Are we alone in the universe? How can we discover if we share the universe with other intelligent beings? These questions have long pricked the human imagination.
We are now searching our own solar system for basic biological lifeforms beyond Earth and the US-based Search for Extraterrestrial Intelligence (SETI) programme listens to the wide universe beyond our solar system for, mainly, radio signals that might betray intelligent life out there.
SETI has also calculated how deep into space radio signals from intelligent life on Earth can travel into space to possibly be detected by alien intelligence elsewhere.
To date, SETI has detected no incoming signals to Earth of intelligent alien life elsewhere in the universe. Another fruitful exercise it has pursued is to ponder the capacity of alien life beyond our galaxy to detect intelligent life on Earth. This question was expertly explored by astronomer Phil Plait in Scientific American recently.
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Astronomers have so far discovered nearly 6,000 planets that orbit stars other than our sun. This figure extrapolates to hundreds of billions of planets in our Milky Way galaxy, which means very many habitable planets must exist if even a small percentage of these planets are Earth-like.
And Plait reminds us that, given the right conditions, it seems life can arise easily. Life arose on Earth as soon as temperatures cooled enough for oceans to form. This is why scientists believe it’s reasonable to expect that life exists on other planets beyond Earth.
Before critically thinking about detecting intelligent aliens by scanning the universe for signals betraying their existence, we must first make some assumptions.
First, it seems sensible to assume that any intelligent aliens are as technologically developed as we are and, secondly, they would almost certainly use radio waves for communications.
These waves pass at the speed of light through interstellar space. Thirdly, we must also take into account that our galaxy, not to mention the wider universe, is almost unimaginably huge – the closest star to our sun in our galaxy is Proxima Centauri, 4.25 light years away, the next nearest star, Barnard’s Star, is 5.96 and Sirius is 8.7 light years away.
SETI monitors radio waves incoming from space in the hope of detecting signals from intelligent aliens and also broadcasts “we are here” radio messages into the universe. Aliens searching for signals of intelligent life elsewhere in the universe would surely monitor incoming radio-wave transmissions also. These higher-power transmissions can be detected 12,000 light years from Earth and several billion stars lie within that range.
We also send radio messages to the planetary probes we sent into deep space. These broadcasts can be detected up to 65 light years away, within the range of thousands of stars. Emissions from our radio/TV stations and from cell phone towers also leak into space, but these signals can only be detected up to four light years away.
While radio waves are clearly the method of choice when searching for incoming signals of intelligent alien life, or broadcasting our existence to the wide universe, other detectable signatures of life also exist.
Civilisations affect the atmospheres of their planets, for example emitting greenhouse gases into the air from industry. The consequent alteration of atmospheric composition is detectable from space. Certain wavelengths of light are absorbed by molecules in the atmosphere creating a telltale pattern that can be measured.
SETI investigators estimate that the signature of the increased nitrogen dioxide levels in our atmosphere, released by burning fossil fuels, could be detected up to 5.7 light years away – within the range of Proxima Centauri.
Of course, there is always the possibility that we, or a distant alien civilisation, would build a telescope enormously bigger than any current telescope on Earth and capable of crisply imaging a planet from a vast distance. That would allow aliens a great distance away to resolve our cities in sufficient detail to see them light up at night for example, thereby revealing our existence.
We have been doing modern astronomy on Earth for little more than a century, and, as Plait points out, we continue to build bigger and better telescopes and to discover new ways of investigating the universe. Who knows what our capacity to probe the universe will be in another 100 years.
William Reville is an emeritus professor of biochemistry at UCC
