A planet is a large, round heavenly body that orbits a star and shines with light reflected from the star. We know of nine planets that orbit the sun in our solar system. Since 1992, astronomers have also discovered many planets orbiting other stars.
|The sun blazes with energy. On its surface, magnetic forces create loops and streams of gas that extend tens of thousands of miles or kilometers into space. This image was made by photographing ultraviolet radiation given off by atoms of iron gas that are hotter than 9 million degrees F (5 million degrees C). Image credit: NASA/Transition Region & Coronal Explorer|
The usual order of the planets in our solar system, outward from the sun, is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. To help remember the order, some people use the phrase My Very Educated Mother Just Sent Us Nine Pizzas as a memory aid. The initial letters of the words in that phrase match the initial letters of the planet names.
Pluto is not always the farthest planet from the sun, however. Its orbit is such a long oval that Pluto moves inside the path of Neptune for about 20 years every 248 years. One such 20-year period lasted from Jan. 23, 1979, to Feb. 11, 1999.
The planets of our solar system can be divided into two groups, except for Pluto. The innermost four planets — Mercury, Venus, Earth, and Mars — are small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets have from 38 to 95 percent of Earth’s diameter and from 5.5 to 82 percent of Earth’s mass (amount of matter).
The next four planets — Jupiter, Saturn, Uranus, and Neptune — are called gas giants or Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other materials also occur, including traces of ammonia and methane in their atmospheres. They range from 3.9 times to 11.2 times Earth’s diameter and from 15 times to 318 times Earth’s mass. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of infrared radiation, which is felt as heat, instead of visible light. Scientists think the source of some of the energy is probably the slow compression of the planets by their own gravity.
The ninth planet, Pluto, is only 19 percent the diameter of Earth and 1/500 of its mass. As a small, rocky planet with a larger orbit than the gas giants, it does not fit in either group. Some astronomers think that Pluto may not be a major planet at all.
This article discusses Planet (Observing the planets) (How planets move) (The planets of our solar system) (Planets in other solar systems) (How the planets formed).
Observing the planets
People have known the inner six planets of our solar system for thousands of years because they are visible from Earth without a telescope. The outermost three planets — Uranus, Neptune, and Pluto — were discovered by astronomers, beginning in the 1780’s. All three can be seen from Earth with a telescope.
To the unaided eye, the planets look much like the background stars in the night sky. However, the planets move slightly from night to night in relation to the stars. The name planet comes from a Greek word meaning to wander. The planets and the moon almost always follow the same apparent path through the sky. This path, known as the zodiac, is about 16¡ wide. At its center is the ecliptic, the apparent path of the sun. If you see a bright object near the ecliptic at night or near sunrise or sunset, it is most likely a planet. You can even see the brightest planets in the daytime, if you know where to look.
Planets and stars also differ in the steadiness of their light when viewed from Earth’s surface. Planets shine with a steady light, but stars seem to twinkle.
The twinkling is due to the moving layers of air that surround Earth. Stars are so far away that they are mere points of light in the sky, even when viewed through a telescope. The atmosphere bends the starlight passing through it. As small regions of the atmosphere move about, the points of light seem to dance and change in brightness.
Planets, which are much closer, look like tiny disks through a telescope. The atmosphere scatters light from different points on a planet’s disk. However, enough light always arrives from a sufficient number of points to provide a steady appearance. How planets move
Planets move in two main ways. They travel around their parent star in paths called orbits. As each planet orbits its star, it also rotates on its axis, an imaginary line through its center.
Viewed from Earth’s surface, the planets of the solar system and the stars appear to move around Earth. They rise in the east and set in the west each night. Most of the time, the planets move westward across the sky slightly more slowly than the stars do. As a result, the planets seem to drift eastward relative to the background stars. This motion is called prograde. For a while each year, however, the planets seem to reverse their direction. This backward motion is called retrograde.
In ancient times, most scientists thought that the moon, sun, planets, and stars actually moved around Earth. One puzzle that ancient scientists struggled to explain was the annual retrograde motion of the planets. In about A.D. 150, the Greek astronomer Ptolemy developed a theory that the planets orbited in small circles, which in turn orbited Earth in larger circles. Ptolemy thought that retrograde motion was caused by a planet moving on its small circle in an opposite direction from the motion of the small circle around the big circle.
In 1543, the Polish astronomer Nicolaus Copernicus showed that the sun is the center of the orbits of the planets. Our term solar system is based on Copernicus’s discovery. Copernicus realized that retrograde motion occurs because Earth moves faster in its orbit than the planets that are farther from the sun. The planets that are closer to the sun move faster in their orbits than Earth travels in its orbit. Retrograde motion occurs whenever Earth passes an outer planet traveling around the sun or an inner planet passes Earth.
In the 1600’s, the German astronomer Johannes Kepler used observations of Mars by the Danish astronomer Tycho Brahe to figure out three laws of planetary motion. Although Kepler developed his laws for the planets of our solar system, astronomers have since realized that Kepler’s laws are valid for all heavenly bodies that orbit other bodies.
Kepler’s first law says that planets move in elliptical (oval-shaped) orbits around their parent star — in our solar system, the sun. An ellipse is a closed curve formed around two fixed points called foci. The ellipse is formed by the path of a point moving so that the sum of its distances from the two foci remains the same. The orbital paths of the planets form such curves, with the parent star at one focus of the ellipse. Before Kepler, scientists had assumed that the planets moved in circular orbits.
Kepler’s second law says that an imaginary line joining the parent star to its planet sweeps across equal areas of space in equal amounts of time. When a planet is close to its star, it moves relatively rapidly in its orbit. The line therefore sweeps out a short, fat, trianglelike figure. When the planet is farther from its star, it moves relatively slowly. In this case, the line sweeps out a long, thin figure that resembles a triangle. But the two figures have equal areas.
Kepler’s third law says that a planet’s period (the time it takes to complete an orbit around its star) depends on its average distance from the star. The law says that the square of the planet’s period — that is, the period multiplied by itself — is proportional to the cube of the planet’s average distance from its star — the distance multiplied by itself twice — for all planets in a solar system.
The English scientist, astronomer, and mathematician Isaac Newton presented his theory of gravity and explained why Kepler’s laws work in a treatise published in 1687. Newton showed how his expanded version of Kepler’s third law could be used to find the mass of the sun or of any other object around which things orbit. Using Newton’s explanation, astronomers can determine the mass of a planet by studying the period of its moon or moons and their distance from the planet.
Planets rotate at different rates. One day is defined as how long it takes Earth to rotate once. Jupiter and Saturn spin much faster, in only about 10 hours. Venus rotates much slower, in about 243 Earth days.
Most planets rotate in the same direction in which they revolve around the sun, with their axis of rotation standing upright from their orbital path. A law of physics holds that such rotation does not change by itself. So astronomers think that the solar system formed out of a cloud of gas and dust that was already spinning.
Uranus and Pluto are tipped on their sides, however, so that their axes lie nearly level with their paths around the sun. Venus is tipped all the way over. Its axis is almost completely upright, but the planet rotates in the direction opposite from the direction of its revolution around the sun. Most astronomers think that some other objects in the solar system must have collided with Uranus, Pluto, and Venus and tipped them.
|The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. Image credit: NASA|
The planets of our solar system
Astronomers measure distances within the solar system in astronomical units (AU). One astronomical unit is the average distance between Earth and the sun, which is about 93 million miles (150 million kilometers). The inner planets have orbits whose diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Because of their different distances from the sun, the temperature, surface features, and other conditions on the planets vary widely. The discovery of other large objects has led some scholars to debate the number of planets in our solar system.
Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close to the sun that temperatures on its surface can climb as high as 800 degrees F (430 degrees C). But some regions near the planet’s poles may be always in shadow, and astronomers speculate that water or ice may remain there. No spacecraft has visited Mercury since the 1970’s, when Mariner 10 photographed about half the planet’s surface at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by Mercury three times before going into orbit around the planet in 2011.
|Thick clouds of sulfuric acid cover Venus. Image credit: NASA|
Venus is known as Earth’s twin because it resembles Earth in size and mass, though it has no moon. Venus has a dense atmosphere that consists primarily of carbon dioxide. The pressure of the atmosphere on Venus’s surface is 90 times that of Earth’s atmosphere. Venus’s thick atmosphere traps energy from the sun, raising the surface temperature on Venus to about 870 degrees F (465 degrees C), hot enough to melt lead. This trapping of heat is known as the greenhouse effect. Scientists have warned that a similar process on Earth is causing permanent global warming. Several spacecraft have orbited or landed on Venus. In the 1990’s, the Magellan spacecraft used radar — radio waves bounced off the planet — to map Venus in detail.
|Earth, our home planet, has oceans of liquid water, and continents that rise above sea level. Image credit: NASA/Goddard Space Flight Center|
Earth, our home planet, has an atmosphere that is mostly nitrogen with some oxygen. Earth has oceans of liquid water and continents that rise above sea level. Many measuring devices on the surface and in space monitor conditions on our planet. In 1998, the National Aeronautics and Space Administration (NASA) launched the first of a series of satellites called the Earth Observing System (EOS). The EOS satellites will carry remote-sensing instruments to measure climate changes and other conditions on Earth’s surface.
|The planet Mars has clouds in its atmosphere and a deposit of ice at its north pole. Image credit: NASA/JPL/Malin Space Science Systems|
Mars is known as the red planet because of its reddish-brown appearance, caused by rusty dust on the Martian surface. Mars is a cold, dry world with a thin atmosphere. The atmospheric pressure (pressure exerted by the weight of the gases in the atmosphere) on the Martian surface is less than 1 percent the atmospheric pressure on Earth. This low surface pressure has enabled most of the water that Mars may once have had to escape into space.
The surface of Mars has giant volcanoes, a huge system of canyons, and stream beds that look as if water flowed through them in the past. Mars has two tiny moons, Phobos and Deimos. Many spacecraft have landed on or orbited Mars.
|The layers of dense clouds around Jupiter appear in a photograph of the planet taken by the Voyager 1 space probe. Image credit: JPL|
Jupiter, the largest planet in our solar system, has more mass than the other planets combined. Like the other Jovian planets, it has gaseous outer layers and may have a rocky core. A huge storm system called the Great Red Spot in Jupiter’s atmosphere is larger than Earth and has raged for hundreds of years.
Jupiter’s four largest moons — Io, Europa, Ganymede, and Callisto — are larger than Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter’s equator are three thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a probe into Jupiter’s atmosphere. Galileo orbited Jupiter from 1995 to 2003.
|Saturn is encircled by seven major rings. Image credit: NASA/JPL/Space Science Institute|
Saturn, another giant planet, has a magnificent set of gleaming rings. Its gaseous atmosphere is not as colorful as Jupiter’s, however. One reason Saturn is relatively drab is that its hazy upper atmosphere makes the cloud patterns below difficult to see. Another reason is that Saturn is farther than Jupiter from the sun. Because of the difference in distance, Saturn is colder than Jupiter. Due to the temperature difference, the kinds of chemical reactions that color Jupiter’s atmosphere occur too slowly to do the same on Saturn.
Saturn’s moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up views of Saturn and its rings and moons.
The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was designed to be dropped into Titan’s atmosphere.
|Uranus appears in true colors, left, and false colors, right, in images produced by combining numerous pictures taken by the Voyager 2 spacecraft. Image credit: JPL|
Uranus was the first planet discovered with a telescope. German-born English astronomer William Herschel found it in 1781. He at first thought he had discovered a comet. Almost 200 years later, scientists detected 10 narrow rings around Uranus when the planet moved in front of a star and the rings became visible. Voyager 2 studied Uranus and its rings and moons close-up in 1986.
|The blue clouds of Neptune are mostly frozen methane. The other object shown is Neptune’s moon Triton. Image credit: NASA/JPL|
Neptune was first observed in 1846 by German astronomer Johann G. Galle after other astronomers predicted its position by studying how it affected Uranus’s orbit. In 1989, Voyager 2 found that Neptune had a storm system called the Great Dark Spot, similar to Jupiter’s Great Red Spot. But five years later, in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished. Neptune has four narrow rings, one of which has clumps of matter. Neptune’s moon Triton is one of the largest in the solar system and has volcanoes that emit plumes of frozen nitrogen.
|Pluto is so far from Earth that even powerful telescopes reveal little detail of its surface. The Hubble Space Telescope gathered the light for the pictures of Pluto shown here. Image credit: NASA|
Tiny, distant Pluto has been difficult to study because it is so far from Earth. The American astronomer Clyde W. Tombaugh discovered Pluto in 1930. Only in 1978, when astronomers discovered a moon orbiting Pluto, could they determine the planet’s mass. They found that Pluto was much less massive than expected. The Hubble Space Telescope found a dozen areas of contrasting light and dark on Pluto.
How many planets?
In the early 2000’s, the discovery of nearly Pluto-sized objects in our outer solar system caused increasing debate over whether Pluto should be considered a planet. In 2002, astronomers discovered an object called Quaoar (KWAH oh wahr) that, like Pluto, lies beyond the orbit of Neptune. Quaoar measures about 780 miles (1,250 kilometers) in diameter, larger than any previously known asteroid and roughly the size of Pluto’s moon Charon. In 2004, astronomers found another planetlike object three times farther from the sun than Pluto. The object, which the discoverers called Sedna, appeared to be as large as 1,100 miles (1,800 kilometers) in diameter, about three-fourths the size of Pluto. Many people wondered whether these objects should be called planets, and, if not, whether Pluto should be called a planet.
Nearly all astronomers agree that a planet is a body that orbits a star and does not orbit another body. The moon, for example, cannot be a planet because it orbits Earth. However, millions of objects, including all the known asteroids, orbit the sun. For this reason, much of the debate has centered around the minimum mass such an object should have to be considered a planet.
Some astronomers have proposed that a planet should have a mass greater than the total mass of all other bodies in nearby orbits. Some asteroids, for example, orbit the sun at nearly the same distance as Earth does. Of these objects, only Earth would be considered a planet, because it has more mass than all of these asteroids combined. Pluto, on the other hand, would not be considered a planet, because its mass is not greater than the combined mass of Quaoar and other objects that share similar orbits. Other astronomers have proposed that an object is large enough to be a planet if it has enough mass to be shaped into a ball by its own gravity. This definition would include Pluto as well as Quaoar, Sedna, the asteroid Ceres, and perhaps many other objects.
Many astronomers have chosen to continue classifying Pluto as a planet at least until a more formal definition of a planet can be established. They continue to describe Quaoar, Sedna, and similar objects as planetoids or minor planets.
Planets in other solar systems
How planets are detected
Even with the most advanced telescopes, astronomers cannot see planets orbiting other stars directly. The planets shine only by reflected light and are hidden by the brilliance of their parent stars. The planets and their stars are also much farther away than our sun. The nearest star is 4.2 light-years away, compared to 8 light-minutes for the sun. One light-year is the distance that light travels in one year — about 5.88 trillion miles (9.46 trillion kilometers). Thus, it takes light 4.2 years to reach Earth from the nearest star beyond the sun and only 8 minutes to reach Earth from the sun.
Scientists know of more than 100 stars other than the sun that have planets. Astronomers cannot see planets around distant stars. However, they can detect the planets from tiny changes in the stars’ movement and tiny decreases in the amount of light coming from the stars. The changes in a star’s movement are caused by the slight pull of the planet’s gravity on its parent star. To find new planets, astronomers use a technique called spectroscopy, which breaks down the light from stars into its component rainbow of colors. The scientists look for places in the rainbow where colors are missing. At these places, dark lines known as spectral lines cross the rainbow. The spectral lines change their location in the rainbow slightly as a star is pulled by the gravity of an orbiting planet toward and away from Earth. These apparent changes in a star’s light as the star moves are due to a phenomenon known as the Doppler effect. The changes not only show that a planet is present but also indicate how much mass it has.
The amount of light coming from the star decreases when the planet passes in front of the star. The planet blocks some of the starlight, dimming the star.
The first discoveries
Astronomers announced the discovery of the first planets around a star other than our sun in 1992. The star is a pulsar named PSR B1257+12 in the constellation Virgo. Pulsars are dead stars that have collapsed until they are only about 12 miles (20 kilometers) across. They spin rapidly on their axes, sending out radio waves that arrive on Earth as pulses of radio energy. Some pulsars spin hundreds of times each second. If a pulsar has a planet, the planet pulls the star to and fro slightly as it orbits. These pulls cause slight variations in the radio pulses. From measurements of these variations, the Polish-born American astronomer Alexander Wolszczan and American Dale A. Frail discovered three planets in orbit around PSR B1257+12. The star emits such strong X rays, however, that no life could survive on its planets.
Astronomers soon began to find planets around stars more like the sun. In 1995, Swiss astronomers Michel Mayor and Didier Queloz found the first planet orbiting a sunlike star, 51 Pegasi, in the constellation Pegasus. American astronomers Geoffrey W. Marcy and R. Paul Butler confirmed the discovery and found planets of their own around other stars. In 1999, astronomers announced the first discovery of a multiple-planet system belonging to a sunlike star. They determined that three planets orbit the star Upsilon Andromedae, which is 44 light-years from Earth in the constellation Andromeda.
Also in 1999, American astronomer Gregory W. Henry first detected a dimming of starlight due to the presence of a planet. The star that Henry observed is known as HD 209458, and it is located in Pegasus. Henry measured the star’s brightness at the request of Marcy, Butler, and American astronomer Steven S. Vogt, who had previously used the spectroscopic technique to identify this star as a parent of a planet.
Some stars have a planet orbiting them at a distance at which living things could exist. Most scientists consider liquid water essential for life, so a region that is neither too hot nor too cold for liquid water is known as a habitable zone. Although astronomers have found stars with planets in their habitable zones, all the planets found so far are probably gaseous with no solid surface. But they may have solid moons.
In 2001, Marcy announced the discovery of a solar system containing an extremely unusual object. That object and an ordinary planet orbit the star HD 168443, which is 123 light-years away in the constellation Serpens. The object is so unusual because of its mass. It is at least 17 times as massive as Jupiter.
Astronomers are not yet sure how to classify the object. They had not thought that a planet could be as massive as the object is. Before this discovery, the only known heavenly bodies of such mass were dim objects called brown dwarfs. But brown dwarfs form by means of the same process that forms stars, not planets.
Astronomers also have been surprised to find that other solar systems have huge, gaseous planets in close orbits. In our own solar system, the inner planets are rocky and small, and only the outer planets, except for Pluto, are huge and gassy. But several newly discovered planets have at least as much mass as Jupiter, the largest planet in our solar system. Unlike Jupiter, however, these massive planets race around their stars in only a few weeks. Kepler’s third law says that for a planet to complete its orbit so quickly, it must be close to its parent star. Several of these giant planets, therefore, must travel around their stars even closer than our innermost planet, Mercury, orbits our sun. Such close orbits would make their surfaces too hot to support life as we know it.
Some newly discovered planets follow unusual orbits. Most planets travel around their stars on nearly circular paths, like those of the planets in our solar system. But a planet around the star 16 Cygni B follows an extremely elliptical orbit. It travels farther from its star than the planet Mars does from our sun, and then draws closer to the star than Venus does to our sun. If a planet in our solar system traveled in such an extreme oval, its gravity would disrupt the orbits of the other planets and toss them out of their paths.
Throughout the early 2000’s, astronomers continued to improve techniques for detecting planets, enabling them to discover an increasing variety of planets around other stars. In 2004, astronomers announced the first discoveries of planets much smaller than Jupiter. The newly discovered planets were about the size of Uranus or Neptune. Despite the planets’ huge size, astronomers theorized that some of them might be rocky planets rather than gas giants.
How the planets formed
Astronomers have developed a theory about how our solar system formed that explains why it has small, rocky planets close to the sun and big, gaseous ones farther away. Astronomers believe our solar system formed about 4.6 billion years ago from a giant, rotating cloud of gas and dust called the solar nebula. Gravity pulled together a portion of gas and dust at the center of the nebula that was denser than the rest. The material accumulated into a dense, spinning clump that formed our sun.
The remaining gas and dust flattened into a disk called a protoplanetary disk swirling around the sun. Protoplanetary disks around distant stars were first observed through telescopes in 1983. Rocky particles within the disk collided and stuck together, forming bodies called planetesimals. Planetesimals later combined to form the planets. At the distances of the outer planets, gases froze into ice, creating huge balls of frozen gas that formed the Jovian planets.
Hot gases and electrically charged particles flow from our sun constantly, forming a stream called the solar wind. The solar wind was stronger at first than it is today. The early solar wind drove the light elements — hydrogen and helium — away from the inner planets like Earth. But the stronger gravity of the giant outer planets held on to more of the planets’ hydrogen and helium, and the solar wind was weaker there. So these outer planets kept most of their light elements and wound up with much more mass than Earth.
Astronomers developed these theories when they thought that rocky planets always orbited close to the parent star and giant planets farther out. But the “rule” was based only on our own solar system. Now that astronomers have learned something about other solar systems, they have devised new theories. Some scientists have suggested that the giant planets in other solar systems may have formed far from their parent stars and later moved in closer.