Hello? Is anybody out there?
Several exoplanets have been identified and quite a few are in the habitable zones of stars. Some of them might just be harbouring life
Astronomers refer to the “Goldilocks Principle”. In the fairy tale, the little girl discovered that some of the food in the bears’ house was too hot, some too cold. But some of it was just the right temperature.
Similarly, solar systems have ideal habitable, or Goldilocks zones where temperatures are “just right” for liquid water. Since life on Earth (the only kind we know exists for sure) evolved on a rocky planet with liquid water, it’s thought that similar planets, with similar atmospheres, at the right distance from the parent star have a high probability of harbouring life.
In 1961, astronomer Frank Drake formulated an “equation” estimating the probability of contacting intelligent life. His calculation estimates the number of stars, which may possess planets capable of supporting life. Then it estimates the fraction of such planets, which may actually develop life, and then the small fraction of such life-bearing planets where intelligent civilisations may develop. Finally, it estimates how many such civilisations will develop communication systems with detectible signals.
Drake’s equation was written more or less as provocation. But the number of stars is very, very large, effectively tending towards infinity. Infinity (or a very large number) divided by a small number is still infinity (or a very large number). The Drake Equation thus suggests, there could be a very large number of communicating civilisations, even if a very small fraction of stars develop life.
Drake’s logic inspired the Search for Extra Terrestrial Civilisation (Seti) project. Seti monitors radio waves for signals. It is a collaborative project; people donate computing time to process radio signals for anomalies. There was recently a lot of excitement when some “signals” were picked up in Russia. Sadly, this was a false alarm.
The counterargument to Drake’s Equation is embedded in a simple question asked by Enrico Fermi: “So, where is everybody?” If so many civilisations exist, why have they not sent unambiguous messages, or come visiting? There are many possible explanations for the silence.
One is that there is no life beyond Earth. Another is that the nearest civilisation is so distant that we haven’t picked up signals yet. Or, such civilisations are so advanced that we have not even realised they were “chatting”. Or, most popularly, extraterrestrials have indeed contacted Earth and governments have conspired to conceal it.
The last few years have removed one particular element of doubt from the Drake Equation. Exoplanets exist. Many have been identified and quite a few are in habitable zones of stars. The Kepler Space Telescope has discovered more than a thousand exoplanets.
Kepler went operational in 2009, launched into a heliocentric orbit, which trails a little behind the Earth. It has revolutionised our understanding of planetary formation and solar system behaviour. Kepler uses “transit”, finding planets by detecting changes in starlight as planets pass in front of their stars. Exoplanets are also detected by studying stellar “wobbles”. Star motion is disturbed by the gravitational effect of orbiting planets and instruments can pick up velocity changes of one centimetre per second at large distances. This data can be used to infer planetary radius, the length of year, distance from the star, composition, temperature, etc.
Most of the exoplanets detected are gas giants like Jupiter and Saturn. Surprisingly, some of those are within habitable zones. There is an exoplanet database (exoplanets.org) and many websites list the more unusual exoplanets.
A 2013 paper, “Prevalence of Earthsize planets orbiting sun-like stars” from University of California, Berkeley, identified 10 Earth-like planets within habitable zones and postulated that 22 per cent of sun-like stars (Type G minus) would have such rocky planets. It also suggested that the nearest such planet may be 12 light years away.
That was a pessimistic estimate. This August, the European Southern Observatory in La Silla, Chile, announced a rocky planet, in the Goldilocks zone of Proxima Centauri. As every geek knows, that is the star nearest our solar system and it’s only four light years away. It’s not known if this planet has an atmosphere. But just maybe, the Drake Equation will turn out to be more than provocation. The truth is out there!