Our asteroid belt, which is situated between Jupiter and Mars, has traditionally been seen as something of a nuisance. Every once in awhile one of these rocks dislodges itself and heads straight for Earth, what often results in a cataclysmic impact. But ironically, as a new study from the University of Colorado suggests, we may owe our very existence to these chunks of displaced rocks. And according to the researchers, our asteroid belt appears to be unique as far as these things go - what may be contributing to the dearth of life in the galaxy.
Astronomers and astrobiologists are increasingly coming to see asteroid belts as an important component to solar system composition, planet formation, and the emergence of life.
Despite the astronomical chaos produced by impact events, asteroids delivered water, organic compounds, and heavy elements to Earth - what are all crucial for the emerge of life. They were also likely responsible for the formation of our moon (which we know is crucial for seasonal stability), and even the introduction of life itself (via panspermia). Moreover, by virtue of their ability to cause mass extinctions, asteroids may have contributed to crucial periodic phases in the evolution of complex life - the rebooting of life such that intelligence had a chance to get started (what biologist Stephen J. Gould referred to as ‘punctuated equilibrium' phases).
And according to researchers Rebecca Martin (University of Colorado) and Mario Livio (Space Telescope Science Institute in Baltimore, Md.), not every solar system has an asteroid belt like ours - and not by a long shot. In fact, only 4% of all observed solar systems have an asteroid belt that sits past the so-called "snow line" - a celestial demarcation point that divides the inner solar system from the colder outlying regions where volatile materials such as water ice are far enough from the sun to remain intact.
The reason why our asteroid belt resides beyond the snow line is no mystery: Jupiter. And as many astronomers are now discovering, most solar systems have a giant planet that resides well inside the snow line - what may account for the rarity of asteroid belts like ours.
Soon after forming from the sun's primordial protoplanetary disk, the gas giant's tremendous gravity would have prevented nearby materials inside its orbit from coalescing and turning into a planet. Instead, Jupiter caused these materials to collide and break apart - creating fragmented rocks that settled into what eventually became the asteroid belt.
But not only that, the presence of Jupiter was, owing to its size, orbit, and time of formation, crucial to the composition of the asteroid belt itself - a structure that contains millions of rocks, metals, and bits of ice. Its orbit is such that it only gently perturbs the asteroid belt. This is important because if it ventured too close, or even through the asteroid belt, it would have scattered the asteroids (no asteroids, no life on planets in the habitable zone - as contradictory as that might sound). Similarly, if it was too far, the asteroid belt would have remained massive and dense - which would have resulted in far too many impact events (thus making evolution difficult, if not impossible).
And what's fascinating about Martin and Livio's analysis is their suggestion that every solar system has an asteroid belt at roughly the exact same location just beyond the snow line. What varies, however, is whether or not a solar system has a gas giant to mould its composition. The scientists theorize that, owing to Jupiter, our asteroid belt is 1% the size of its original mass - a kind of Goldilocks figure that may be a key factor to life emerging and prospering in the solar system.
Consequently, the researchers suggest that astrobiologists should concentrate their searches in those solar systems where a giant planet resides outside the snow line.
You can read the entire study at Monthly Notices of the Royal Astronomical Society.
Image: NASA/ESA/A. Feild, STScI.