Our Solar System: An Island of Calm in a Violent Universe (and it’s special, too)

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We humans like to think we’re special, but astronomically speaking we’ve been shot down quite severely and humbly put in our place. We’re not at the center of our solar system, nowhere near the center of our galaxy and certainly not at the center of the universe. But now comes great news for the human psyche from scientists trying to explain solar system formation. As far as solar systems go, we have thought ours was just average and that all solar systems were like ours. But in looking at the 300 plus extrasolar planets that have been discovered and the systems they are in, none so far are anything like our home solar system. In fact, say scientists at Northwestern University, we may be special after all. In a study using computer simulations (this is the week for computer simulations, see here and here), researchers ran more than a hundred simulations, and the results show that the average planetary system’s origin was full of violence and drama but that the formation of something like our solar system required conditions to be “just right” and quite special indeed.

The study illustrates that if early conditions had been just slightly different, very unpleasant things could have happened — like planets being thrown into the sun or jettisoned into deep space. This was the first simulation to model the formation of planetary systems from beginning to end, starting with the generic disk of gas and dust that is left behind after the formation of the central star and ending with a full planetary system.

Before the first exoplanets were discovered in the early 1990’s we only had our own solar system from which to create a model, and astronomers had no reason to think our solar system unusual.

“But we now know that these other planetary systems don’t look like the solar system at all,” said Frederic A. Rasio, a theoretical astrophysicist and professor of physics and astronomy at Northwestern.
“The shapes of the exoplanets’ orbits are elongated, not nice and circular. Planets are not where we expect them to be. Many giant planets similar to Jupiter, known as ‘hot Jupiters,’ are so close to the star they have orbits of mere days. Clearly we needed to start fresh in explaining planetary formation and this greater variety of planets we now see.”

The simulations suggest that an average planetary system’s origin is extremely dramatic. The gas disk that gives birth to the planets also pushes them mercilessly toward the central star, where they crowd together or are engulfed. Among the growing planets, there is cut-throat competition for gas, a chaotic process that produces a rich variety of planet masses.

Also planets orbiting close to each other can create a slingshot encounter that flings the planets elsewhere in the system; occasionally, one is ejected into deep space. Despite its best efforts to kill its offspring, the gas disk eventually is consumed and dissipates, and a young planetary system emerges.

“Such a turbulent history would seem to leave little room for the sedate solar system, and our simulations show exactly that,” said Rasio. “Conditions must be just right for the solar system to emerge.”

Too massive a gas disk, for example, and planet formation is an anarchic mess, producing “hot Jupiters” and noncircular orbits galore. Too low-mass a disk, and nothing bigger than Neptune — an “ice giant” with only a small amount of gas — will grow.

“We now better understand the process of planet formation and can explain the properties of the strange exoplanets we’ve observed,” said Rasio. “We also know that the solar system is special and understand at some level what makes it special.”

“The solar system had to be born under just the right conditions to become this quiet place we see. The vast majority of other planetary systems didn’t have these special properties at birth and became something very different.”

So, go ahead. Feel special.

Original News Source: Northwestern University