Asteroid belts similar to the one around the Sun, but not ones that are much sparser or denser, might be crucial to the appearance of life on any Earth-like worlds. If they are, then inhabited planets like our own might be a rarity. That’s the conclusion of astronomers Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and Mario Livio of the Space Telescope Science Institute in Baltimore.
Asteroid impacts are thought to have played a role in getting terrestrial life off the ground by delivering water and organic materials to the young Earth. Later, they may have triggered bursts of evolution by challenging life to adapt under tough circumstances. The rise of mammals and, ultimately, of humans is partly the consequence of a run-in with a 10-kilometer-wide asteroid about 65 million years ago. But if the bombardment of Earth over the past four billion years or so had been too heavy or too light, life may have been wiped out or never had the chance to start.
Key to determining the size and mass of the Sun’s asteroid belt, and hence the level of bombardment suffered by the inner planets, is Jupiter. The gravitational influence of the biggest world in the solar system prevented nearby material inside its orbit from coalescing into another planet-sized body. But what Jupiter did next, in those early times just after the planets formed, was pivotal. Because of gravitational interactions with other objects in the solar system, it moved toward the Sun – its orbit migrated inward – but only a bit. Jupiter’s migration was just right to thin out the asteroid belt so that the Earth wouldn’t be pummelled too much in the eons to come. But the big planet stopped short of plowing through the belt and scattering so many asteroids that life on Earth couldn’t be nurtured by future encounters.
Martin and Livio started by assuming that asteroid belts around other stars will always be located around the ‘snow line’ – the distance from the central star at which it’s cool enough for volatile stuff like water ice to remain solid. Then they looked at infrared observations by the Spitzer Space Telecope of 90 stars around which warm dust has been detected, suggesting the presence of asteroid belts in the making. They found that the temperature of the dust matched that of the predicted snow line.
Next they turned to observations of 520 giant exoplanets. Of these they found that only 19 – a mere four percent – lay just outside the snow line, in relatively the same position as Jupiter is to the Sun. It seems that most giant planets migrate either too far or not enough to give rise to the kind of Goldilocks asteroid belt that is just right to foster life on inner, Earth-like worlds. While this may be bad news for astrobiologists it does at least point to the kind of planetary systems on which they should focus their attention.