FACTS500 INSIDE! Your guide to planets, stars, and galaxies by Richard Talcott A supplement to Astronomy magazine © 2012 Kalmbach Publishing Co. This material may not be reproduced in any form 618129 without permission from the publisher. www.Astronomy.com Saturn Saturn’s rings consist of icy particles ranging in size from tiny motes to house-sized icebergs. NASA/THE HUBBLE Planets HERItaGE TEam (STSCI/AURA) of the solar system arth may seem extraordinary to those who call it home, but it’s not a land of superlatives. Earth is neither too hot nor too cold, too big nor too small. It’s just right in so many ways — the perfect “Goldilocks” planet. Of course, as the only known abode of life in the universe, Earth doesE have one major claim to being special. The other planets in the solar system leave their marks in different ways. The planets divide into two broad categories: terrestrial and jovian. The small, rocky terrestrial planets include Mercury, Venus, Earth, and Mars. Mercury, the closest to the Sun, bakes Mars at temperatures up to 800° Fahrenheit at noon. But Mercury’s razor-thin atmosphere can’t hold heat; at night, the temperature plummets far below freezing. Venus most resembles Earth in mass and diameter, but a thick atmosphere of carbon dioxide has led to a runaway greenhouse effect. Venus’ surface remains a scorching 865° F year-round. Earth and Mars are the water worlds of the solar system. Our home planet is the only one with liquid water at the surface now, but spacecraft observations during the past 15 years leave no doubt that Mars once had loads of surface water. Even now, Mars has permafrost and permanent polar caps of water ice. Winds up to 70 mph blow around the ubiquitous martian dust, creating shifting seasonal patterns. FUN The jovian planets — Jupiter, Saturn, Uranus, and Neptune Mars boasts the largest vol- — are all gaseous behemoths. They consist mostly of hydrogen canoes in the solar system, and helium, the most abundant elements in the universe. Jupiter although they’re all extinct. The biggest — Olympus dwarfs the others: It contains more than twice as much matter Mons — spans nearly 400 as all the other planets combined. All the jovian planets pos- miles and rises 13 miles sess ring systems, but only Saturn’s appears bright. Its icy rings above the surrounding plains. span 170,000 miles and measure just 100 feet thick. Uranus FACT and Neptune are the true twin planets of the solar system, with nearly equal diameters, masses, compositions, and rotations. Most scientists no longer consider small, distant Pluto to be a major planet. A mixture of ice and rock, this world more closely resembles the thousands of so-called Kuiper Belt objects that lurk beyond Neptune. In 2006, astronomers demoted Pluto to a Mars’ ruddy appearance arises “dwarf planet,” a category that also includes the asteroid Ceres. because the sand on the planet’s surface consists largely of iron 2 Your guide to planets, stars, and galaxies oxides — rust. NASA/JPL/MSSS Jupiter Jupiter is so big that it would take 11 Earths wedged side by side to cross the giant’s girth and more than 1,000 Earths to fill its volume. NASA/JPL/UNIVERSITY OF ARIZONA Thick clouds blanket Venus, so astronomers use radar to see Venus its surface. The atmospheric pressure there is nearly 100 times that at Earth’s surface. NASA/JPL FUN Saturn has the lowest density of any planet. In fact, if you filled a solar-system-sized basin with water, the ringed world would float. FACT Mercury Earth Pluto & Charon FUN Sunlight takes just eight min- utes to reach Earth but more than four hours to cross the void to Neptune and Pluto. FACT Mercury's high density means Nearly three-quarters of more than half of it must be made Earth’s surface is covered with of the heavy elements iron and water. It’s what makes our home The orbit of dwarf planet Pluto (left; along with its moon, Charon) brings it closer nickel. NASA/JPL/USGS world conducive to life. NASA to the Sun than Neptune for 20 years out of its nearly 250-year-long circuit. ESA/NASA Uranus Neptune Solar system planets Planet Distance from Sun Orbital period Diameter Mass Density (Earth=1) (Earth=1) (Earth=1) (water=1) Mercury 0.39 87.97 days 0.383 0.055 5.43 Venus 0.72 224.70 days 0.949 0.815 5.24 Earth 1.00 365.26 days 1.000 1.000 5.52 Mars 1.52 686.98 days 0.532 0.107 3.93 Ceres 2.77 4.60 years 0.075 0.0002 1.93 Jupiter 5.20 11.86 years 11.209 317.832 1.33 Saturn 9.58 29.46 years 9.449 95.159 0.69 Uranus’ bland cloud tops mask Storms rage in Neptune’s atmos- Uranus 19.20 84.01 years 4.007 14.536 1.27 the fact that its rotation axis lies in phere, as they do in the massive Neptune 30.05 164.79 years 3.883 17.147 1.64 its orbital plane, so night and day at atmospheres of most of the jovian Pluto 39.48 247.68 years 0.187 0.002 1.75 the poles last 40 years each. NASA/JPL planets. NASA/JPL Note: Ceres and Pluto are officially considered to be dwarf planets. Titan Titan’s hazy atmosphere glows as it scatters incom- ing sunlight. The atmosphere of Saturn’s moon is thicker than Earth’s and, like ours, contains mainly nitrogen. NASA/JPL/SSI Small bodies Europa of the solar Ridges crack the surface of Jupiter’s moon Europa. Such ridges could be sites where slushy water erupted through the icy surface system and then froze. NASA/JPL fter the Sun and planets, there’s not much else in the Io solar system — certainly not in terms of mass. But More than 100 active volcanoes dot the surface in sheer number (and in a few notable instances, of Jupiter’s moon Io. The plumes can reach 100 prominence), the small objects hold their own. The miles high and spread debris over thou- biggest of the small bodies actually outrank the small- sands of miles. NASA/JPL est planet.A Both Ganymede, a Jupiter moon, and Titan, a Saturn moon, have diameters larger than Mercury. More than 170 moons have been discovered orbiting the eight planets, although the vast majority are little more than flying boulders. The smaller planets tend to have fewer moons. Earth has just one, which formed when an object the size of Mars struck the proto-Earth, ejecting debris that eventually coalesced. Mercury and Venus have no moons, and Mars possesses just two small ones. Oddly enough, Pluto’s large moon, Charon, is half the diam- eter of the dwarf planet — the largest ratio in the solar system. The hefty moons of the gas giants garner most of the atten- tion. Jupiter’s four big moons — Io, Europa, Ganymede, and Callisto — form a miniature solar system. Io ranks as the most volcanically active object in the solar system. Europa hides an ocean of liquid water — perhaps larger than all of Earth’s oceans — beneath its frigid ice crust. Giant Ganymede also may harbor an ocean and has a surface covered with intriguing grooved ter- rain. And Callisto sports more craters than any other object in the solar system. At the top of Saturn’s family of moons is Titan, which possesses a significant atmosphere and methane lakes. More than half a million asteroids also inhabit the solar system. The biggest, Ceres, has a diameter of 600 miles. Yet most are far smaller: If you add them all up, asteroids don’t equal the weight of Earth’s Moon. Most asteroids circle the Sun between the orbits of Mars and Jupiter, although a few wander into Earth’s vicinity. Perhaps the most spectacular small bodies are comets. Billions of these “dirty snowballs” lurk in the outer solar sys- tem. If their long, looping orbits bring them close to the Sun’s warmth, they shed gas and dust. The Sun then blows this mate- rial back to create a long tail. Although a comet’s nucleus may be only a mile or two across, its tail can stretch millions of miles. 4 Your guideguide to to planets, planets, stars, stars, and and galaxies galaxies Triton Callisto Geyser-like plumes deposited the dark streaks seen on Nep- tune’s moon Triton. At a FUN temperature of –390° F, Triton Saturn’s enigmatic outer has the coldest surface known in moon, Iapetus, has a split the solar system. NASA/JPL personality: Half of its surface appears as dark as freshly laid asphalt while the opposite Enceladus hemisphere reflects as much light as newly fallen snow. FACT Saturn’s icy moon Enceladus reflects more than 90 percent of the sunlight Multi-ringed impact basins, some stretching more than that reaches it, the highest percentage 1,000 miles, formed on Jupiter’s Callisto when massive of any object in the solar system. NASA/JPL impacts left concentric fractures and faults. NASA/JPL Io Hale-Bopp Eros One of the brightest comets of the past 40 years, Hale-Bopp wowed observers in 1997. It had a nucleus 25 miles wide and a tail that stretched more than 100 million miles. BILL AND SALLY FLETCHER Potato-shaped asteroid Eros, some 20 miles long, looks like a lot of other modest-sized asteroids, but this object might one day wander dangerously close to Earth.