9:59:59.000,9:59:59.000 Consider the spot where you’re sitting. 9:59:59.000,9:59:59.000 Travel backwards in time 9:59:59.000,9:59:59.000 and it might’ve been submerged at [br]the bottom of a shallow sea, 9:59:59.000,9:59:59.000 buried under miles of rock, 9:59:59.000,9:59:59.000 or floating through a molten, [br]infernal landscape. 9:59:59.000,9:59:59.000 But go back far enough— 9:59:59.000,9:59:59.000 about 4.6 billion years, 9:59:59.000,9:59:59.000 and you’d be in the middle of an enormous [br]cloud of dust and gas 9:59:59.000,9:59:59.000 orbiting a newborn star. 9:59:59.000,9:59:59.000 This is the setting for some of the [br]biggest, smallest mysteries of physics: 9:59:59.000,9:59:59.000 the mysteries of cosmic dust bunnies. 9:59:59.000,9:59:59.000 Seemingly empty regions [br]of space between stars 9:59:59.000,9:59:59.000 actually contain clouds of gas and dust, 9:59:59.000,9:59:59.000 usually blown there by supernovas. 9:59:59.000,9:59:59.000 When a dense cloud reaches a certain [br]threshold called the Jeans mass, 9:59:59.000,9:59:59.000 it collapses in on itself. 9:59:59.000,9:59:59.000 The shrinking cloud rotates faster [br]and faster, and heats up, 9:59:59.000,9:59:59.000 eventually becoming hot enough to burn [br]hydrogen in its core. 9:59:59.000,9:59:59.000 At this point a star is born. 9:59:59.000,9:59:59.000 As fusion begins in the new star, 9:59:59.000,9:59:59.000 it sends out jets of gas that blow [br]off the top and bottom of the cloud, 9:59:59.000,9:59:59.000 leaving behind an orbiting ring of gas [br]and dust called a protoplanetary disk.’ 9:59:59.000,9:59:59.000 This is a surprisingly windy place; 9:59:59.000,9:59:59.000 eddies of gas carry particles apart, [br]and send them smashing into each other. 9:59:59.000,9:59:59.000 The dust consists of tiny metal fragments,[br]bits of rock, and, further out, ices. 9:59:59.000,9:59:59.000 We’ve observed thousands of these disks [br]in the sky, 9:59:59.000,9:59:59.000 at various stages of development [br]as dust clumps together 9:59:59.000,9:59:59.000 into larger and larger masses. 9:59:59.000,9:59:59.000 Dust grains 100 times smaller than the [br]width of a human hair stick to each other 9:59:59.000,9:59:59.000 through what’s called [br]the van der Waals force. 9:59:59.000,9:59:59.000 That’s where a cloud of electrons [br]shifts to one side of a molecule, 9:59:59.000,9:59:59.000 creating a negative charge on one end, [br]and on positive charge on the other. 9:59:59.000,9:59:59.000 Opposites attract, but van der Waals can [br]only hold tiny things together. 9:59:59.000,9:59:59.000 And there’s a problem: once dust [br]clusters grow to a certain size, 9:59:59.000,9:59:59.000 the windy atmosphere of a disk should [br]constantly break them up 9:59:59.000,9:59:59.000 as they crash into each other. 9:59:59.000,9:59:59.000 The question of how they continue to grow[br]is the first mystery of dust bunnies. 9:59:59.000,9:59:59.000 One theory looks to electrostatic charge [br]to answer this. 9:59:59.000,9:59:59.000 Energetic gamma rays, x-rays, [br]and UV photons 9:59:59.000,9:59:59.000 knock electrons off of gas [br]atoms within the disk, 9:59:59.000,9:59:59.000 creating positive ions [br]and negative electrons. 9:59:59.000,9:59:59.000 Electrons run into and stick to dust, 9:59:59.000,9:59:59.000 making it negatively charged. 9:59:59.000,9:59:59.000 Now, when the wind pushes [br]clusters together, 9:59:59.000,9:59:59.000 like repels like and slows them down [br]as they collide. 9:59:59.000,9:59:59.000 With gentle collisions [br]they won’t fragment, 9:59:59.000,9:59:59.000 but if the repulsion is too strong, [br]they’ll never grow. 9:59:59.000,9:59:59.000 One theory suggests that high energy [br]particles 9:59:59.000,9:59:59.000 can knock more electrons off of some [br]dust clumps, 9:59:59.000,9:59:59.000 leaving them positively charged. 9:59:59.000,9:59:59.000 Opposites again attract, [br]and clusters grow rapidly. 9:59:59.000,9:59:59.000 But before long we reach [br]another set of mysteries. 9:59:59.000,9:59:59.000 We know from evidence found in meteorites 9:59:59.000,9:59:59.000 that these fluffy dust bunnies [br]eventually get heated, melted 9:59:59.000,9:59:59.000 and then cooled into solid [br]pellets called chondrules. 9:59:59.000,9:59:59.000 And we have no idea how [br]or why that happens. 9:59:59.000,9:59:59.000 Furthermore, once those pellets do form,[br]how do they stick together? 9:59:59.000,9:59:59.000 The electrostatic forces from before [br]are too weak, 9:59:59.000,9:59:59.000 and small rocks can’t be held together [br]by gravity either. 9:59:59.000,9:59:59.000 Gravity increases proportionally to the [br]mass of the objects involved. 9:59:59.000,9:59:59.000 That’s why you could effortlessly escape [br]an asteroid the size of a small mountain 9:59:59.000,9:59:59.000 using just the force generated [br]by your legs. 9:59:59.000,9:59:59.000 So if not gravity, then what? 9:59:59.000,9:59:59.000 Perhaps it’s dust. 9:59:59.000,9:59:59.000 A fluffy dust rim collected around the [br]outside of the pellets 9:59:59.000,9:59:59.000 could act like Velcro. 9:59:59.000,9:59:59.000 There’s evidence for this in meteors, 9:59:59.000,9:59:59.000 where we find many chondrules surrounded[br]by a thin rim of very fine material– 9:59:59.000,9:59:59.000 possibly condensed dust. 9:59:59.000,9:59:59.000 Eventually the chondrule pellets get [br]cemented together inside larger rocks, 9:59:59.000,9:59:59.000 which at about 1 kilometer across 9:59:59.000,9:59:59.000 are finally large enough to hold [br]themselves together through gravity. 9:59:59.000,9:59:59.000 They continue to collide and grow [br]into larger and larger bodies, 9:59:59.000,9:59:59.000 including the planets we know today. 9:59:59.000,9:59:59.000 Ultimately, the seeds of [br]everything familiar– 9:59:59.000,9:59:59.000 the size of our planet, its position [br]within the solar system, 9:59:59.000,9:59:59.000 and its elemental composition– 9:59:59.000,9:59:59.000 were determined by an uncountably large [br]series of random collisions. 9:59:59.000,9:59:59.000 Change the dust cloud just a bit, 9:59:59.000,9:59:59.000 and perhaps the conditions wouldn’t [br]have been right 9:59:59.000,9:59:59.000 for the formation of life on our planet.