History of Astronomy - astronomy.

History of Astronomy - astronomy. I INTRODUCTION History of Astronomy, history of the science that studies all the celestial bodies in the universe. Astronomy includes the study of planets and their satellites, comets and meteors, stars and interstellar matter, star systems known as galaxies, and clusters of galaxies. The field of astronomy has developed from simple observations about the movement of the Sun and Moon into sophisticated theories about the nature of the universe. See also Cosmology. II OVERVIEW Advances in astronomy over the centuries have depended to a great extent on developments in technology. Initially, ancient peoples could only view the sky with their eyes. With careful attention to the changing positions of the Sun, Moon, planets, and stars, they were able to develop calendars and ultimately predictions of rare events, including eclipses. Instruments that allowed the measurement of the precise positions of celestial objects were the first major technological development, and those measurements formed the basis of models of the solar system. The invention of the telescope in the early 1600s completely changed scientists' ideas about the structure of the solar system and led to the discovery of new planets around our own sun. The telescope was also key to the measurement of distances to nearby stars and thereby provided the first clues to just how vast the universe is. The invention of the spectroscope combined with photography led to the discovery that the stars are made of the same elements found here on Earth. Astronomy is different from most other sciences in that, apart from the planets we have visited by spacecraft, researchers cannot do experiments in the laboratory with the objects that they want to study. Instead, astronomers must learn about these distant objects by relying entirely on the visible light and other forms of energy--electromagnetic radiation--that are given off by them. The great breakthroughs of the 20th century were the development of spacecraft that allowed scientists to observe the universe from outside the distorting effects of Earth's atmosphere, and the development of new sensors sensitive to forms of energy our eyes cannot detect. Examples are X rays , gamma rays, infrared or heat energy, and radio waves. These new windows on the universe have greatly expanded astronomical knowledge. See also Space Exploration. III ANCIENT ORIGINS Ancient astronomers had only their eyes with which to view the sky, but they had a very practical reason for studying the skies. Thousands of years ago, changes in the heavens were the only available clocks and calendars. The stars could also be used for navigation. See also Archaeoastronomy. Ancient Babylonian, Assyrian, and Egyptian astronomers all knew the approximate length of the year. The Egyptians of 3,000 years ago adopted a calendar with a year that was 365 days long, very near the modern value of 365.242 days. The Egyptians also used the rising of the star Sirius in the pre-dawn sky to mark the time when the Nile River could be expected to flood. The Chinese determined the approximate length of the year at about the same time as the Egyptians. The Maya of Central America kept a continuous record of days from day zero, which occurred on our equivalent of August 13, 3114 BC. They also kept track of years, eclipses, and the motions of the visible planets. Their year consisted of 18 months, each 20 days long, plus one 5-day month to total 365 days. Occasional adjustments were made to allow for the extra quarter of a day. The adjustments required in the Maya calendar illustrate a common problem faced by ancient astronomers. Neither an entire month nor an entire year contains an exact whole number of days; to keep calendar years in step with the seasons, which were important for planting crops, the calendar makers assigned different numbers of days to successive months or years. Even though individual months or years were not the same length, they averaged out to approximately the true value. See also Calendar. In the British Isles, ancient people used stone circles to keep track of the motions of the Sun and Moon. The best-known example is Stonehenge, a complex array of massive stones, ditches, and holes laid out in concentric circles. Stonehenge was built over an extended period of time lasting from about 2800 to 1500 BC. Some of the stones are aligned with the directions in which the Sun rises and sets at critical times of the year, such as when it reaches its most northerly and southerly points in the sky (the summer and winter solstices). Ancient astronomers also observed five bright planets (the ones we call Mercury, Venus, Mars, Jupiter, and Saturn). These bodies, together with the Sun and Moon, move relative to the stars within a narrow band called the zodiac. The Moon moves around the zodiac quickly, overtaking the Sun about once every 29.5 days. The Sun and Moon always move along the zodiac from west to east. The five bright planets--Mercury, Venus, Mars, Jupiter, and Saturn--also have a generally eastward motion against the background of the stars. However, ancient astronomers in many different places around the globe noted that Mars, Jupiter, and Saturn sometimes move westward, in a backwards or retrograde direction. These planets, therefore, appear to have an erratic eastward course, with periodic loops in their paths. In ancient times, people imagined that celestial events, especially the planetary motions, were connected with their own fortunes. This belief, called astrology, encouraged the development of mathematical schemes for predicting the planetary motions and thus furthered the early progress of astronomy. However, none of the systems of astrology has been shown to be at all effective in making verifiable predictions. Stars provide the background against which the motions of the planets are measured. Ancient Chinese, Egyptians, Greeks, and others gave names to patterns of stars. We call these patterns constellations. Some are very familiar, such as the Big Dipper, the Pleiades, and Orion. Few constellations look like their namesakes. Rather, ancient astronomers probably simply named areas of the sky with prominent groupings of stars after important characters in their mythology. IV GREEK ASTRONOMY Modern astronomy can trace its heritage directly back to the ancient Greeks, who began to develop explanations for their observations of the sky. The writings of Aristotle summarize the knowledge of that era. He attributed the phases of the Moon--that is, the changes in its apparent shape--to the fact that we see different portions of its sunlit surface during the month. He also knew that the Sun is farther away from the Earth than the Moon because the Moon occasionally passes between the Sun and Earth and blocks the Sun's light (a solar eclipse). Aristotle cited two observations to show that Earth is a sphere. The first is that the shadow of Earth, which is seen during an eclipse of the Moon (when Earth is directly between the Sun and Moon), is always round. Only a sphere always has a round shadow no matter how it is viewed. If the Earth were a disk, we would sometimes see the shadow edge-on, and it would look like a straight line. The second observation was that travelers who journeyed a long distance south reported seeing stars not visible from Greece. If Earth were flat, all travelers anywhere would see the same stars. On a spherical Earth, travelers at different latitudes (different distances north or south) view the sky from different angles and see different constellations. The Greek astronomer and mathematician Eratosthenes measured the size of the spherical Earth in about 200 BC. He noticed that on the first day of summer in Syene, Egypt, the Sun was directly overhead at noon. On the same date and time in Alexandria, Egypt, the Sun was about 7 degrees south of zenith. With simple geometry and knowledge of the distance between the two cities, he estimated the circumference of the Earth to be 250,000 stadia. (The stadium was a unit of length, derived from the length of the racetrack in an ancient Greek stadium. We have an approximate idea of how big an ancient Greek stadium was, and based on that approximation Eratosthenes was within 20 percent, and possibly within 1 percent, of the correct answer.) Probably the most original ancient observer of the heavens was Aristarchus of Sámos, a Greek. He believed that motions in the sky could be explained by the hypothesis that Earth turns around on its axis once every 24 hours and, along with the other planets, revolves around the Sun. This theory, however, makes an important prediction that ancient Greeks could not verify. If Earth moves in an orbit around the Sun, then we look at the stars from different directions at different times of the year. As Earth moves along, nearby stars should shift their positions in the sky relative to more distant ones. The Greeks tried to measure this effect for the stars but were unsuccessful. It was only in 1838 that astronomers' equipment could make measurements with the accuracy required to measure the very small shift of the stars, which turn out to be much, much farther away than the Greeks could imagine. Perhaps the greatest of the ancient astronomers was Hipparchus, who lived around 150 BC and did most of his work at an observatory he built in Rhodes. There he recorded accurate positions of about 850 bright stars and classified them according to their brightness. The brightest stars he said were of the first magnitude, a term astronomers still use today. Because our planet is not an exact sphere, but bulges at the equator, the gravitational pulls of the Sun and Moon cause it to wobble like a top. It takes about 26,000 years for Earth's axis to complete one full circle. Hipparchus estimated that the Earth's axis shifts its position relative to the stars by 46 seconds of arc per year, which is very close to the modern value of 50.26 seconds of arc per year. This is known as the precession of the Earth. The last of the great ancient astronomers was Ptolemy, who worked in Alexandria in about the year AD 140. Ptolemy's greatest contribution was a geometrical model of the solar system that made it possible to predict the positions of the planets at any date and time. His model was used for about 1,400 years, until the time of Copernicus. Ptolemy's challenge was to explain the complex motions of the planets, including the fact that they sometimes appear to move westward or backward in their orbits. In order to explain the observation, he assumed that each planet revolved in a small orbit called an epicycle. The center of the epicycle then revolved about the Earth on a much larger circle. At the time, circles were thought to be the perfect shape. It was assumed that the heavenly bodies would follow the most perfect shape. See also Ptolemaic System. Astronomers now know that the planets do not follow circular orbits but rather elliptical ones, and they orbit around the Sun, not Earth. The backward or westward motion is explained by the fact that Earth moves more rapidly in its orbit than do Mars, Jupiter, and Saturn. When the Earth overtakes them during its yearly circuit around the Sun, these planets appear to move backwards relative to the stars. For an analogy, think of passing a slowly moving car on the freeway. As you overtake it, the car appears to be moving backward relative to the scenery beyond the side of the road. V COPERNICUS AND GALILEO Astronomy took a dramatic turn in the 16th century as a result of the contributions of the Polish astronomer Nicolaus Copernicus. Educated in Italy and made a canon (member of the clergy) of the Roman Catholic Church, Copernicus spent most of his life pursuing astronomy. His greatest contribution is entitled On the Revolution of Heavenly Bodies (1543), in which he analyzed critically the Ptolemaic theory of an Earth-centered universe and showed that the planetary motions can be explained much more simply by assuming that all the planets, including Earth, orbit the Sun. His ideas were not widely accepted until more than 100 years later. The Italian astronomer Galileo ushered in a new era of science, one in which observations and experiments play the key role in testing models and hypotheses. Most historians believe that Dutch spectacle-maker Hans Lippershey invented the first telescope in the year 1608, but Galileo built one of his own in 1609, shortly after news of this invention reached him. Others had used telescopes to observe objects on Earth, but Galileo was the first to report astronomical observations, and his observations confirmed that Copernicus was right and that Ptolemy's model of the planetary motions was wrong. Copernicus had predicted that if Venus orbits the Sun rather than Earth, Venus should go through phases just as the Moon does. Galileo discovered the phases of Venus. He also detected four moons orbiting Jupiter, which showed that not everything orbits Earth. One argument against the idea that Earth orbits the Sun was that the Moon would be left behind. Galileo's observations clearly disproved that argument. After all, Jupiter's moons were able to keep up with Jupiter. Convinced that at least some planets did not circle Earth, Galileo began to speak and write in favor of the Copernican system. His attempts to publicize the Copernican system caused him to be tried by the Inquisition for heresy, and he was condemned to house arrest. Although he was forced to repudiate his beliefs and writings, Galileo and other Renaissance scientists showed that nature can be studied and understood through experiments and observations. VI KEPLER AND NEWTON From the scientific viewpoint, the Copernican theory was only a rearrangement of the planetary orbits. The ancient Greek theory that planets move in perfect circles at fixed speeds was retained in the Copernican system. Precise new observations, however, showed that this could not be the case. From 1580 to 1597 Danish astronomer Tycho Brahe observed the Sun, Moon, and planets from his island observatory near Copenhagen, Denmark, and later in Germany. Based on the data compiled by Brahe, his German assistant, Johannes Kepler, showed that the planets revolve around the Sun, not in circular orbits with uniform motion, but in elliptical orbits at varying speeds. He also discovered that their relative distances from the Sun can be calculated from the observed periods of revolution. The English physicist Sir Isaac Newton was the genius who developed the mathematical equations that describe the motions of the planets. He had to invent new forms of mathematics, including calculus, to help him solve this problem. What Newton showed was that the most natural state of motion is a straight line. Since planets move along curved (elliptical) paths, some force must be acting on them. Newton called this force gravity. He showed that the force of gravity between two objects must be directly proportional to their mass and inversely proportional to the square of the distance between them. Newton was able to prove mathematically that if gravity behaved in this way, then the only orbits permitted were exactly those described by Kepler. In Newton's day, gravity had been associated with the Earth alone; if you drop something, it falls to the ground. Newton's great insight showed that this force is universal. It acts everywhere, including on the planets. VII TOWARD MODERN ASTRONOMY The telescopes used by Galileo were made with lenses that typically were only about 2.5 cm (1 in) in diameter. Over the next 400 years, developments in technology made it possible to build ever larger telescopes with greater light-gathering power to detect ever fainter objects. Mirrors replaced lenses as the main optical elements in telescopes. The largest single telescopes in the world today, the twin Keck telescopes at the Mauna Kea Observatory in Hawaii, are each 10 m (400 in) in diameter, and astronomers are developing plans to build telescopes that are 3 to 5 times larger still. Discoveries with telescopes from the 1600s through the 1800s laid the basis for modern astronomy. Many new members of the solar system were identified, including the planet Uranus in 1781 by the British astronomer Sir William Herschel and the planet Neptune in 1846, which was discovered independently by the British astronomer John Couch Adams and the French astronomer Urbain Jean Joseph Leverrier. Using telescopes astronomers also discovered the first asteroids between the orbits of Mars and Jupiter. Newton's colleague Edmond Halley used the new theory of gravity to calculate the orbits of comets. Based on his calculations, he noted that bright comets observed in 1531, 1607, and 1682 might well be the same comet, reaching the point in its orbit closest to the Sun every 76 years. He predicted that this comet would return in about 1758. Although Halley had died by 1758, when the comet did indeed appear as he had predicted it was given the name Halley's Comet. Telescopic studies of double stars, also known as binary star systems, provided evidence that gravity applies outside the solar system. The two members of a double star system follow elliptical orbits around their common center of gravity, just as the planets orbit the Sun. This proof that the law of gravity is truly universal meant that the same physical processes that we can study here on Earth can be applied to studies of distant objects, including stars. The distances to stars were first measured in 1838. In this year, three astronomers reported distances for three different stars--61 Cygni, Alpha Centauri, and Vega. The distances were calculated from measurements of the very slight shift in position of these nearby stars relative to much more distant background stars when viewed from opposite sides of Earth's orbit. This is the calculation that the Greeks tried to perform in order to test whether the Earth orbits the Sun. The Greeks failed because the shift in position, which is called parallax, is only about 1.5 seconds of arc for even the nearest bright star. This degree of separation is about equal to the apparent size of a quarter when viewed from a distance of 2.3 km (1.4 mi). It was much too small to be measured with the techniques available to the Greeks. The nearest of the first three stars measured, Alpha Centauri, is at a distance of about 42 trillion km (26 trillion mi). Obviously astronomers needed a new unit to measure such large distances, and one that eventually became widely used is the light-year. One light-year is equal to the distance that light travels in one year at the speed of light, which is about 300,000 km/sec (186,000 mi/sec). So one light-year equals 9.5 trillion km (5.9 trillion mi). The distance to Alpha Centauri from Earth is about 4.4 light-years. In the mid-1800s astronomers also obtained information about what stars are made of. They used a technique called spectroscopy. When the light from a star is spread out into its rainbow of colors and passed through an instrument known as a spectroscope, some of the colors are found to be missing. These missing colors are referred to as dark lines. Laboratory experiments showed that the pattern of dark lines can be used to identify what hot gases--hydrogen, helium, even iron--are present in the star. Each element produces its own unique pattern. In 1864 British astronomer Sir William Huggins was the first to show that the pattern of dark lines in the spectrum of a star matched the patterns produced by elements known here on Earth. Huggins's discovery was another important example showing that the physical processes that we study here on Earth can be used to study the whole universe. Spectroscopy also provides information about the temperatures of stars, their masses, and their motions in space. VIII THE FOUNDATIONS OF MODERN ASTRONOMY A Einstein and Relativity As the 20th century began, the German-born physicist Albert Einstein advanced his general theory of relativity, which fundamentally changed our understanding of gravity. Einstein described gravitation as the curvature of space and time. His theory explained certain things that Newton's theory of gravity could not. For example, certain peculiarities in Mercury's orbit of the Sun could not be adequately described by Newton's theory. In 1919 a team of astronomers led by British astronomer Sir Arthur Stanley Eddington used the occasion of a solar eclipse to measure the deflection of starlight as it passed by the Sun and arrived at numbers that agreed with Einstein's predictions. B Edwin Hubble and the Scale of the Universe The 1920s proved to be a breakthrough decade for astronomers who were attempting to learn more about the size, or scale, of the universe. In 1920 two American astronomers--Heber D. Curtis of the Lick Observatory and Harlow Shapley of the Mount Wilson Observatory--debated whether so-called spiral nebulae were part of the Milky Way Galaxy or were themselves distant galaxies. Curtis argued that they were "inconceivably distant galaxies of stars," while Shapley placed them near the Sun. In 1923 American astronomer Edwin Hubble, using the largest telescope in existence at the time--the 2.5-m (100-in) Hooker telescope at the Mount Wilson Observatory--discovered two Cepheid variable stars in a spiral nebula known as Andromeda. The intrinsic or true brightness of these stars was already known as a result of earlier work by American astronomer Henrietta Leavitt. The distance to Andromeda could then be calculated by a comparison of the apparent brightness of the Cepheids with their intrinsic brightness. Over the next six years, Hubble found a total of 40 Cepheids in Andromeda, and in 1929 he published a paper in which he calculated that the Andromeda nebula was about 900,000 light-years from Earth (current estimates of this distance are about 2.2 million light-years). Hubble's observations therefore proved that Andromeda was a vast distance from the Milky Way Galaxy, which had a diameter of 100,000 light-years, and so must be a separate galaxy. In 1929 Hubble published another and even more astounding discovery. His studies of distant galaxies revealed that the universe was not static, as had been previously believed, but was expanding in size. In 1927, the Belgian scientist Georges Lemaître had proposed a new model for the universe based on Einstein's theory of general relativity. In this model, Lemaître assumed that the universe is expanding, a result that is consistent with the equations of general relativity. Hubble's measurements of the red-shifts of distant galaxies, however, were the first to demonstrate that Lemaître's assumption was indeed correct. This finding paved the way for the big bang theory of the origin of the universe. C Hans Bethe and Solar Energy By the mid-1900s, astronomers had finally worked out the source of the energy radiated by the Sun and stars. The Sun produces 3.86 × 1026 watts of power each second, a very large number indeed. Geological evidence shows that simple forms of life have existed on Earth for nearly 4 billion years, indicating that solar energy must have been expended at about its present rate for that length of time. In 1939 American physicist Hans Bethe advanced the theory that solar energy is produced by the fusion of four hydrogen atoms to form helium. In that process, some mass is converted to energy according to the famous equation E = mc2 formulated by Einstein. In this equation, E stands for energy, m for mass, and c for the speed of light. Since the speed of light is a very large number, very little mass is required to keep the Sun shining for billions of years. Building on the work of Bethe, the American astronomer William Fowler, along with British astronomers Sir Fred Hoyle and Geoffrey and Margaret Burbidge, showed in 1957 that the heavy chemical elements, such as carbon, nitrogen, and oxygen, are made in stars as a result of nuclear fusion processes. Astronomers thus discovered that all the heavy elements in the universe originated in stars. Understanding nuclear fusion within stars also enabled astronomers to obtain a better grasp of a star's evolution. Knowing the mass of a star, astronomers could calculate its stellar lifetime. The Indian American astrophysicist Subrahmanyan Chandrasekhar calculated the amount of mass, known as the Chandrasekhar limit, that would determine a star's fate. Stars with masses less than 1.4 times the mass of the Sun when fusion ended could complete their evolution as white dwarf stars. More massive stars would implode and end their lives as either neutron stars or black holes. Rapidly spinning neutron stars were later detected by British radio astronomers Jocelyn Bell, who was then a graduate student, and her adviser, Antony Hewish. IX THE GOLDEN AGE OF ASTRONOMY The second half of the 20th century was truly a golden age for astronomy. Rapid advances in technology made it possible to build very large optical telescopes on the ground. By the early 21st century astronomers were using about a dozen telescopes with mirrors larger than 8 m (300 in) in diameter. Because it is much cheaper to build telescopes on the ground than in space, large ground-based telescopes with their ability to gather large amounts of light (think of a telescope as a bucket for collecting light; the bigger the bucket, the more light collected) are particularly valuable for studying the faintest objects. The most distant objects tend to be very faint, but they are very important for understanding the evolution of the universe. Since light takes a long time to reach us, the universe gives us a kind of time machine so that we can see what it was like when it was much younger than it is now. For the most distant objects observed so far, it took nearly 13 billion years for their light to reach Earth, so we are seeing them as they existed 13 billion years ago. Radio astronomy is also best done from the ground. All forms of electromagnetic radiation with wavelengths longer than infrared wavelengths are called radio waves. Radio waves are not sound waves like the ones you hear when you listen to your MP3 player. In fact, we cannot detect them with our senses but must use electronic equipment. In a radio telescope, radio waves are reflected by a metallic surface and brought to a focus. They are then sent to an electronic receiver, where they can be recorded and analyzed. Radio astronomy is especially useful for studying spectral lines produced by cold gas atoms and molecules and also for studying high-energy particles moving rapidly in strong magnetic fields. A Radio Astronomy and the Big Bang Radio astronomy proved to be instrumental in verifying the big bang theory of the origin of the universe. In the 1940s the Russian American theoretical physicist George Gamow proposed that the universe originated in a hot, dense state from which it exploded, setting off the observed expansion of the universe. British astronomer Fred Hoyle dismissed the theory derisively as a "big bang" in contrast to his own theory of a steady-state universe, which assumed that the universe was eternal and unchanging with time. Two of Gamow's students--Ralph Alpher and Robert Herman--predicted that a relic of this explosive event would take the form of radiation emanating at a uniform temperature from all directions in the sky. In 1965, using a radio telescope, American astrophysicists Arno Penzias and Robert Wilson detected and identified this cosmic background radiation, providing the first observational evidence for the big bang theory. B New Windows on the Universe and New Mysteries The ability to launch spacecraft opened up new windows on the universe. Astronomical objects not only give off radio waves and light of the kind that our eyes are sensitive to. They also emit other forms of energy--electromagnetic radiation--ranging from high-energy gamma rays and X rays, to infrared or heat radiation. Much of this electromagnetic radiation is absorbed by Earth's atmosphere and does not reach the ground. However, technology again came to the rescue by making it possible to launch telescopes above Earth's atmosphere to observe these different types of electromagnetic radiation. During the last quarter of the 20th century, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) launched many spacecraft designed to exploit the advantages of being outside Earth's atmosphere. Particularly powerful were three great observatories: the Chandra X-ray Observatory, the Spitzer Space Telescope, and the Hubble Space Telescope (HST). Turbulence in the Earth's atmosphere blurs astronomical images. Because the Hubble Space Telescope is unaffected by this blurring, it can take superbly sharp images and has given astronomers both scientifically important and stunningly beautiful images of planets, star clusters, and galaxies. The pace of discovery enabled by these new facilities, both in space and on the ground, has been truly remarkable. Astronomers not only know that the expansion of the universe began about 13.8 billion years ago, but they have also learned that the expansion is not occurring at a steady pace but is accelerating (increasing its speed) as the universe ages. Some form of energy is powering this acceleration. Since no physical theory predicted the existence of this form of energy, scientists call it dark energy. There is also dark matter in the universe--dark in the sense that it gives off no electromagnetic radiation but does exert a gravitational force. One of the challenges for astronomers in the 21st century will be to try to determine the properties of both dark matter and dark energy. Astronomers know that stars are found in giant systems called galaxies, which are held together by gravity. Stars in each galaxy orbit around the center of the galaxy, obeying Newton's law of gravity. The Milky Way is the galaxy that contains our own sun and solar system. Our sun is, however, only one rather ordinary star among the 100 billion or so stars that make up the Milky Way. And our galaxy is only one of billions of galaxies in the universe. Astronomers have also verified that black holes exist in large numbers. Predicted by Einstein's theory of general relativity, a black hole is a region in space where matter is very highly concentrated and the force of gravity is so great that nothing--neither matter nor light--that ventures too close can escape from its gravitational pull. The existence of black holes can be detected by measuring the motions of objects orbiting nearby but just out of reach. Black holes are commonly found at the centers of galaxies and provide the explanation for another curious class of objects discovered in the 1960s--the quasars. Quasars are at the distances of galaxies and produce more energy than typical galaxies in a volume of space no bigger than our own solar system. Astronomers have shown that the engine that powers the quasar is a black hole surrounded by swirling gas heated to a very high temperature as it spirals toward the black hole. Eventually this gas will be swallowed up by the black hole and disappear from view. We know that the first stars began to form about 13.4 billion years ago and that star formation continues to the present day. Stars form from dense clouds of dust and gas. A region of slightly higher density within a large cloud can begin to attract dust and gas from nearby and eventually collapse to form a star. The nearest stellar nursery is in the direction of the constellation Orion, where there are hundreds of stars (so faint they can be seen only with a telescope) that are no more than a few hundred thousand years old. Closer to home, NASA has now sent spacecraft to orbit all of the planets. The dwarf planet known as Pluto, which was formerly classified as a planet, has not yet been visited by a spacecraft. Pluto was discovered by American astronomer Clyde Tombaugh in 1930. A spacecraft launched in 2006 is expected to rendezvous with Pluto in 2015. Perhaps most exciting of all, we have discovered that our own solar system is not the only one. As of mid-2006, astronomers had found 194 planets orbiting other stars. About 10 percent of the nearby stars with compositions like that of our own Sun have at least one planet. Most of the planets discovered to date have masses similar to that of Jupiter or are even larger. Techniques are not yet good enough to detect relatively low-mass planets like Earth, but over the next 50 years or so, we should have new technologies that will allow us to learn whether planets suitable for life are common or rare. See also Extrasolar Planets; Planet. Contributed By: Fred Lawrence Whipple Vera C. Rubin Reviewed By: Sidney C. Wolff Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.