Reasons to Believe

How Did Migration Affect Our Solar System?

Try to imagine the last collision you witnessed that improved the objects involved. As much as I like collisions, they almost always bring destruction. Similarly, consider what happens when a bowling ball moves through a field of tennis balls. Everything the bowling ball encounters gets scattered all over the place. Yet as scientists study the environment of the early solar system, they find that the collisions and bowling ball actually helped make Earth a home for life. Here’s how.

In 1995, astronomers discovered 51 Pegasi b, the first exoplanet orbiting a sunlike star. Aside from the historical nature of the discovery, the exoplanet’s characteristics caused quite a stir. With a mass at least half of Jupiter’s, 51 Pegasi b is clearly a gas giant. However, it orbits its host star with a period of four days at a distance of one-twentieth the Earth-Sun distance (1 astronomical unit, or AU)! Finding such a massive planet so close to its star made astronomers realize that planets can actually move from the location where they form. The discovery of hundreds of these “hot Jupiters” means that any realistic model must include planet migration. So how did migration affect our solar system?

The discovery of exoplanets made scientists reassess the final phases of planet formation. In our solar system, the terrestrial planets (Mercury, Venus, Earth, and Mars) orbit closer to the sun and the gas giants (Jupiter, Saturn, Uranus, and Neptune) are farther out. This arrangement arises because the ices necessary for rapid planet growth (required by the gas giants) only form outside a certain distance from the sun called the frost or snow line. Inside that distance, the sun provides enough radiation to prevent ice formation. Currently, the distance to the snow line is around 5 AU. However, the presence of gas and debris in the early solar system as well as the reduced output of the sun brought the snow line into a distance of 2.7 AU—roughly the orbit of the asteroid belt.

Cleaning Out the Debris

The most accepted model for the formation of the solar system, the so-called Nice model, starts with the gas giants forming in more tightly packed orbits (5.5–17 AU) than they currently occupy (5–30 AU). A large disk of ice and rock planetesimals (roughly 35 times Earth’s mass) orbited in the region from 17 to 35 AU. Gravitational encounters of the most distant gas giant would scatter these planetesimals inward while the gas giant moved outward. The inward moving planetesimals would then interact with the next gas giant until they encounter Jupiter. The enormous gravitational influence of Jupiter scatters the planetesimals into highly elliptical orbits or even ejects them from the solar system, causing Jupiter to migrate closer to the sun. The net effect of this model results in (1) the clearing of 99 percent of the planetesimal disk mass; (2) a small inward migration of Jupiter to its current orbit; (3) the outward migration of Saturn, Neptune, and Uranus to their current orbits; (4) Uranus and Neptune swapping orbits; and (5) the possible ejection of a hypothetical fifth gas giant.

lhborbitsImage: Orbits of gas giants before (left), during (middle), and after (right) migration.
Image credit: Wikimedia Commons/AstroMark

The first and second effects figure prominently in Earth’s capacity to support life. Because the migration in the Nice model clears so much mass from the solar system, the rate of asteroids and comets hitting Earth drops by roughly a factor of 1,000. Consider the consequences of an impact rate this much larger.

Sixty-six million years ago, a six-mile-wide object slammed into Earth near the Yucatán Peninsula. The impact, the ejected debris raining back onto Earth, and the ensuing climate change wiped out three-fourths of all animal life, including the dinosaurs, from the planet. Scientists estimate that such impacts currently happen every 50–100 million years. Had the migration of the giant planets not reduced this rate by a factor of 1,000, these events would happen every 50,000–100,000 years. In other words, since humanity arrived on Earth roughly 100,000 to 150,000 years ago, one would expect two or three of these extinction level impacts just within that time frame. Given that it seems to take something on the order of 1 million years to recover from just one of these extinction events, an impact rate 1,000 times higher would dramatically affect Earth’s capacity to support life!

Making the Moon

The moon plays an important role in Earth’s capacity to maintain oceans of liquid water. It minimizes the wobbling of Earth’s rotation axis. It provides an abundant supply of heat to Earth’s interior so that Earth keeps an active plate tectonic cycle. As mentioned earlier, the last step in the planet formation process involves a larger object growing by absorbing the remaining planetesimals in its neighborhood. In Earth’s case, the last major collision involved a Mars-sized object that led to the formation of the moon.

Consider what would happen if Mars collided with Earth. What an explosion! If the collision occurs under the right conditions, the iron core and heavier elements (like uranium and thorium) in Mars would incorporate into Earth’s core, while the mantle material from both bodies would mix. A significant fraction of that material would be flung into orbit around Earth and form into the moon. The collision obviously had a large impact on the moon, but it also affected Earth by making it more massive (by about 10 percent), enriching Earth’s iron core, adding radioactive elements that heat the planet, and speeding the rotation up so that one day lasted five to six hours. While the giant impact scenario explains much of the data from the Earth-Moon system, as scientists make more detailed models, they continue to find anomalies. One unexpected find is the similar composition of the moon and Earth since the original models show that the moon largely forms from the collider material. However, the model answers far more questions than its competitors.

An intriguing thought comes to mind when considering the other terrestrial planets in the solar system. Mars does have two moons, Phobos and Deimos, but they have significantly less mass and little impact on conditions on the red planet. Recent work indicates that the two moons likely formed from an impact that generated a third, even larger moon. However, tidal interactions with Mars over a couple million years sent the larger moon crashing back into the planet’s surface, leaving only Phobos and Deimos.

The collision process that produced the moon should also occur on Venus and form debris disks there. However, Earth’s sister planet has no moon. Venus does show evidence of large impacts late in its history. The rotation axis of Venus is nearly sideways compared to the rest of the planets, and it rotates only once every 243 Earth days. It may be that Venus’s rotation results from interactions with other planets or solar tides. Or the last collisions may have knocked Venus’s rotation axis to its weird angle and slowed its rotation rate—yet these collisions did not produce a moon. Additionally, a collision may have produced a moon, only to have a later impact to cause that moon to crash back into Venus.

Surveying the terrestrial planets reveals that only Earth has a large moon. Additionally, Earth’s habitability may actually depend on having such a large moon!

A growing body of research indicates that events early in the history of the solar system led to the formation of Earth’s unusual moon while dramatically reducing the frequency of collisions experienced by our planet. Without these two critical events, Earth’s capacity to support life would have been radically diminished, maybe even to the point of being uninhabitable. Instead, Earth’s history shows a remarkable string of orchestrated events leading to a place that hosts an amazing diversity and abundance of life.

Subjects: Early Earth, Solar System Design