Although we’re now searching for extraterrestrial life in other galaxies, there was once a time when we couldn’t even imagine looking outside our own solar system. In 1992, when two scientists discovered the first planet orbiting a star that wasn’t our sun, they probably had no idea that in the next quarter of a century the scientific community would have discovered over 3,500 new exoplanets in 2,700 solar systems. There was a lot of hope that Mars would be harbouring ‘little green men’ as it shared our ‘Goldilocks zone’, an area which is a certain distance from a star where scientists believe planets must be to have a chance of supporting life. Astronomers now have their sights set on much more distant galaxies and have developed a multitude of techniques for detecting life, whatever form it may take, on other planets.
During the heady days of the first exoplanet investigations, scientists imagined many sophisticated civilisations out there, waiting for humans to get their act together and develop the ability to communicate through space. It was simply a case of sending out a signal to let others know we were interested, and waiting for the call to join an alien utopia across the galaxy. One attempt at communication involved a plate, designed by Carl Sagan, which was placed on the Pioneer spacecrafts in the 1970s. Inscribed upon it was a diagram of a hydrogen atom, a diagram of the shuttle’s journey through space, and a picture of a naked man and woman in an attempt to communicate as much information about Earth and the human race as possible. This rudimentary and hard-to-decipher message was widely criticised, and decades later we are, unsurprisingly, still waiting on the first reply.
Scientists have now lowered their expectations of the complexity of life we hope to find and its enthusiasm for communication with us. Secondary signals, derived from the amount of light absorbed or reflected by planets, potentially indicating the presence of complex life or water are currently seen as our best bet for easily checking many planets. This form of planetary spectroscopy makes use of light waves from a star going through the atmosphere of a nearby planet. Certain chemicals absorb specific wavelengths of light, resulting in small gaps in the light spectrum that is reflected, indicating the presence of chemicals like oxygen, water, and methane. Finding these chemicals all together means a planet has a high chance of supporting life, and discovering the presence of chemicals that are produced by burning hydrocarbons could be an indicator of how technologically advanced this life may be.
Even more crucial for a living planet is finding gases in combinations that are uniquely produced by life forms. The presence of oxygen and methane together is highly suggestive of respiring organisms as, in normal circumstances, oxygen burns up methane, meaning that without life, methane in an atmosphere would be removed completely in a matter of decades.
The most probable medium in which life can begin is water, around which the search for life has revolved for many years now. A technique for spotting distant watery planets that appear as no more than a few pixels on a telescope display is being developed in one of NASA’s laboratories. Thousands of high-resolution pictures of Earth are turned into a single or a few pixels to create an ‘average pixel’ of what a watery planet would look like; this light is put through a noise filter to simulate the interface expected from an exoplanet that might be of interest to astronomers.
However, water is not the only crucial factor, as a rocky environment is also essential for life on exoplanets. Phosphorus in rocks must be washed into a body of water by rain to create the correct chemical soup for life to flourish in. If the water-to-rock ratio is too high then these crucial reactions will never happen. Similarly, if the planet is covered in too much water, the high pressure from the oceans condense the core of the planet, preventing it from melting and stopping magma from driving geological activity. Without this activity, the planet remains stagnant, still, and lifeless.
In the search for other planets with life, we only have one data point to use as a reference: Earth. Many scientists are now trying to think outside the bubble of our own atmosphere and postulate alternative chemicals that could create and support life. Our closest neighbours could be surviving on gas molecules that we haven’t discovered and completely passing us by. When we still have so much to discover, our search for extraterrestrial life seems daunting at the very least. Almost all astronomers believe that life is out there, but whether it’s willing to be found is another question altogether.
Izzy Sturt is studying for an MSc in Science Communication at Imperial College London
Banner image: The NGC 4414 Galaxy, The Hubble Heritage Team, Wikipedia