9781422283707

T H E S O L A R S Y S T E M

Far Planets and Beyond

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First printing 9 8 7 6 5 4 3 2 1 Series ISBN: 978-1-4222-3547-8 ISBN: 978-1-4222-3550-8 ebook ISBN: 978-1-4222-8370-7

Library of Congress Cataloging-in-Publication data is on file with the publisher.

T H E S O L A R S Y S T EM Comets and Meteors • Far Planets • Giant Planets • Near Planets Our Home Planet • Space Exploration • The Sun

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CONTENTS Introduction 4 Far, Far Away 6 Uranus, the New World 8 Uranus Inside and Out 10 Many Moons 12 Miranda’s a Miracle! 14 Neptune, the Blue World 16 The Stormy Atmosphere 18 Rings and Moons 20 Cold, Cold Triton 22 Pluto, the Tiniest World? 24 A Planet no Longer 26 New Horizons 28 Amazing Discoveries 30 Planets Beyond 32 Exoplanets 34 Nothing but the Facts 36 Time Line 38 Words to Understand 39 Index 40

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or thousands of years stargazers knew of only five planets besides the Earth. They could see these bodies with the naked eye, looking like bright stars wander- ing across the night sky. It was not until 1781 that another planet was spotted, which was named Uranus. The discovery of Uranus set astronomers looking for other new planets, and two more were eventually found— Neptune in 1846 and Pluto in 1930. Uranus and Neptune are large planets, each about four times bigger across than the Earth. And they are quite different from the Earth. Like the giant planets Jupiter and Saturn, they are made up mainly of gas and Int roduct ion F

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liquid. Also like those giants, they have rings and many moons circling them. Pluto, since 2006 re-classified as a dwarf planet, is a vastly different world from Uranus and Neptune and indeed from the planets in our Solar System . It is a tiny ball of rock and ice, smaller even than our own Moon. It is so far away that it takes nearly 250 years to circle once around the Sun. Pluto and other nearby dwarf planets are at the outer edge of the Solar System. But some astronomers are convinced that other planets exist far beyond Pluto, orbiting their own suns in their own systems.

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  the far planets before   the Space Age began.   Little was known about Far, Far Away

the naked eye under ideal viewing conditions. It looks like a very faint star. A telescope shows Uranus as a round disc, with a bluish-green color. But it does not show any markings, such as the bands that can be seen on Jupiter and Saturn. Five satellites , or moons, can also be seen circling round the planet. Neptune and Pluto are so far away and so faint that they are invisible to the naked eye. A telescope shows Neptune as a bluish disc, with two moons circling round it. But even a powerful telescope will show Pluto only as a pinpoint of light. Deep space Voyager To find out more about the planets, astronomers have enlisted the help of space scientists. In August 1977, a space probe named Voyager 2 blasted off the launch pad at Cape Canaveral in Florida. Its mission was to fly to the giant planets Jupiter and Saturn, then on to Uranus and Neptune. It was making use of a chance

Saturn is the most distant planet that we can see easily with the naked eye . Uranus, the nearest of the far planets, is much smaller than Saturn and very much farther away. But it can, just barely, be seen with

∆ Voyager 2 captured a photo of Neptune and its large moon Triton as it nears the planet.

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Three years later, Voyager 2 homed in on Neptune, passing closest in August 1989. It sent back spectacular pictures of storms and spotted icy volcanoes erupting on Triton, Neptune’s largest moon. Eyes in orbit Since the early 1990s, astronomers have continued to keep an eye on Uranus and Neptune using the Hubble Space Telescope (HST). The HST gets a much clearer view of the two planets than telescopes on Earth. Next stop, the stars Neptune was Voyager 2 ’ s last port of call. Since the early 2000s, it has soared far beyond, reaching more than 9 billion miles from the Earth. It is heading for interstellar space—the space between the stars. It will not get near another star for tens of thousands of years.

∆ The Voyager 2 probe beams radio signals back to Earth using its large dish antenna.

lineup of the outer planets that would not happen again for 176 years. A sister craft, Voyager 1 , set off a month later. Both Voyagers visited Jupiter and Saturn in turn, and then Voyager 1 began to head out of the Solar System. But Voyager 2 sped on to Uranus, flying past the planet in January 1986. It made many new discoveries, showing sets of rings around the planet and a swarm of tiny new moons .

Uranus, Neptune, and Pluto lie billions of miles from the Sun. Pluto lies 40 times farther away from the Sun than the Earth does. It was finally visted by a space probe, New Horizons , in 2015.

∆

Scientists use telescopes—and rocks and coconuts—to teach about distant planets.

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  size of the known Solar System.   The discovery of Uranus doubled the Uranus, the New World

But the object was neither a curious star nor a comet. It was a new planet, the first planet to be discovered since ancient times. It was named Uranus. When the orbit of the new planet was calculated, it was found to lie more than 1.78 billion miles away from the Sun. It was twice as far away as Saturn. A smaller giant

On the night of March 13, 1781,

William Herschel was stargazing. A musician by profession, he was a keen astronomer as well. William Herschel lived in Bath, in southwest England. That night he was

∆ Uranus symbol

∆ William Herschel

looking at the stars in the constellation Gemini, the Twins. He noticed that one of the “stars” appeared as a disc, not as a point, like all the other stars. It was, he wrote in his notebook, “a curious either nebulous star or perhaps a comet .”

Uranus is the third largest planet, after Jupiter and Saturn. With a diameter of about 31,500 miles (51,200 km), it is less than half the size of Saturn. But it is still four times the size of the Earth. Uranus is so far away that

it takes 84 Earth-years to travel once around the Sun. From the Earth,

we can watch it make its way

slowly through the heavens against the background of stars. Like all the planets, it travels through the constellations of the Zodiac .

∆ Uranus, the seventh planet out from the Sun, lies beyond Saturn.

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∆ Uranus is four times bigger across than our own planet.

All topsy–turvy Every planet spins around like a top as it travels in its orbit around the Sun. Most planets spin around in a nearly upright position in relation to the direction they are traveling. Their axis —the imaginary line around which they spin—is nearly at right angles to the direction they travel. Uranus is different. Its axis leans right over and lies nearly in the direction the planet travels. So, compared with the other planets, Uranus spins on its side. This means that, as it travels around the Sun, its poles face toward and away from the Sun in turn. Each pole spends 42 years in the sunlight, followed by 42 years in darkness.

∆ In Greek mythology, Uranus was god of the heavens. He was killed by Saturn, who was his youngest son.

∆ The axis of Uranus is tilted right over. No other planet’s axis is tilted over like this.

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Uranus Inside and Out

  greenish-blue gas giant.   Faint rings surround this

Clouds and winds When Voyager 2 visited Uranus, the

Like Jupiter and Saturn, Uranus is made up mainly of gas and liquid. It has no solid surface. On top, it has a deep atmosphere of hydrogen and helium gases, along with a little methane. (On Earth, methane is the main component of natural gas, which is piped into homes for cooking and heating.) The methane gives Uranus’s atmosphere its color. The gas absorbs the red light from sunlight, making the atmosphere appear bluish-green. Underneath the atmosphere is a deep, warm ocean of water, methane, and ammonia. And at the center of the planet is a core of solid rock .

atmosphere looked featureless in ordinary light. There were no bands of clouds like those on Jupiter. The clouds of methane that must have been present were hidden by a thick haze. Careful computer processing of the images made a few isolated clouds show up, together with faint bands that were presumably cloud belts. Winds forced the clouds along at speeds up to about 340 miles (550 km) an hour. Since then, the Hubble Space Telescope has spotted strings of bright clouds on the planet.

Uranus has an atmosphere thousands of miles deep, above a vast ocean and a rocky core.

∆

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∆ This false-color picture shows the many ringlets in two parts of the Epsilon ring.

Rings around Uranus In 1977, the year Voyager 2 was

launched, astronomers discovered that Uranus had a set of rings circling it. This made Uranus the second planet known to have rings, after Saturn. But Uranus’s rings are too faint to be seen from Earth. Nine rings in all were spotted when they passed in front of a star and one by one blocked its light. Voyager 2 discovered two more rings and pictured the whole ring system clearly. Compared with Saturn’s broad, bright shining rings, the rings around Uranus are narrow and very dark—as black as coal. Most of the other rings are less than 6 miles (10 km) wide. The outermost ring, the Epsilon, is the widest at up to 62 miles (100 km) across. ∆ In normal photographs, Uranus looks the same all over. But a false-color image shows a hazy region (orange).

Listen to the radio Voyager 2 made another discovery when it flew past Uranus. It picked up radio signals from around the planet. To scientists on the Earth, this could only mean one thing—that Uranus has a magnetic field around it, just like the Earth and the giant planets. Radio signals are set up when electrically charged particles whizz about inside a planet’s magnetic field. The magnetic field of a planet rotates as the planet itself rotates. By studying the radio signals the planet gives off, the

time of rotation can be found. The radio signals from Uranus showed that the planet rotates once every 17 hours 14 minutes. This means that its ‘day’ is about 7 hours shorter than our own (24 hours).

∆ True color

Background image: Six of the ten rings around Uranus.

∆ False color

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  Uranus’s icy moons are cracked and cratered. Many Moons

From the Earth, only five of Uranus’s moons can be seen in a telescope. Going out from the planet, they are Miranda, Ariel, Umbriel, Oberon, and Titania. Except for Umbriel, the moons were named after characters in William Shakespeare’s plays. When the Voyager 2 probe flew past Uranus in 1986, it discovered another ten tiny moons that were nearer the planet than Miranda. And in 1997 astronomers found two more moons very far out, making 17 in all. These new moons, too, have been given the names of characters in Shakespeare’s plays.

∆ This Hubble Space Telescope picture shows Uranus and seven of its moons.

∆ Deep valleys and steep cliffs scar the surface of Ariel.

Little shepherds The moons Voyager 2

discovered really are tiny. Even the biggest, Puck, is only about 93 miles (150 km) across. The two smallest, Cordelia and Ophelia, are only about 18 miles (30 km) across. These two moons circle each side of the outer ring of Uranus, the Epsilon. They are known as shepherd moons because of the way they seem to “ herd ” the particles in the ring and keep them in place.

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A large, fresh crater named Wunda shows up at the edge of this picture of Umbriel.

∆

Valleys and canyons

The old moons The five large moons of Uranus seem to be made up of a mixture of rock and ice. Astronomers work this out

Ariel and Titania have fewer craters and are cut by network of valleys, or canyons, several miles deep. These canyons must have been formed when the crusts of the moons moved and cracked long ago. Volcanoes would then have erupted in the cracks, or faults . But they would not have given off hot liquid rock, but ice-cold water. The water would then have frozen, producing the smooth floors the canyons have today.

from the density, or size and weight, of the bodies. The moons may have a large core of rock with a layer of ice on top, or they may be made up of a jumbled mass of rock and ice. But they all seem to have an icy surface. With a diameter of about 980 miles (1,580 km), Titania is the biggest moon, slightly bigger than Oberon. Ariel and Umbriel are smaller, both about 720 miles (1,160 km) across. They are more than twice the size of Miranda. Craters and cracks Craters are found all over the surface of the moons. They have been made by meteorites smashing into the surface. Some of the craters are bright, which shows that they are recent. The meteorites have exposed fresh white ice in the otherwise darker, dirtier surface. The most craters are found on Oberon and Umbriel, so these two moons probably have old surfaces.

∆ A great canyon called Messina Chasma cuts across Titania.

∆ The five main moons of Uranus compared in size with the Earth’s Moon at the bottom.

Earth’s Moon

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  oddest in the Solar System.   Miranda’s patchwork surface is the Miranda's a Miracle!

By chance, when Voyager 2 visited Uranus, the moon it passed closest to was Miranda. It flew within 1,860 miles (3000 km) of the smallest of Uranus’s moons that we can see from Earth. This was a stroke of luck, because Miranda is by far the most interesting of Uranus’s moons and one of the strangest moons in the Solar System. Miranda has all kinds of surface features jumbled up together, like the different patches in a patchwork quilt. Here is a patch of old cratered plains; there, a patch with young steep cliffs. Here is a peculiar, V-shaped, patch; there, a strange circular grooved region that looks like a gigantic racetrack.

∆ Different kinds of features meet on Miranda’s surface.

Geologists baffled Planetary geologists—the scientists who study the surface and makeup of planets—had never seen anything like Miranda’s surface before. They are not sure how it came about . One idea is that long ago Miranda suffered a catastrophe. It was hit by another huge object and was smashed to bits. In time, gravity pulled the bits together, and Miranda became a single moon once more. And the patches we now see are the tops of the separate bits that came together. The cratered plains are part of the surface of the old moon.

∆ Miranda’s icy cliffs, called Verona Rupes, soar to a height of more than 12 miles (20 km).

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