Jupiter Migration Limits Mars’ Mass

Jupiter Migration Limits Mars’ Mass

“Drawing the short stick.” Playing “One of these things is not like the others”. Getting “black-balled.” Each of these concepts or games hinges on something unusual or unexpected. Mars plays a similar role as astronomers try to understand how the solar system formed.

The solar system’s four rocky planets—Mercury, Venus, Earth, and Mars—have masses (relative to the mass of Earth) of 0.06, 0.81, 1.00 and 0.11, respectively. Notice that Venus and Earth dwarf the other two. Mercury formed in the region close to the Sun where astronomers expect little material to contribute to planet formation, so its small mass is not surprising. Mars, on the other hand, should have a mass similar to Earth’s and Venus’ considering where it formed. Until recently, astronomers lacked a satisfactory explanation for this discrepancy between Mars’ actual mass and its expected mass. Then scientists discovered a mechanism by which Jupiter steals material from the region where Mars formed.1

In order for planets to grow, dust (and ice) must stick together in the nebula surrounding the forming Sun. As these planetesimals collect together and grow large enough, they gravitationally attract other material from the nebula and start growing even faster. Eventually, the planetesimals collapse to form a protoplanet and when the protoplanet exceeds the size of the Moon, it starts accumulating an extended atmosphere. While this basic process applies to all planets, there is a region around the Sun, called the “snow line,” where ice forms more easily. Planets formed beyond the snow line grow much more rapidly and turn into gas giants. Because Jupiter likely formed just beyond the snow line, it grew to its current size well before the other planets finished forming.

However, Jupiter did not simply remain in its formation orbit. Solar system formation models demonstrate that, as Jupiter continued to interact with the gas and dust orbiting the Sun, its orbit decreased in size until it was only 1.5 astronomical units (or AU, the Earth-Sun distance). With such a small orbit, it stole a large portion of the material that would otherwise have increased Mars’ size.

Such a small orbit for Jupiter would have also left Earth uninhabitable. However, Saturn also migrated inward as it grew and eventually reached an orbit where it went around the Sun twice for every three orbits of Jupiter. This arrangement caused both Jupiter and Saturn’s migration to switch directions, and they eventually ended up in their present orbits where they minimize the number of comets and asteroids that hit Earth. Not only do computer models presented in the research paper explain the Mars’ size and the present orbits of all the planets, it also properly accounts for the composition and location of the asteroid belt.  

In one sense, these models show how the behavior of the gas giants our solar system resembles those found around other stars, namely that Jupiter and Saturn migrated from where they formed. However, the migration inward (to clear out the inner solar system) and subsequent migration outward (where Jupiter and Saturn protect Earth from destructive-to-life impact events) reflects fine-tuning. Such fine-tuning fits comfortably in a creation model where a benevolent Designer carefully prepares a planet to support life.

  1. Kevin J. Walsh et al., “A Low Mass for Mars from Jupiter’s Early Gas-driven Migration,”Nature, published online June 5, 2011, doi:10.1038/nature10201.