Many of the parameters in the Drake equation could only be guessed at…until now

Back in 1961, a small group of scientists met at the National Radio Astronomy Observatory in Greenbank, West Virginia, to discuss the search for extraterrestrial intelligence for the first time. The group was an eclectic mix including the astronomer Carl Sagan, the neuroscientist John Lilly who worked on Dolphin communication and the radio astronomer Frank Drake who organised the meeting.

Before the meeting, Drake wrote down all the factors that determine the likelihood of extraterrestrial intelligence elsewhere in the universe. These include the fraction of stars with planets, the average number of these planets that can potentially support life, the fraction of these that actually develop life and so on. He realised that multiplying these numbers together produced an important figure: the number of detectable civilisations in the galaxy.

Since then, Drake’s equation has become a famous rallying point in the search for extraterrestrial intelligence. But it has never been an accurate one. Drake was well aware at the time that many of the parameters in his equation were extremely hard to quantify and that has led some critics to say that the equation is little better than a guess or even of no use at all.

But in recent years, astronomers have begun to gather data that has a significant impact on some of the parameters in the Drake equation. In particular, NASA launched a space telescope called Kepler in 2009 that was designed to look for exoplanets orbiting other stars and produce an estimate of the proportion that have planets.

During its mission, Kepler has identified some 2000 exoplanet candidates, a huge increase on the 400 or so that were known before it was launched. What’s more, before Kepler, the known exoplanets were mainly Jupiter-sized, making it hard to estimate the number of Earth-like planets.

By contrast, most of the planets discovered by Kepler are Neptune-sized or smaller. Indeed, the Kepler data has led to a dramatic change in astronomers’ understanding of the likely number of Earth-like planets around other stars.

So an interesting question is how the new data changes the Drake equation. Today, we get an answer thanks to the work of Amri Wandel at The Hebrew University of Jerusalem in Israel, who has made a number of inferences based on the new data. “The recent results of the Kepler mission significantly reduce the uncertainty in the astronomical parameters of the Drake equation,” he says.

First, some background about the equation itself, which comes in a number of forms. The biotic Drake equation, which describes the number of planets with life, takes the following form:

Nb = R*.Fs. Fp. Fe. Nhz. Fb. Lb

Where Nb is the number of biotic planets in the Milky Way, R*is the rate of star birth, Fs is the fraction of stars suitable for the evolution of life, Fp is the fraction of stars that have planets, Fe the fraction of Earth-sized planets, Nhz is the number of such planets within the habitable zone , Fb is the probability of evolution of life and Lb is how long biotic life survives on average.

Some of these numbers are straightforward to estimate. For example, the rate of star formation, R*, is well known. In the Milky Way, it is about 10 stars per year. However, the fraction of stars suitable for the evolution of life, Fs, is less clear. Many astronomers assume that life is limited to stars similar to the Sun, in which case, Fs is about 0.1.

This is clearly a conservative estimate given that many stars seem to have habitable zones. Wandel points out that if red giants are included in this number then Fs is probably closer to 1.

The Kepler mission has significantly improved astronomers understanding of the next term Fp. The data indicates that most stars have planetary systems so Fp is probably about 1 as well.

The Kepler data also suggests that between 7 and 15 per cent of Sun-like stars have an Earth -sized planet in the habitable zone so Fe.Nhz is about 0.1. However, Wandel points out that if biotic life is not restricted to Earth-like planets but includes places like Jupiter’s moon Europa, then Fe.Nhz could be much closer to 1.

All that allows Wandel to extrapolate about the density of planets with life in the Milky Way. His conclusion, assuming that life often evolves on habitable planets, is that life-bearing planets could be remarkably close. “The extended analyses, updated by the Kepler results, suggests that our nearest biotic neighbor exoplanets may be as close as 10 light years,” he says.

And even if life is less likely, the chances are good that biotic planets like close by. “Even with a less optimistic estimate of the biotic probability, for example that biotic life evolves on one in a thousand suitable planets, our biotic neighbor planets may be expected within 100 light years,” he says.

But the existence of life is very different from the existence of an intelligent civilisation. And on this point, Wandel is less optimistic. Little if anything can be reliably said about the likelihood of life evolving to a civilised stage.

Nevertheless, Wandel says the new data suggests that we are unlikely to have any close neighbours of this type. “The distance to the nearest putative civilizations, even for optimistic values of the Drake parameters, is estimated to be thousands of light years,” he says.

That’s an interesting update on an equation that has fascinated people for decades and is likely to continue to do so. If life-bearing planets really do exist within 10 light years of here, the chances of seeing biomarkers, such as methane or oxygen, in their atmospheres are relatively good (provided, of course, that life there is anything like life here).

All that contributes to the growing sense among astrobiologists and others that humanity is close to detecting other lifeforms for the first time, perhaps even within the lifetimes of people who are alive today.

That’s a goal that is surely worth pursuing aggressively given that such a discovery would be one of the greatest in the history and future of science.

Ref: arxiv.org/abs/1412.1302 : On The Abundance Of Extraterrestrial Life After The Kepler Mission