Norm Murray first noticed something was amiss in the solar system while peering through a telescope in grade three. He was using a filter to look at the Sun, and he noticed the seemingly uniform sun actually has spots on it.
He didn’t know it at the time, but the discovery of sunspots by Galileo 350 years earlier had shaken the prevailing notion of a perfect, unchanging cosmos. Today, Murray is helping to explode the 20th century’s standard picture of a placid, clockwork solar system through his research at the Canadian Institute for Theoretical Astrophysics at U of T.
In Murray’s universe, giant planets play tug-of-war with comets. Sometimes they lose their grip and fling one into the sun or into deep space. Even the orbits of the planets themselves aren’t fixed, but shrink and grow over time. In some solar systems, planets move so far inwards that they end up crashing into their parent star.
There is evidence that such titanic struggles took place in our own solar system. One sign is the group of comets at the outer edge of the solar system that orbit in precise harmony with Neptune’s orbit. Murray says the best explanation for this is that Neptune collected them into this orbit one by one while it was migrating outwards from the sun.
But what caused Neptune to migrate? Moving planets away from the Sun is no small feat. And it now looks as though the other gas giants migrated too—including Jupiter, which is three hundred times more massive than the Earth. What could be tossing planets around like so many ping-pong balls?
Ejections
Murray explains by going back to the beginning of the solar system, 5 billion years ago. Back then, there were a lot more comets around, perhaps three hundred Earth-masses’ worth. And they didn’t reside at the edge of the solar system the way most do today. They began life much closer in, between the solar system’s biggest giants, Jupiter and Saturn.
“What happens is that most of the bodies you plunk down in there are on unstable orbits,” Murray said.
“The orbit of the comet gets tickled a little bit by Jupiter and Saturn. And it’s tickled in such a way that its orbit is, in fact, chaotic.”
Eventually, Jupiter ejected most of these objects from the solar system. As a result, Jupiter’s own orbit changed.
“When Jupiter was ejecting these bodies, it had to put energy into the comets, which means that Jupiter had to lose energy.” This caused Jupiter to move inward towards the sun.
“The other giant planets were trying to eject comets as well, but what they mostly did was push them down to Jupiter, and then Jupiter ejected them. So the other giant planets ended up moving outward.”
A similar effect explains the source of the asteroids that strike the Earth every now and then. Astronomers have long known that there must be some outside supply of these asteroids, because if there weren’t, the Earth would have collided with the existing ones long ago. Asteroids on the inner edge of the asteroid belt between Jupiter and Mars get pushed into chaotic orbits by Mars’ gravity. Over time, their orbits can change enough that they cross Earth’s orbit, “and then you’ve got a near-Earth asteroid, said Murray.”
Like a boat in water
Theories like Murray’s may also explain some puzzling features of newly-discovered solar systems. One mystery is why so many of these planets orbit so close to the sun.
Astronomers are pretty sure that planets can’t be created so close to the sun, so something must be pushing them inward after they form. The comet ejection theory is one explanation, but Murray thinks most planets migrate inward by another mechanism. The key, he says, is the giant gas disk from which planets form.
Astronomers once thought the gas disks played no role after the planets had formed. But recent work is changing that view.
“The gas disk has a mass that’s bigger than the planets that form out of it. So whatever it does, the planets go along for the ride,” Murray said.
Murray compares the effect to that of trying to push a boat through water.
“If you put a boat in the water and try to push it through the water, there’s a drag that you have to overcome,” Murray said.
This isn’t simply due to friction between the boat and the water, Murray says.
“What happens is the boat launches a wake, so there’s a wave that comes off the boat that travels away from the boat. And that wave, of course, carries energy with it. And that energy came out of the boat’s motion.”
“The same thing’s happening with a planet in orbit around a star with a gas disk. The planet launches waves in the gas disk, not from direct physical contact but from a gravitational force that it exerts on the gas. So there’s a gravitational force that exerts a force on the gas and that launches a wave in the disk that will propagate away from the planet.”
According to Newton’s third law, for every action there must be an equal and opposite reaction. By launching a wave in one direction, the planet gets kicked in the opposite direction.
“So it can move the planet inward if the wave propagates out, or it can push the planet outward if the wave propagates in,” he said.
A much more dynamic picture of solar systems has emerged from these theories. Murray is already thinking of other ways that orbits could be altered over time. Astronomers had best get used to change in the solar system.
