Detecting Extraterrestrial Life with Light? Exoplanet Astronomy’s Revolutionary Method

At some point in our lives, we’ve all lain awake at night wondering if aliens are real. We’ve all watched a sci-fi movie about an alien invasion or rolled our eyes at a flashy “UFO SPOTTED” headline from a sketchy news website. But no matter what state-of-the-art information source the idea was raised from, we’ve all thought back to the quintessential “are we alone in this universe?” This question, typically dismissed as too futuristic to be truthfully answered in our lifetime and generally diminished to pure conspiracy, is actually not as impossible to answer as most people might think. And as advances are made in the field of exoplanet astronomy, we move one step closer to solving it. 

The field of exoplanet astronomy has boomed since the discovery of the first exoplanet orbiting a main sequence star (51 Pegasi) in 1995. Since then, astronomers have developed numerous methods for not only detecting exoplanets, but for characterizing and analyzing them as well. One of these methods is called transmission spectroscopy, which allows scientists to identify what molecules are in a planet's atmosphere using only the spectrum of light obtained when a planet transits and eclipses its star. The first spectroscopic exoplanet measurements were taken in the early 2000s, and since then, advancements in observational technology have only provided scientists with increasingly precise data.

So, how does transmission spectroscopy work? Essentially, when a planet transits its star, meaning it passes in front of it from Earth’s perspective, a small portion of the star’s light travels through the planet’s atmosphere and back towards telescopes on or orbiting Earth. The light is intercepted by exceptionally reflective mirrors on these telescopes that then direct the light to a spectrograph, which separates the beam into a prism of colors that astronomers can analyze. 

In the planet’s spectrum, gaps of light can be observed at different wavelengths. These are absorption lines, and they are formed because certain molecules in the planet’s atmosphere absorb starlight at specific wavelengths. Every molecule absorbs light at different wavelengths, providing a unique fingerprint for each molecule that scientists can read. Therefore, using the just the planet’s spectrum, astronomers can determine what molecules exist in its atmosphere.

There are three main types of transmission spectra: flat, featureless, and sloped. A flat spectrum indicates that there isn’t much absorption or scattering, which means the atmosphere is either cloudless or has a thick layer of high-up clouds that prevent detection of molecular absorption. A featureless spectrum shows a constant decline in a graph of amount of light vs. wavelength. This indicates that there is haze in the planet’s atmosphere which can scatter the light that passes through it. Finally, a sloped spectra, which has a gradual increase or decrease in a graph of amount of light vs. wavelength, suggests that there is molecular absorption (Jovian).

Exoplanet spectroscopy is key in helping scientists characterize exoplanets, as well as in the modern day search for extraterrestrial life. It allows astronomers to remotely detect the presence of biosignature gasses, which are gasses that indicate potential life. So, biosignature gasses on Earth like oxygen, methane, and N2O could signify the existence of life on other planets. There are also other gasses that could also suggest life such as methanethiol, which could be produced by bacteria. Spectroscopy can also help astronomers characterize planets, predict their physical features and atmospheric composition, and speculate as to how they formed and evolved. 

It’s clear that transmission spectroscopy is the most promising method in the future search for extraterrestrial life. Already, it has helped us analyze hundreds of exoplanets light years away. With current technology, it is still difficult to obtain data on potentially habitable exoplanets because they are much smaller than gas giants, which are much easier for telescopes to detect. However , there are a few projects in the works right now that could hopefully provide scientists with more data on potentially habitable exoplanets. The Nancy Grace Roman Space Telescope, set to launch in 2027, will carry a coronagraph that can block the glare from stars, enabling the discovery and direct imaging of smaller exoplanets. Also to look forward to is the European Space Agency’s Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), which is set to launch in 2029. This mission plans to observe around 1000 diverse exoplanets, including rocky and potentially habitable ones. The future of exoplanet spectroscopy and its potential for biosignature gas detection looks bright. But whether we do find life or not, it is undeniable: transmission spectroscopy is the most promising method for answering what has historically been deemed “unanswerable”.