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Compact Cosmos

27 26 PLANETSANDMOONS forming solar systems According to current theory,some 4.6 billion years ago a huge cloud of dust and gas grew lumpy and collapsed under gravity, spinning and flattening into a disk. At the center the largest lump became so massive that hydrogen fused into helium,igniting the nuclear fires of our sun. Smaller planetessimals, orbiting in the surrounding accretion disk,coagulated into a collection of planets opposite top. Like a giant mixer on full speed, some parts ferociously collided while others were flung out into the Oort Cloud or farther into deep space. All this may have occurred in a mere 100,000 years, though recent models suggest that it could have happened even quicker in the wink of a cosmic eye. Our own solar system below is just one of many. Hundreds of exoplanets in extraordinary orbital systems have been detected from the slight wobblings of nearby stars lower opposite. Manyhave masses greater than our largest planet,Jupiter,and may be brown dwarfs, protostars not quite big enough to burst into nuclear fusion. Someof these giants have orbits of days rather than months, whipping around extremely close to their parent star. Quietly behaved systems like ours seem quite rare.
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