By Ian Steadman, Wired UK

If we want to find intelligent life elsewhere in the universe, it might be wise to look for stars with asteroid belts similar the one in our own Solar System.

According to the theory of punctuated equilibrium, evolution goes faster and further when life has to make rapid changes to survive new environments – and few things have as dramatic an effect on the environment as an asteroid impact. If humans evolved thanks to asteroid impacts, intelligent life might need an asteroid belt like our own to provide just the right number of periodic hits to spur evolution on. Only a fraction of current exoplanet systems have these characteristics, meaning places like our own Solar System – and intelligent aliens – might be less common than we previously thought.

[partner id="wireduk"]Astronomers Rebecca Martin of the University of Colorado in Boulder and Mario Livio of the Space Telescope Institute in Baltimore have hypothesised that the location of the Solar System's asteroid belt – between Jupiter and Mars – is not an accident, and actually necessary for life. As the Solar System formed, the gravitational forces between Jupiter and the Sun would have pulled and stretched clumps of dust and planetoids in the inner Solar System. The asteroid belt lies on the so-called "snow line" – fragile materials like ice will stay frozen further out, but closer in they will melt and fall apart.

During the formation of the Solar System, cold rock and ice coalesced into the planets as we know them. However, as Jupiter formed, it shifted in its orbit closer to the Sun just a little bit before stopping. The tidal forces at work between Jupiter and the Sun would have torn apart the material on the snow line, preventing a planet forming and leaving behind an asteroid belt – which today has a total mass only one percent of that which would have been there originally.

Those asteroids would have bombarded the inner Solar System – including Earth – and, in theory, provided the raw materials needed for life (like water) and also giving evolution a kickstart by drastically changing the early Earth's climate and environment. To check that this wasn't just something restricted to our Solar System, Martin and Livio looked at data from Nasa's Spitzer telescope, which has so far found infrared signals around 90 different stars which can indicate the presence of an asteroid belt. In every case, the belts were located exactly where Martin and Livio had predicted the snow line should be relative to each star's mass, supporting their snow line theory of asteroid belt formation.

If these are the circumstances which allow intelligent life to evolve somewhere, then it will make the task of finding aliens we can chat with a lot harder – few stars with exoplanets that we've found so far have the right setup of a dusty asteroid belt on the snow line with a gas giant parked just outside it.

If the gas giant has formed but not shifted in slightly, as Jupiter did, then the belt will become so full of large objects that the inner planets will be bombarded too frequently for life to fully take hold; if the gas giant continues to move inwards as it orbits, it won't just stop the belt turning into a planet – it'll suck everything of any serious size up and leave behind only minor fragments of space rock and dust, including any planets life could evolve on.

Martin and Livio then looked at 520 gas giants found orbiting other stars – in only 19 cases were they outside of where that star's snow line would be expected to be. That means fewer than four percent of exoplanet systems will have the right setup to support the evolution of advanced, intelligent life in accordance with the punctuated equilibrium theory.

Martin, the lead author of the research, published in Monthly Notices of the Royal Astronomical Society, writes: "Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet. Our study suggests that our Solar System may be rather special."

Source: Wired UK

Image: NASA/JPL-Caltech