbit once around the Sun. This image of Jupiter was taken by Voyager 2 in The colors were later enhanced to bring out more details. Jupiter is named for the king of the gods in Roman mythology.
The planet is enormous, the largest object in the solar system besides the Sun. Like the other gas giants, it is much less dense than Earth. Astronauts trying to land a spaceship on the surface of Jupiter would find that there is no solid surface at all!
Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements Figure below. Deeper within the planet, pressure compresses the gases into a liquid.
Some evidence suggests that Jupiter may have a small rocky core of heavier elements at its center. These bands rotate around the planet, but also swirl around in turbulent storms. This storm is more than three times as wide as the entire Earth. Clouds in the storm rotate in a counterclockwise direction, making one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least years, since astronomers could first see the storm through telescopes. Do you think the Great Red Spot is a permanent feature on Jupiter?
How could you know? The white storm just below the Great Red Spot is about the same diameter as Earth. Jupiter has a very large number of moons — 63 have been discovered so far.
Four are big enough and bright enough to be seen from Earth, using no more than a pair of binoculars. These moons — Io, Europa, Ganymede, and Callisto — were first discovered by Galileo in , so they are sometimes referred to as the Galilean moons Figure below. The Galilean moons are larger than the dwarf planets Pluto, Ceres, and Eris.
Ganymede is not only the biggest moon in the solar system it is even larger than the planet Mercury! This composite image shows the four Galilean moons and their sizes relative to the Great Red Spot. From top to bottom, the moons are Io, Europa, Ganymede, and Callisto. Sizes are to scale. Scientists are particularly interested in Europa because it may be a place to find extraterrestrial life.
What features might make a satellite so far from the Sun a candidate for life?
0コメント