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The Genesis of Planets

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The Genesis of Planets ASTRONOMY Long viewed as a stately procession to a foregone conclusion, planetary formation turns out to be startlingly chaotic lthough they are, in cosmic terms, of us trying to fathom their origins. When I mere scraps—insignificant to the was in graduate school in the 1970s, we tended A grand narrative of heavenly expan- to think of planet formation as a well-ordered, sion—planets are the most diverse and intricate deterministic process—an assembly line that class of object in the universe. No other celestial turns amorphous disks of gas and dust into bodies support such a complex interplay of copies of our solar system. Now we are realiz- astronomical, geologic, and chemical and bio- ing that the process is chaotic, with distinct logical processes. No other places in the cosmos outcomes for each system. The worlds that could support life as we know it. The worlds of emerge are the survivors of a hurly-burly of our solar system come in a tremendous variety, competing mechanisms of creation and de- and even they hardly prepared us for the discov- struction. Many are blasted apart, fed into the eries of the past decade, during which astrono- fires of their system’s newborn star or ejected mers have found more than 200 planets. into interstellar space. Our own Earth may The sheer diversity of these bodies’ masses, have long-lost siblings that wander through the sizes, compositions and orbits challenges those lightless void. CREDIT 50 SCIENTIFIC AMERICAN May 2008 © 2008 SCIENTIFIC AMERICAN, INC. By Douglas N. C. Lin Illustrations by Don Dixon The study of planet formation lies at the in- abrupt whoosh as the prenatal disk of gas and tersection of astrophysics, planetary science, dust breaks up—a process that replicates, in statistical mechanics and nonlinear dynamics. miniature, the formation of stars. This hypoth- Broadly speaking, planetary scientists have de- esis remains contentious because it assumes the veloped two leading theories. The sequential- existence of highly unstable conditions, which accretion scenario holds that tiny grains of may not be attainable. Moreover, astronomers dust clump together to create solid nuggets of have found that the heaviest planets and the BABY GIANT PLANET swoops up gas from the disk rock, which either draw in huge amounts of gas, lightest stars are separated by a “desert”—a around a newborn star. becoming gas giants such as Jupiter, or do not, scarcity of intermediate bodies. The disjunc- becoming rocky planets such as Earth. The tion implies that planets are not simply little main drawback of this scenario is that it is a stars but have an entirely different origin. slow process and that gas may disperse before Although researchers have not settled this it can run to completion. controversy, most consider the sequential-ac- The alternative, gravitational-instability cretion scenario the most plausible of the two. CREDIT scenario holds that gas giants take shape in an I will focus on it here. www.SciAm.com SCIENTIFIC AMERICAN 51 © 2008 SCIENTIFIC AMERICAN, INC. An Interstellar Cloud Collapses ets. Newly formed disks contain mostly hydro- 1 . Time: 0 (starting point of planet gen and helium gas. In their hot and dense inner formation sequence) regions, dust grains are vaporized; in the cool Our solar system belongs to a galaxy of some and tenuous outer parts, the dust particles sur- 100 billion stars threaded with clouds of gas and vive and grow as vapor condenses onto them. dust, much of it the debris of previous genera- Astronomers have discovered many young tions of stars. “Dust” in this context simply stars that are surrounded by such disks. Stars be- means microscopic bits of water ice, iron and tween one million and three million years old have other solid substances that condensed in the gas-rich disks, whereas those older than 10 mil- cool outer layers of stars and were blown out lion years have meager, gas-poor disks, the gas into interstellar space. When clouds are suffi- having been blown away by the newborn star or ciently cold and dense, they can collapse under by bright neighboring stars. This span of time de- the force of gravity to form clusters of stars, a lineates the epoch of planet formation. The mass process that takes 100,000 to a few million of heavy elements in these disks is roughly com- years [see “Fountains of Youth: Early Days in parable to the mass of heavy elements in the plan- the Life of a Star,” by Thomas P. Ray; Scientif- ets of the solar system, providing a strong clue ic American, August 2000]. that the planets indeed arose from such disks. Surrounding each star is a rotating disk of left- Ending point: Newborn star surrounded over material, the wherewithal for making plan- by gas and micron-size dust grains [STAGE 2] COsmic DUst BUnnies Even the mightiest planets have humble roots: as micron-size the newborn star, defining a “snow line” beyond which water dust grains (the ashes of long-dead stars) embedded in a swirl- stays frozen. In our solar system, the snow line marks the ing disk of gas. The disk's temperature falls with distance from boundary between the inner rocky planets and outer gas giants. ●3 At the snow line, local conditions are such that the drag force reverses direction. Grains tend to accumulate and readily coagulate into larger ●1 Grains collide, clump and grow. ●2 Small grains are swept along by the gas, but bodies called planetesimals. those larger than a millimeter experience a drag force and spiral in. 2–4 AU Dust spirals inward e lin Protosun Snow Disk of gas and dust 52 SCIENTIFIC AMERICAN © 2008 SCIENTIFIC AMERICAN, INC. May 2008 [STAGE 3] The Disk Sorts Itself Out 2. Time: About 1 million years THE rise OF THE OLIGarcHS Dust grains in the protoplanetary disk are Billions of kilometer-size planetesimals, built up during stage 2, then stirred by nearby gas and collide with one agglomerate into moon- to Earth-size bodies known as embryos. Relatively another, sometimes sticking together, some- few in number, embryos dominate their respective orbital zones; this times breaking apart. The grains intercept star- “oligarchy” of embryos competes for the remaining material. light and reemit lower-wavelength infrared light, ensuring that heat reaches even the dark- est regions of the disk’s interior. The tempera- ture, density and pressure of gas generally decrease with distance from the star. Because of the balance of pressure, rotation and gravity, gas orbits the star slightly slower than an inde- pendent body at the same distance would. Consequently, dust grains larger than a few millimeters in size tend to outpace the gas, there- by running into a headwind that slows them down and causes them to spiral inward, toward Planetesimals collide and adhere. the star. The bigger the grains grow, the faster they spiral. Chunks a meter in size can halve their distance from the star within 1,000 years. As they approach the star, the grains warm up, and eventually water and other low-boiling- point substances, known as volatiles, boil off. The distance at which this happens, the “snow line,” lies between 2 and 4 AU (astronomical units) from the star, which in our solar system falls between the orbits of Mars and Jupiter. (The radius of Earth’s orbit is 1 AU.) The snow line di- A few bodies undergo runaway growth. They stir up the orbits of the rest. vides the planetary system into an inner, volatile- poor region filled with rocky bodies and an out- er, volatile-rich region filled with icy ones. At the snow line itself, water molecules tend to accumulate as they boil off grains. This build- up of water triggers a cascade of effects. It pro- duces a discontinuity in gas properties at the The embryos run out of raw material and stop growing. snow line, which leads to a pressure drop there. The balance of forces causes the gas to speed up its revolution around the central star. Conse- tion, planetesimals have swept up almost all the quently, grains in the vicinity feel not a head- original dust. Planetesimals are hard to see di- wind but a tailwind, which boosts their velocity rectly, but astronomers can infer their presence and halts their inward migration. As grains con- from the debris of their collisions [see “The Hid- tinue to arrive from the outer parts of the disk, den Members of Planetary Systems,” by David they pile up at the snow line. In effect, the snow Ardila; Scientific American,April 2004]. line becomes a snowbank. Ending point: Swarms of kilometer-size Crammed together, the grains collide and building blocks known as planetesimals grow. Some break through the snow line and con- tinue to migrate inward, but in the process they Planetary Embryos Germinate become coated with slush and complex molecules, 3 . Time: 1 million to 10 million years which makes them stickier. Some regions are so The cratered landscapes on Mercury, the moon thick with dust that the grains’ collective gravita- and the asteroids leave little doubt that nascent tional attraction also accelerates their growth. planetary systems are shooting galleries. Colli- In these ways, the dust grains pack them- sions between planetesimals either build them selves into kilometer-size bodies called planetes- up or break them apart. A balance between imals. By the end of the stage of planet forma- coagulation and fragmentation leads to a distri- www.SciAm.com © 2008 SCIENTIFIC AMERICAN, INC. SCIENTIFIC AMERICAN 53 bution of sizes in which small bodies account of gaps in the disk, where planetesimals also for most of the surface area in the emerging sys- tend to accumulate.
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