Solar system turned inside out by speck of dust

Every now and then someone makes a scientific discovery that threatens to turn what we thought we knew inside out.

Such was the case in 2006 when a small capsule containing samples of a comet's tail parachuted to Earth.

The fact that NASA had been able to manoeuvre a spacecraft (Stardust) into the tail of a comet (Wild 2) orbiting out near Jupiter, snag thousands of dust particles in a trap made of a puffy, glassy material and have Stardust send it back to Earth is mind-boggling enough in itself, but what researchers found when they examined the grains of dust turned out to be astounding beyond what almost anyone imagined.

What most of us thought we knew, was that our familiar solar system formed approximately 4.5 billion years ago from a spinning disc of dust and gas we call the solar nebula. The long-accepted model has been that the objects orbiting the sun accreted from the available material roughly where they are now.

Comets, which generally orbit well beyond Neptune in the unimaginable frigidness of space, should be an accretion of ice and carbon-rich dust according to the model.

The new evidence from Wild 2 (pronounced Vilt 2) told a very different story. Under nearly a million times magnification the dust specks revealed tiny black grains, some mere nanometres across, of metal and minerals such as tungsten and titanium nitride whose origin could only be very near to our fledgling sun at temperatures greater than 3,000 degrees Fahrenheit.

This was very good news for a group of scientists led by Harold Levison of the Southwest Research Institute, who, in 2004 proposed a much more chaotic and violent birth scenario for the solar system.

Building on work that posits the odd elliptical orbits of planets is not consistent with the orderly accretion theory for the early solar system, which should have generated stable, roughly circular orbits, Levison and his colleagues developed a hypothesis now known as the Nice model (from Nice, France where the group met).

In a nutshell, the group hypothesized that the four gas giants, Jupiter, Saturn, Uranus and Neptune started out much more tightly packed together with the latter three closer to the sun than they are now. For several hundred million years the gravitational dance of the giants cleared the solar disc of early debris by either sucking it in or flinging it away.

Of course they were also tugging away at each other. Levison et. al. proposed that early on the giants' orbits slowly, but surely, shifted. Jupiter moved inward and the other three outward until approximately 3.8 to 4 billion years ago when Saturn started completing precisely one orbit for every two of Jupiter's. That "resonance" between the two elongated their orbits causing Saturn to move so close to Uranus and Neptune that those two were catapulted outward, perhaps even exchanging places.

As Uranus and Neptune moved outward through areas of the outer solar system still littered with debris, they created chaos flinging icy, rocky objects (comets and asteroids) in every direction.

It's a much more chaotic and violent origin story than what we've become accustomed to, but one that is supported by the Wild 2 evidence. It also suggests that the solar system is still much more dynamic and unstable than the human imagination can comprehend.

In all of human recorded history very little has changed about the solar system, except our understanding of it. But human history doesn't even register on the space-time scale of solar system history.

Even Sir Isaac Newton knew from his calculations that planetary orbits were ultimately unstable, but could not fathom how there was no evidence they had ever changed. As people so frequently do when they can't explain something, Newton invoked God saying the Creator must step in from time to time to stabilize the orbits. I'm not sure how he would explain why a perfect God, would create something so imperfect that He would have to keep fixing it all the time.

In any event, what we do know is that the future will undoubtedly be just as violent and chaotic as the past. We know that our sun will eventually (approximately five billion years from now) go Super Nova incinerating the inner planets, including Earth, if indeed our little rocky home is still around to be incinerated.

Some models predict orbital instability could cause the orbits of Mercury and Venus to cross, or that the orbits could be perturbed enough for planets to crash into each other or at least get close enough to cause all manner of havoc well before the sun explodes.

Given enough time, crazy things can, and will happen in the universe, and, whatever happens, it does not end well for Earth.

My reference for this column is the article "Our Wild Wild Solar System" from the July 2013 issue of National Geographic.