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Not so long ago — as recently as the mid-1990s, in fact — there was a theory so beautiful that astronomers thought it simply had to be true.
They gave it a rather pedestrian name: the core-accretion theory. But its beauty lay in how it used just a few basic principles of physics and chemistry to account for every major feature of our Solar System. It explained why all the planets orbit the Sun in the same direction; why their orbits are almost perfectly circular and lie in or near the plane of the star's equator; why the four inner planets (Mercury, Venus, Earth and Mars) are comparatively small, dense bodies made mostly of rock and iron; and why the four outer planets (Jupiter, Saturn, Uranus and Neptune) are enormous, gaseous globes made mostly of hydrogen and helium. And because the same principles of physics and astronomy must apply throughout the Universe, it predicted that any system of 'exoplanets' around another star would look pretty much the same.
But in the mid-1990s, astronomers actually started finding those exoplanets — and they looked nothing like those in our Solar System. Gas giants the size of Jupiter whipped around their stars in tiny orbits, where core accretion said gas giants were impossible. Other exoplanets traced out wildly elliptical orbits. Some looped around their stars' poles. Planetary systems, it seemed, could take any shape that did not violate the laws of physics.
Following the launch of NASA's planet-finding Kepler satellite in 2009, the number of possible exoplanets quickly multiplied into the thousands — enough to give astronomers their first meaningful statistics on other planetary systems, and to undermine the standard theory for good. Not only were there lots of exoplanet systems bearing no resemblance to ours, but the most commonly observed type of planet — a 'super-Earth' that falls between the sizes of our world and Neptune, which is four times bigger — does not even exist in our Solar System. Using our planetary family as a model, says astronomer Gregory Laughlin of the University of California, Santa Cruz, “has led to no success in extrapolating what's out there”.
Read more here. (Nature)
They gave it a rather pedestrian name: the core-accretion theory. But its beauty lay in how it used just a few basic principles of physics and chemistry to account for every major feature of our Solar System. It explained why all the planets orbit the Sun in the same direction; why their orbits are almost perfectly circular and lie in or near the plane of the star's equator; why the four inner planets (Mercury, Venus, Earth and Mars) are comparatively small, dense bodies made mostly of rock and iron; and why the four outer planets (Jupiter, Saturn, Uranus and Neptune) are enormous, gaseous globes made mostly of hydrogen and helium. And because the same principles of physics and astronomy must apply throughout the Universe, it predicted that any system of 'exoplanets' around another star would look pretty much the same.
But in the mid-1990s, astronomers actually started finding those exoplanets — and they looked nothing like those in our Solar System. Gas giants the size of Jupiter whipped around their stars in tiny orbits, where core accretion said gas giants were impossible. Other exoplanets traced out wildly elliptical orbits. Some looped around their stars' poles. Planetary systems, it seemed, could take any shape that did not violate the laws of physics.
Following the launch of NASA's planet-finding Kepler satellite in 2009, the number of possible exoplanets quickly multiplied into the thousands — enough to give astronomers their first meaningful statistics on other planetary systems, and to undermine the standard theory for good. Not only were there lots of exoplanet systems bearing no resemblance to ours, but the most commonly observed type of planet — a 'super-Earth' that falls between the sizes of our world and Neptune, which is four times bigger — does not even exist in our Solar System. Using our planetary family as a model, says astronomer Gregory Laughlin of the University of California, Santa Cruz, “has led to no success in extrapolating what's out there”.
Read more here. (Nature)
