There's an assumption made by scientists who study the cosmos that our sun, planet, solar system, galaxy, and our location in these are all pretty much the norm in the universe. There's nothing special about where we are or the conditions which exist in our galactic neighborhood. Our planet and our place in the universe are, it has been said, mediocre.
Unfortunately, for adherents to the Principle of Mediocrity, this idea has become more difficult to maintain of late, having suffered numerous insults at the hands of research scientists themselves over the past several decades. Books like Rare Earth and Privileged Planet summarize their findings and make a powerful case that the earth is special indeed in a host of important ways.
Now comes news that not only are planets like our earth probably quite rare in the universe so are solar systems like ours. Science Daily reports on an article scheduled to appear in an upcoming edition of the journal Science:
Prevailing theoretical models attempting to explain the formation of the solar system have assumed it to be average in every way. Now a new study by Northwestern University astronomers, using recent data from the 300 exoplanets discovered orbiting other stars, turns that view on its head.
The solar system, it turns out, is pretty 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.
Solar systems must be similar to ours if life is going to flourish in them. For instance, ours is a relatively stable system compared to what almost every other system is like. Also, the large gas giants in the outer regions of our solar system act as vacuum sweepers attracting meteors and comets into their bosoms and protecting the earth from their impact. The distribution of the planets and the size of the sun ensure that gravitational effects do not exert a destructive influence on the earth's orbit. The list goes on, but read what Science Daily says:
The [computer] 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, as the planets approach each other, they frequently lock into dynamical resonances that drive the orbits of all participants to be increasingly elongated. Such a gravitational embrace often results in 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."
The more we learn about the cosmos and about life the more difficult it is to think that it's all just a fortuitous accident and the easier it is to think that conscious purpose lies behind it all.
Artist's rendition of the formation of the solar system
Graphic representation of the solar system (not to scale)
RLC