Story from Science News. (H/T ECM)

Excerpt:

According to the most popular formation theory, planets coalesce from a swirling disk of gas and dust that surrounds young stars. Since the disk rotates in the same direction as the star, the planets spawned by the disk should revolve in the same direction. But in an overcrowded planetary system, where a gravitational game of billiards is all but inevitable, orbits can get scrambled. A close encounter between planetary siblings can push one body outward while flinging the other inward, elongating and tilting the inner planet’s orbit.

In this scenario, the solar system may have been unusually lucky. Either it avoided catastrophic gravitational encounters between massive planets or it suffered such interactions so long ago that most of the planets had the chance to resettle into nearly circular orbits with little or no tilt, says Frédéric Pont of the University of Exeter in England.

“The presence of advanced life on Earth may be contingent on our planetary system having avoided the brunt of planet-planet scatter,” keeping Earth on a circular, Goldilocks-style orbit—neither too hot nor too cold for life as we know it, he speculates.

The circularity of the orbit is crucial for maintaining liquid water at the surface. If the the planet’s orbit is too eccentric, then the temperature variations will either freeze the water, or evaporate it into the atmosphere. Either condition is fatal to complex life.