Odds on aliens

If there are other planets like Earth out there, at least one in three probably harbours life, say two physicists in Australia. If life can arise on planets unlike ours, then the odds on finding life are even more favourable.

Charles Lineweaver and Tamara Davis of the University of New South Wales in Sydney argue that, despite our sample of habitable planets being but one, we are not as ignorant about life elsewhere as it might seem.

We can glean important knowledge from the fact that life on Earth seems to have evolved very quickly, say the pair. According to the earliest fossil records, life took no more than about half a billion years to gain a foothold, once the planetary conditions were amenable. This time scale might actually have been much less – even instantaneous in geological terms.

This rapidity tells us that the probability of life developing on an Earth-like planet is high, Lineweaver and Davis reckon. If a gambler wins the lottery within the first three days of buying tickets, it is likely, although not certain, that the chance of winning is high. The fact that “we won soon after life became possible on Earth”, say the researchers, points to there having been a good chance of life developing.

Several unknowns might demolish the researchers’ statistical argument. For example, life may have to develop rapidly if it is to develop at all. Then we are back to knowing nothing from our sample of one. Or Earth may be more finely tuned to nurturing life than we think – a truly habitable planet may have to be not just similar to Earth but virtually identical.

It is also possible that life might appear on planets that are radically unlike Earth. In this case, the researchers’ optimistic conclusion represents a minimum probability, making life elsewhere even more likely.

The Drake equation
Lineweaver and Davis used the geological record of life on Earth to pin down a key unknown in a famous formula that calculates the likely fraction of stars in our galaxy on which life has appeared1.

Called the Drake Equation, the formula was devised by astronomer Frank Drake in 1961. It summarizes all the factors involved in the likelihood of our detecting signals broadcast by an intelligent alien civilization.

Lineweaver and Davis considered a simpler version of the equation, which asks merely what proportion of stars have evolved any kind of life, not necessarily intelligent. This question can be broken down into three parts: what fraction of stars have planetary systems, what fraction of those planetary systems contain a habitable planet, and on what fraction of those habitable worlds life has actually appeared.

“NASA’s Terrestrial Planet Finder will scan for Earth-like extra-solar planets”
In the past decade, astronomical observations of planets around stars other than our own sun have started to tell us about the first two terms in the equation. NASA has just announced a new programme called the Terrestrial Planet Finder (TPF), which will use Earth-based astronomy to scan for Earth-like planets outside our Solar System. The design of the TPF mission will be finalized in 2006.

But it is the mysterious third factor that Lineweaver and Davis have put a figure on – the fraction of habitable worlds on which life has actually appeared. Some scientists argue that it is extremely small; others contend that life is virtually inevitable on any potentially habitable world – that the fraction is very close to one.

For Earth-like planets that are older than about a billion years (Earth is about 4.5 billion years old), this fraction is indeed probably close to one, say the duo. They think there is a high probability that it is at least 0.33.

This doesn’t in itself mean that life elsewhere is very likely, however. Quite apart from the assumptions that the researchers have made, we don’t yet know the other terms in the equation.