Figure 9.31: Jupiter is a giant planet with a mass 1/1000 that of the sun, and 2.5 times that of all the other planets in the solar system combined. A prominent feature of the planet is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen. Jupiter has 67 moons; most of these are very small, and are probably captured asteroids that got too close. This ‘family portrait’ includes its four largest moons: from top to bottom Io, Europa, Ganymede and Callisto, which all formed around Jupiter long ago. They are known as the Galilean moons, because they were first discovered by Galileo. Io was named after a priestess of the Roman goddess Juno whom Jupiter fell in love with. It is covered with giant volcanoes which erupt very frequently, and which have filled in almost all of its impact craters. It is the only celestial body in the solar system besides Earth known to have volcanic activity. Io has an iron core and a brown silicate outer layer. Its red and orange colours come primarily from sulphur, which condenses on the surface after being outgassed by the volcanoes. Europa was initially a Cretan moon goddess, who was incorporated into Greek mythology as a virgin Phoenician princess. She had an affair with Zeus (Jupiter in Roman mythology) but he asked her to marry Asterion, the king of stars. Europa is about the size of Earth’s moon and its surface and crust are made almost entirely of water ice. Europa has a high degree of reflectivity, making it among the brightest moons in the solar system. Because the surface is smooth, it has re-covered it since the Heavy Bombardment Period by adding layers of ice. The surface may be only a few million years old and it is believed that beneath the icy outer shell lies a liquid water ocean, twice as large as Earth’s oceans combined, heated by Europa’s hot core. Does the ocean harbour life? The moon Ganymede was named after a handsome Trojan prince who Zeus (Jupiter) took away to serve as a cupbearer for the gods, as well as a lover for him. Zeus eventually turned Ganymede into the constellation Aquarius (the water-bearer) in the sky. Recently, it has been discovered that Ganymede, like Europa, probably has a liquid water ocean underneath its icy exterior. It is the largest of the four moons, about the size of Mercury. Its outstanding characteristic is that it is the only moon to have its own magnetic field. Callisto was named after a nymph, loved by Zeus but hated by his wife Hera. Hera changed her into a bear, and Zeus then placed her in the sky as the constellation the Great Bear, also known as Ursa Major. Callisto is the most heavily cratered object in the solar system. Its icy landscape has essentially remained unchanged since its formation four billion years ago.
9.2.1 Hot Jupiters
Te last decade has seen a bonanza of exoplanet discoveries – planets that orbit a star other than the sun. Many of these planets belong to a class known as ‘hot Jupiters’, or gas giants physically similar to Jupiter but much hotter, with orbits that take them feverishly close to their stars. While Jupiter orbits the sun at a distance of 5.2 AU, the hot Jupiters have very short orbital radii with semi-major axes of 0.015–0.5 AU. One of the best-known hot Jupiters is 51 Pegasi b. Discovered in 1995, it was the first extrasolar planet found orbiting a sun-like star. 51 Pegasi b has an orbital period of about four days. WASP-18b (Figure 9.30) is possibly an ill-fated planet, seen to be gradually moving inwards towards its sun, and its doom. At first, since we don’t have anything like them in our own
solar system, hot Jupiters were considered goofy oddballs. But as more were found, they went from oddity to normalcy. For a while, it started to look like our own solar system was the real misfit. And how do hot Jupiters wind up so close to their stars? Let’s take a closer look at the formation of Jupiter. Around
4.6 billion years ago, the solar system was a cloud of dust and gas known as the solar nebula. Gravity collapsed the material towards its centre, it began to spin, and the sun formed in the centre of the nebula. Te remaining material began to clump together. Close to the sun the solar wind swept away the lighter elements such as hydrogen and helium leaving only heavy, rocky materials to create Earth-like planets. But at greater distances from the sun where the solar winds were calmer, the lighter elements were given the chance to coalesce into
314
gas giants. A relatively new theory suggests that in the outer regions, clumps of dust and gas are bound together early in the life of the solar system. Over time, perhaps thousands to a million years, these clumps slowly compacted into a giant planet, thereby trapping the rapidly vanishing lighter gases. In this way, Jupiter could have been born and, as the king in
the solar system, it most likely impacted the creation and paths of other planets. Te planet itself would have had sufficient mass to alter the path of other ‘baby’ planets that travelled near it, sending them veering either into the outermost reaches of the solar system or toward a fiery death near the sun. Tere are two schools of thought regarding the origin of
hot Jupiters: either formation at a distance followed by inward migration from a more distant orbit to a much tighter one typical of hot Jupiters, or formation at the distance at which they are currently observed. Te leading theory is the first since it is thought that hot Jupiters are too massive to form in situ because of a lack of building materials close to a star. Terefore, we believe that these features initially form as gas giants in distant orbits where building materials are sufficient, and become hot Jupiters when the gravitational influences from nearby stars or planets drive them into closer orbits. Te planets start out in eccentric orbits; then, over a period of hundreds of millions of years, they are believed to gradually settle down into tight, circular orbits.
9.2.2 Three Puzzles At school we were told that planets orbiting a star like the sun
NASA/JPL
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272 |
Page 273 |
Page 274 |
Page 275 |
Page 276 |
Page 277 |
Page 278 |
Page 279 |
Page 280 |
Page 281 |
Page 282 |
Page 283 |
Page 284 |
Page 285 |
Page 286 |
Page 287 |
Page 288 |
Page 289 |
Page 290 |
Page 291 |
Page 292 |
Page 293 |
Page 294 |
Page 295 |
Page 296 |
Page 297 |
Page 298 |
Page 299 |
Page 300 |
Page 301 |
Page 302 |
Page 303 |
Page 304 |
Page 305 |
Page 306 |
Page 307 |
Page 308 |
Page 309 |
Page 310 |
Page 311 |
Page 312 |
Page 313 |
Page 314 |
Page 315 |
Page 316 |
Page 317 |
Page 318 |
Page 319 |
Page 320 |
Page 321 |
Page 322 |
Page 323 |
Page 324 |
Page 325 |
Page 326 |
Page 327 |
Page 328 |
Page 329 |
Page 330 |
Page 331 |
Page 332 |
Page 333 |
Page 334 |
Page 335 |
Page 336 |
Page 337 |
Page 338 |
Page 339 |
Page 340 |
Page 341 |
Page 342 |
Page 343 |
Page 344 |
Page 345 |
Page 346 |
Page 347 |
Page 348 |
Page 349 |
Page 350 |
Page 351 |
Page 352
