Once upon a time (in the mid-1990s, to be more precise), astronomers had a beautiful theory of how planets could form. However, since then they have found “thousands of star systems wildly different from our own.” Now the old theory is very dead.
A Nature news article describes the not-so-old-but-already discredited theory:
“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.”
However, exoplanets “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.”
Now astronomers are asking why our system is so different. They have a plethora of questions but few if any answers.
It looks like our solar system has been fine-tuned to enable life to flourish on our planet. Reminds me of a book called Genesis, which describes the origin of all things with the words “In the beginning God created the heavens and the earth” (Gen. 1:1, NKJV).
Finkbeiner, Ann. 2014. Astronomy: Planets in chaos. Nature news. (2 July).