In the past, the amount of information that astronomers have been able to discern about an exoplanet has been limited. The exoplanet transit method looks for slight dips in a star’s brightness as a planet passes in front of the star. The regularity of the transit tells astronomers the distance of the planet from the star (one year for a planet as far away from their star as the Earth is from the sun). The amount of light blocked gives us the volume of the planet. Using the radial velocity method, astronomers look for gravitational effects a planet has on their host star in order to calculate the planet’s mass. By utilizing both these methods, the astronomer can conclude whether or not an exoplanet is terrestrial (rocky) and in the habitable zone.
While a planet that fits both criteria may certainly be capable of hosting alien life, the amount of unconsidered criteria is almost endless. For example, Mars and Venus are both terrestrial and in the habitable zone, but Mars’ lack of a magnetic field allowed the solar wind to chip away at its atmosphere, and a runaway greenhouse effect on Venus has raised its surface temperature enough to melt lead. To top it all off, much of the criteria for the emergence of life are unknown; not a single biologist could tell you how life started on Earth.
One cannot claim “aliens!” from the characteristics of the planet alone. The only sure way to deduce whether or not a planet has life is to look for direct observational evidence of organisms. While direct imaging of exoplanets is a long way off, organisms leave behind traces of their existence. Very recently, the precision of telescopes has improved enough to determine a new characteristic of exoplanets: their composition. This is transit spectroscopy. As a planet passes in front of its star, an unimaginably small amount of light from the star also passes through the atmosphere of the planet before reaching our telescopes. The patterns of this light tell the story of the substance that it passed through. When astronomers analyze the light from the star during an exoplanet transit, they see traces of elements that were “added on” by the atmosphere of the planet. This method is brand-new and is groundbreaking for astrobiologists.
Using the method of transit spectroscopy, astronomers can look for substances that signify life’s presence. When picking a substance to look for, the obvious choice is oxygen. However, finding oxygen in a planet’s atmosphere concludes nothing because oxygen can easily be present without being the byproduct of an alien plant breathing. In my opinion, the holy grail for astrobiologists, the discovery that would verify, or at least give very strong evidence for, the presence of organisms on an extrasolar planet would be the simultaneous presence of carbon dioxide and methane. Methane and carbon dioxide can be created by non-organic processes, but cannot remain the atmosphere together. Methane and carbon dioxide react and produce hydrogen and carbon monoxide. Therefore, the two molecules cannot share the same atmosphere for long periods of time. The only way a planet can have both in the atmosphere is if an organic process is resupplying the atmosphere with both carbon dioxide and methane. This is the case for the Earth. Both molecules are a byproduct of animal life. Animals breathe out carbon dioxide and fart methane. Microbes also create copious amounts of methane through a process called methanogenesis.
After a disappointing delay, NASA’s James Webb telescope is set to launch in March of 2021. It will have extremely advanced spectroscopic capabilities and will be searching for incompatible gases in exoplanet atmospheres. If an Earth-like exoplanet containing both CO2 and CH4 in its atmosphere were to be discovered, the New York Times headline would read “Are We Alone?”.