Planets
Planets are major bodies which orbit stars. They are not massive enough to ignite nuclear reactions at their cores like stars, yet they are massive enough in some cases to support gaseous atmospheres and complex chemistry. Since antiquity, five planets (from the Greek word for "wanderer") were known to move across the background of distant stars: Mercury, Venus, Mars, Jupiter, and Saturn. Early astronomers believed that the universe revolved around the Earth, and found it particularly difficult to describe the motions of the planets. Mars and Jupiter exhibited a puzzling behavior known as "retrograde motion" against the dome of the sky in which, on occasion, they seemed to reverse their direction over several days before returning to their original direction of motion. The ancient Greek astronomer Ptolemy was forced to invent a small circular motion, known as an "epicycle," to account for the variation of the mean motion of the planet on a circular orbit around Earth. It took the work of Nicholas Copernicus, Galileo, Tycho Brahe, Johannes Kepler, and Isaac Newton to offer a simpler, more physically motivated model of the solar system with all bodies, including Earth, orbiting the Sun.
Three more planets were discovered in our own solar system in modern times. In 1781, the British astronomer Sir William Herschel discovered Uranus. In 1846, the French scientist Urbain Leverrier (1811-1877) and the British scientist John Couch Adams (1819-1892) predicted the existence and location of Neptune--the first time such a prediction had ever been made. Lastly, in 1930, the American Clyde Tombaugh (1906-1997) discovered Pluto. While no other planets have been found in our solar system, despite numerous searches, a large number of minor bodies, including comets and asteroids, have been discovered over the years.
Most of what is known about the planets in the solar system comes from a variety of unmanned space probes launched since the 1960s. A number of American and Russian probes were sent out to our nearest neighbors, the inner rocky planets of Mercury, Venus, and Mars. Notably, the Viking missions to Mars in the 1970s could not detect signs of life on the planet today. The outer planets were surveyed with a grand tour of the solar system conducted by Voyager I and Voyager II, which used gravity-assist trajectories to visit multiple planets. Voyager I visited both Jupiter and Saturn, and Voyager II visited Jupiter, Saturn, Uranus, and Neptune, sending back images and measurements of unprecedented quality from each of these planets.
Our knowledge of planetary bodies had been extremely biased until just a few years ago. All of the known planets in our solar system had formed from the same primordial conditions around the same star. Astronomers had constructed formation hypotheses to explain the origin of the solar system. The prevailing notion was that planets formed from a flattened, spinning disk of dust and gas that formed at the same time as our Sun. The inner planets formed by coagulation of rocky planetismals, and the outer planets, lying in the colder outer portions of the disk, accreted a substantial amount of their mass in the form of gas from the surrounding primordial disk.
The scientific world was stunned in 1995 when Michel Mayor and Didier Queloz announced the discovery of the first extrasolar planet orbiting a nearby sun-like star, 51 Pegasi, which lies about 40 light years from Earth. Since then, a total of 34 extrasolar planets have been discovered, with masses ranging from about one-fifth to ten times that of Jupiter. In a most remarkable detection, Geoff Marcy and Paul Butler announced an eclipsing extrasolar planet orbiting the star HD 209458. From this detection, scientists now know both the mass and radius of the object, which has a mass intermediate between Saturn and Jupiter, and a radius about 60% greater than that of Jupiter. Scientists continue to debate the explanation for these new observations, which have re-invigorated and excited the astronomical community. The outstanding challenge for the future is to detect a small rocky planet like our own Earth, with a gaseous atmosphere and possibly even life.
This is the complete article, containing 673 words
(approx. 2 pages at 300 words per page).
