Planets

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" rowspan="2" width=210 BGCOLOR="#9999FF" TEXT="#080000" bgproperties="fixed" background="09376ba3.gif"> <div> <A HREF="10th_planet.htm" TARGET="_top" TITLE="The 10th Planet"><U>The 10th Planet</U></A></div> <bR> <div> <A HREF="planets_m.htm#top_of_page" TARGET="TRLX_Middle" TITLE="Planets"><U><I><B>Top of page</B></I></U></A></div> <bR> <DIV ALIGN="LEFT"></DIV> <div> <B>Links in this page:</B></div> <div> <A HREF="planets_m.htm#other_planetary_systems" TARGET="TRLX_Middle" TITLE="Planets"><U>Other planetary systems</U></A></div> <bR> <div> <A HREF="planets_m.htm#how_the_planets_move_" TARGET="TRLX_Middle" TITLE="Planets"><U>How the planets move</U></A> </div> <div> <A HREF="planets_m.htm#orbiting_the_sun" TARGET="TRLX_Middle" TITLE="Planets"><U>Orbiting the sun</U></A></div> <div> <A HREF="planets_m.htm#rotation" TARGET="TRLX_Middle" TITLE="Planets"><U>Rotation</U></A></div> <bR> <div> <A HREF="planets_m.htm#conditions_on_the_planets" TARGET="TRLX_Middle" TITLE="Planets"><U>Conditions on the planets</U></A></div> <div> <A HREF="planets_m.htm#temperature" TARGET="TRLX_Middle" TITLE="Planets"><U>Temperature</U></A></div> <div> <A HREF="planets_m.htm#atmosphere" TARGET="TRLX_Middle" TITLE="Planets"><U>Atmosphere</U></A></div> <div> <A HREF="planets_m.htm#surface_features" TARGET="TRLX_Middle" TITLE="Planets"><U>Surface features</U></A></div> <bR> <div> <A HREF="planets_m.htm#studying_the_planets_" TARGET="TRLX_Middle" TITLE="Planets"><U>Studying the planets</U></A> </div> <div> <A HREF="planets_m.htm#explaining_the_motion_of_the_planets" TARGET="TRLX_Middle" TITLE="Planets"><U>Explaining the motion of the planets</U></A></div> <div> <A HREF="planets_m.htm#improved_observations" TARGET="TRLX_Middle" TITLE="Planets"><U>Improved observations</U></A></div> <div> <A 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HREF="solar_system.htm" TARGET="_top" TITLE="Solar system Data"><U>Solar system Data</U></A></div> <div> Back to: <A HREF="astronomy.htm" TARGET="_top" TITLE="Astronomy"><U><B><Astronomia></B></U></A></div> </td> </tr><tr> <td valign="top" height=390 colspan=2 BGCOLOR="#99CCFF" TEXT="#080000" bgproperties="fixed" background="09476ba3.gif"> <div> Planets</div> <bR> <div>  <A NAME="top_of_page"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Planet is any of nine large objects that orbit the sun.  A planet may also be a similar body that revolves about a star other than the sun.  Earth is a planet that travels around the sun once a year.  The sun, the planets that orbit it, and the planets' satellites (moons) are parts of the solar system.  Going outward from the sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Pluto, and Neptune.  </div> <bR> <div> The sun and other stars that shine are giant balls of hot gases.  The planets of the solar system are much smaller than the sun and these other stars, and they are either solid or probably have solid cores.  Planets do not produce their own visible light.  The planets of the solar system can be seen only because they reflect sunlight.  Mercury, Venus, Mars, Jupiter, Saturn, and Uranus are visible from Earth without a telescope.  </div> <bR> <div> Planets and stars look much alike in the night sky, but there are two ways to tell them apart.  First, the planets shine steadily, but the stars seem to twinkle.  Second, the planets move in relation to the stars.  The word planet comes from a Greek word meaning to wander.  </div> <bR> <div> The planets differ greatly in size and in distance from the sun.  All the planets together weigh less than a hundredth as much as the sun.  The diameter of Jupiter, the largest planet, is about a tenth of the sun's diameter.  Yet Jupiter is more than 60 times as large as Pluto, the smallest planet.  Earth and the three other planets nearest the sun are somewhat similar in size.  They are called the terrestrial (earthlike) planets.  The four largest planets--the giant planets--are much farther from the sun.  Astronomers know little about Pluto, and do not put it in either group.  </div> <bR> <bR> <bR> <div> Pluto is usually the farthest planet from the sun.  Every 248 years, however, Neptune becomes the most distant planet.  Pluto moves inside Neptune's orbit and remains there for about 20 years.  Pluto entered Neptune's orbit on Jan. 23, 1979, and will remain there until March 15, 1999. </div> <bR> <div>  <A NAME="other_planetary_systems"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Other planetary systems</div> <div> Astronomers have detected planets orbiting at least four stars other than the sun.  In 1994, astronomer Alexander Wolszczan of Pennsylvania State University in the United States (U.S.) announced evidence for three planets in orbit around a pulsar called B1257 + 12.  A pulsar is a rapidly spinning neutron star that sends out radio waves.  The waves arrive at Earth as pulses of radio energy.  Pulsar B1257 + 12 is in the constellation Virgo, and is about 1,300 light-years from Earth.  One light-year is the distance that light travels in one year--about 9.46 trillion kilometres.  </div> <bR> <div> In 1995, Swiss astronomers Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland made the first discovery of a planet orbiting a sunlike star.  The star, called 51 Pegasi, is in the constellation Pegasus and is 55 to 60 light-years from Earth.  The planet is surprisingly large for its location.  It is about seven times closer to its star than Mercury is to the sun, yet is at least 2,500 times more massive than Mercury--about half as massive as Jupiter.  </div> <bR> <div> Astronomers Geoffrey Marcy and Paul Butler of San Francisco State University in the U.S. announced in 1996 that they had discovered planets in orbit about two additional stars.  A planet that is about three times more massive than Jupiter is orbiting the star 47 Ursae Majoris.  The planet is about 306 million kilometres from the star--farther than Mars is from the sun.  The star is in Ursa Major and is 40 to 50 light-years from Earth.  </div> <bR> <div> A planet with approximately eight times the mass of Jupiter is in an orbit about 72 million kilometres from the star 70 Virginis.  The orbit is larger than that of Mercury, but smaller than that of Venus.  The star is in Virgo and is 55 to 70 light-years from Earth.  The remainder of this article discusses only the planets in the solar system. </div> <bR> <DIV ALIGN="LEFT"></DIV> <div>  <A NAME="how_the_planets_move_"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>How the planets move </div> <bR> <div> As seen from Earth, the planets and the stars move westward across the sky.  A person using a telescope to observe a planet must turn it constantly to keep the planet in view.  From night to night, in addition to its motion across the sky, each planet shifts its position slightly eastward in relation to the stars.  At certain times, a planet's position may temporarily shift westward, but it always returns to its regular eastward shift.  </div> <bR> <div>  <A NAME="orbiting_the_sun"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Orbiting the sun</div> <div> If we could look down upon the solar system from the north, we would see that most of the planets travel around the sun at more or less the same level in space.  Astronomers say that the planets orbit the sun in the same plane.  Only two planets, Mercury and Pluto, follow paths that are inclined at significant angles to this plane.  Mercury's path has an inclination of 7° to the orbital plane, and Pluto's has an inclination of 17°.  From our position to the north of the solar system, we would see that all the planets travel around the sun in an anticlockwise direction.  Three laws of planetary motion describing the orbits of the planets were published in the early 1600's by the German astronomer and mathematician Johannes Kepler.  </div> <bR> <div> Kepler's first law says that the planets move in elliptical (oval-shaped) orbits.  As a result, the planets are a little closer to the sun at some points in their orbits than at others.  For example, the earth comes within 147,100,000 kilometres of the sun at its perihelion (point of the orbit nearest the sun).  It goes 152,100,000 kilometres from the sun at its aphelion (point farthest from the sun).  </div> <bR> <div> Kepler's second law is also called the law of areas.  It says that an imaginary line between the sun and a planet sweeps across equal areas of space in equal periods of time.  When a planet is at its closest to the sun, it is moving at its fastest speed.  In a given time--for example, ten days--the line joining the planet to the sun sweeps across a short, thick slice of space, which we will call Area 1.  When the same planet is at its farthest, it is travelling at its slowest speed.  In a ten-day period, the line now sweeps across a long, thin slice of space, which we will call Area 2.  Although the dimensions of the two slices are different, careful measurement shows that Area 1 equals Area 2. </div> <bR> <div> Kepler's third law says that a planet's orbital period (the time required for it to go once around the sun) depends on its average distance from the sun.  According to this law, the square of the period (the period multiplied once by itself) divided by the cube of the distance (the distance multiplied twice by itself) is the same for all the planets.  Thus, a planet four times as far from the sun as another planet takes eight times as long to orbit the sun.  This law was once used to find a planet's average distance from the sun after its orbital period had been measured.  </div> <bR> <div>  <A NAME="rotation"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Rotation</div> <div> Each planet rotates, or spins, as it revolves around the sun.  The planets' rotation periods (the times required for individual planets to complete one spin) range from less than 10 hours for Jupiter to 243 days for Venus.  The earth rotates once every 24 hours, or one day.</div> <bR> <div> Each planet spins about its rotational axis, an imaginary line through its centre.  None of the planets has a rotational axis that is precisely perpendicular (at an angle of 90°) to the path of its orbit.  The axis of each planet tilts at an angle from the perpendicular position.  The earth's axis, for example, tilts at about 231/2° away from the perpendicular.  But other planets have quite different angles of axial tilt.  Mercury's axial tilt is less than 1°, in other words almost perpendicular to its orbit.  Uranus's axial tilt is about 98°, with the axis lying almost parallel to the planet's orbital path.  The axial tilt of Venus is about 178°, which means that the north and south poles of the planet are almost reversed compared with those of planets such as the earth or Mars.  The tilting of a planet's axis results in first one pole and then the other facing toward the sun during the planet's orbit.  This gives rise to uneven heating of the planet and to seasons.</div> <bR> <DIV ALIGN="LEFT"></DIV> <div>  <A NAME="conditions_on_the_planets"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Conditions on the planets</div> <bR> <div> The temperature, atmosphere, surface features, length of days and nights, and other conditions on the planets differ widely.  They depend on three things: (1) the planet's distance from the sun, (2) the planet's atmosphere, and (3) the planet's rotation.  </div> <bR> <div>  <A NAME="temperature"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Temperature</div> <div> The planets nearest the sun receive more heat than those far away from it.  The temperature on the closest planet, Mercury, rises to about 340 °C during the day.  On the earth, which is about 21/2 times as far from the sun as Mercury, the daytime temperature averages only about 16 °C. Pluto is over 100 times as far from the sun as Mercury.  The temperature there is probably lower than -180 °C. </div> <bR> <div> The temperature on a planet is estimated from measurements of infrared radiation (heat waves) and radio waves that the planet sends out.  These measurements are difficult to make for objects with low temperatures.  For this reason, temperature estimates for cold planets are less reliable than those for warm planets.  </div> <bR> <div>  <A NAME="atmosphere"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Atmosphere is the mixture of gases that surrounds a planet.  The atmospheres of the terrestrial planets consist chiefly of carbon dioxide and nitrogen.  The atmospheres of the major planets consist mostly of helium, hydrogen, methane, and ammonia.  The earth is the only planet that has a large amount of oxygen in its atmosphere.  </div> <bR> <div> Astronomers determine the kinds of gases in a planet's atmosphere by analysing the light, radio waves, and other radiation coming from the planet.  Different chemicals absorb different parts of this radiation, so, by seeing which parts are missing, astronomers can work out which chemicals are present in a planet's atmosphere.  </div> <bR> <div> The atmospheric pressure (force exerted by the weight of gases) on a planet's surface depends on the amount of gas in the atmosphere.  The earth's atmosphere has enough gas to produce a pressure of 1.03 kilograms per square centimetre.  But the atmosphere of Mars contains so little gas that its surface pressure is only about 1/150 as great as the earth's.  The atmosphere of Venus has so much gas that its surface pressure is as much as 90 times as great as the pressure on the earth.  </div> <bR> <div> Astronomers can estimate the amount of gas in a planet's atmosphere by measuring how the temperature varies throughout the atmosphere.  A much more accurate, but more difficult, method is to measure changes in radio waves sent through the planet's atmosphere by a passing spacecraft.  </div> <bR> <div>  <A NAME="surface_features"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Surface features of a planet like the earth include mountains, valleys, lakes, rivers, flat areas, and craters.  A terrestrial planet's surface is shaped partly by conditions on the planet itself and partly by collisions with meteoroids.  The giant planets do not have surfaces that we can see either from the earth or from space.  Jupiter, Saturn, Uranus, and Neptune are made up mostly of gases and ice.  When we observe their discs, all we see is the top layer of their atmospheres.  The atmosphere of a giant planet is very deep.  Astronomers have calculated that at the very centre of the giant planets there is a rocky core about the size of the earth.  The core may be surrounded by liquid hydrogen that is under such high pressure that it behaves like a metal and conducts immensely powerful electric currents. </div> <bR> <DIV ALIGN="LEFT"></DIV> <div>  <A NAME="studying_the_planets_"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Studying the planets </div> <bR> <div> People began studying the planets thousands of years ago.  They kept records of how the five planets then known moved and how they changed in brightness.  The motion of the planets was not well understood until the 1600's.  Today, there are still many unanswered questions about conditions on the planets and the origin of the planets.</div> <bR> <div>  <A NAME="explaining_the_motion_of_the_planets"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Explaining the motion of the planets brought about one of the most interesting disputes in the history of science.  The dispute involved two important theories.  </div> <bR> <div> One theory of planetary motion was suggested about A.D. 150 by Ptolemy, a Greek astronomer.  He believed the earth was the centre of the universe.  He thought the sun, moon, planets, and stars travelled around the earth, making one complete revolution in a day.  Ptolemy's theory explained what people saw in the sky, and guided their thinking for over a thousand years.  </div> <bR> <div> The dispute began in 1543, when the Polish astronomer Nicolaus Copernicus suggested that the earth and the other planets travelled around the sun.  He also suggested that the earth turned on an axis, making one complete rotation in a day.  Copernicus' theory made it easier to describe the motions of the planets, and astronomers soon began to use it.  But religious leaders called Copernicus a fool for saying that the earth was just another planet.  They banned his writings till 1757. </div> <bR> <div> Discoveries by other astronomers gradually convinced people that the Copernican theory was correct.  One such discovery was that Mercury and Venus, unlike the other planets, show phases like the moon, as increasing or decreasing amounts of their discs are illuminated by the sun.  The Italian astronomer Galileo Galilei made this discovery in 1610, using the newly invented telescope.  Galileo realized that Mercury and Venus must be travelling around the sun and that Copernicus could be right about a sun-centred system.  When he also discovered satellites travelling around Jupiter, he was convinced that Copernicus was right and that the earth was a planet travelling around the sun.  Kepler's laws of planetary motion boosted Copernicus' theory even more.  However, the Copernican theory gained widespread support only after the English scientist Sir Isaac Newton discovered the law of universal gravitation in about 1665.  This law described the pull of the sun on the planets moving around it.</div> <bR> <div>  <A NAME="improved_observations"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Improved observations</div> <div> After the motions of the planets became understood, astronomers began detailed studies of the individual planets.  With better telescopes that could reveal more detail, they measured the sizes, colours, and other characteristics of the planets.  They also discovered the most distant planets--Uranus, Neptune, and Pluto.  The discovery that planets send out radio waves, and the study of these waves, led to greater understanding of conditions on each planet.  </div> <bR> <div> Today, astronomers use a variety of telescopes on the ground and in space to learn about the planets.  Astronomers also have been looking for planets outside the solar system.  A planet as large as Jupiter would exert a weak gravitational pull on its parent star.  Astronomers could detect the presence of such a planet around a nearby star by observing irregularities in the motion of the star produced by this gravitational tug.  </div> <bR> <div> Spacecraft and high-speed computers have contributed much to planetary observations.  Unmanned probes have made close-up observations of all the planets except Pluto.  Unmanned probes have landed on both Mars and Venus and sent back valuable data, including photographs from the planets' surfaces.  Other probes have analysed the atmospheres and climates or weather of the planets.  The U.S. spacecraft Voyager 1 and Voyager 2 confirmed the giant planets all have strong magnetic fields. (Mercury, Venus, and Mars do not.) They also showed that Saturn is not the only planet with a ring system--all the giant planets have rings.  The Voyager probes also sent back images of the planets' satellites and discovered many previously unknown satellites.  Scientists have used advanced computers to analyse the images and other data sent back to the earth by these spacecraft.  </div> <bR> <div>  <A NAME="explaining_the_formation_of_the_planets"><IMG BORDER="0" SRC="1x1.gif" HEIGHT="1" ALIGN="bottom" WIDTH="1" HSPACE="0" VSPACE="0" ></A>Explaining the formation of the planets</div> <div> Most astronomers today believe that the sun, planets, and smaller bodies in the solar system formed from a large cloud of loosely packed gas and dust about 4.6 billion years ago.  The gravitational pull of particles within the spinning cloud caused it to contract a great deal and become more dense.  Most of the material was pulled toward its centre and formed the sun.  Smaller amounts of material remained in orbit around the forming sun and became flattened into a thin protoplanetary disc.  </div> <bR> <div> The gas and dust in the protoplanetary disc could eventually have formed small chunks.  Gentle collisions of these chunks then built up larger objects.  As the objects grew, their gravity increased.  The larger bodies could have pulled in dust, gas, and smaller objects and grown quickly.  Astronomers believe that these large bodies became the planets and their moons.  </div> <bR> <div> This theory of the origin of the solar system also can account for general differences between the rocky, terrestrial planets and the giant planets, which consist mainly of gases and ice.  Astronomers calculate that the protoplanetary disc of dust and gas was hotter near its centre than at its edge, which was far from the forming sun.  In the hottest regions of the disc, only metals and other rocky materials could form chunks.  The sun's heat prevented atoms of hydrogen, helium, and other light elements from becoming solids or liquids.  These hot gases moved quickly and so could escape the gravity of the chunks of rock.  In addition, some astronomers think that the solar wind (a stream of gases flowing from the sun) may have driven the light elements from the inner solar system.  As a result, the inner, terrestrial planets are mostly rocky worlds.  </div> <bR> <div> The sun's heat and the solar wind had much less effect on the outer portions of the disc.  Cooler temperatures allowed water vapour, ice, and such gases as hydrogen, helium, methane, and ammonia to remain.  Astronomers think that the gases and icy material formed Jupiter, Saturn, Uranus, and Neptune.  As these planets formed, they could have had orbiting discs of dust, gas, and ice--much like the sun's protoplanetary disc.  These discs may have formed the ring and satellite systems around the giant planets.  </div> <bR> <div>                     ----  end of article  ----</div> <div> <A HREF="#top_of_page" TITLE="Planets"><U><I><B>Top of page</B></I></U></A></div> <bR> <bR> </td> </tr></table></body>