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2016-07-28. Forbidden planets: Understanding alien worlds once thought impossible.

posted Jul 30, 2016, 9:46 AM by Alan Gould
By Daniel Clery, Science. For GSS A Changing Cosmos chapter 7. Excerpt: ...The planet hunt accelerated with the launch of NASA’s Kepler spacecraft in 2009, and the 2500 worlds it has discovered added statistical heft to the study of exoplanets—and yet more confusion. Kepler found that the most common type of planet in the galaxy is something between the size of Earth and Neptune—a “super-Earth,” which has no parallel in our solar system and was thought to be almost impossible to make.  ...Other planetary systems looked nothing like our orderly solar system, challenging the well-worn theories that had been developed to explain it. ...The traditional model of how stars and their planets form dates back to the 18th century, when scientists proposed that a slowly rotating cloud of dust and gas could collapse under its own gravity. ...This scenario naturally produces a planetary system just like our own: small, rocky planets with thin atmospheres close to the star, a Jupiter-like gas giant just beyond the snowline, and the other giants getting progressively smaller at greater distances because they move more slowly through their orbits and take longer to hoover up material. ...But the discovery of hot Jupiters suggested something was seriously amiss with the theory. ...Theorists have come up with two possible mechanisms for shuffling the planetary deck. The first, known as migration, requires there to be plenty of material left in the disk after the giant planet has formed. ...If the past is anything to go by, modelers will have to keep on their toes. “Nature is smarter than our theories,” Rafikov says.  http://www.sciencemag.org/news/2016/07/forbidden-planets-understanding-alien-worlds-once-thought-impossible