This spring Jupiter and Venus shine close together in the evening sky. To naked-eye observers, the two planets appear as they did to our prehistoric ancestors—as bright stars that change slowly in position and barely in intensity.
The planets' apparent inertness belies their energetic histories. Mercury, Venus, and the other rocky planets formed by accretion. Small dust grains coalesced to make bigger grains, then pebbles, stones, boulders—all the way up to objects the size of Mars. After crossing that threshold—about 1/10 of Earth’s mass—rocky protoplanets ran out of local material to consume. To grow further, Mars-sized protoplanets had to collide with each other and merge.
That scenario comes from theory and simulation, but evidence exists in our solar system for impacts on the scale needed to complete Earth’s formation. The rotation axes of Venus and Uranus have been knocked far out of alignment with those of the other planets. Mars is distinctly asymmetric: Its southern hemisphere features high mountains and ancient impact craters; its northern hemisphere has been scoured flat by massive lava flows that followed an impact. And given that lunar and terrestrial crusts have identical isotopic compositions, the Moon almost certainly formed when a Mars-sized object smashed into Earth.
Mercury, too, appears to have suffered a violent impact. As new results from NASA's MESSENGER spacecraft have confirmed, the planet's core occupies a staggering 61% of the total volume. By contrast, Earth's inner and outer cores together amount to 16% of the total volume. According to prevailing theory, the way to make a core-heavy planet like Mercury is to start with a normally proportioned planet like Earth and smash it with a protoplanetary projectile that ablates most of the crust and mantle.
A song of ice and fire
In 2007 I interviewed Peter Grünberg after he and Albert Fert had been awarded the Nobel Prize in Physics for their independent discoveries of giant magnetoresistance. "When and why did you decided to become a physicist?" I asked him. "In high school," he replied. "I was fascinated that the motions of the planets could be explained by simple physics."
For young Grünberg, Newton's rarefied laws were enough to spark his interest in physics, but other students might need something more awe-inspiring—something, perhaps, like the formation of the planets.
The plot begins with a vast swirling doughnut-shaped cloud of gas and dust surrounding the young Sun. Rocky planets begin to form in the hot zone close to the Sun; the gas giants start farther out, beyond the point at which sunlight is sill strong enough to melt water. As the formation process plays out, protoplanets compete for material, they smash into each other, they change orbits, and they suffer cratering bombardments from asteroids—a world of strife not unlike the one inhabited by Tyrion Lannister (shown here) and the other characters in HBO's series Game of Thrones.
Eventually, far into the future, the Sun turns into a red giant and its puffed-out atmosphere engulfs and destroys all the planets. By then, humanity will have left the solar system and settled in a new one that future planetary scientists identified and selected from what I expect will be a catalog full of habitable worlds.
The continuing discovery of new exoplanets adds to our knowledge of planet formation, but because the new planets and suns are not like our own, they confound the quest to identify general principles, at least at first. Nevertheless, the challenging nature of planet formation should not deter us from evoking it to inspire the next generation of physicists.
Grünberg's and Fert's discovery led to a great leap forward in the capacity, compactness, and cost of data storage devices. Who knows what a student captivated by the solar system's tumultuous history might go on to invent.
