Astronomers on Earth face a similar problem, except we can’t travel to the exoplanet or exosystem to conduct our research – they’re all too far away. The closest exoplanet found thus far is Proxima Centauri b,  at 4.24 light years away. Even if I strapped myself to the front of the fastest travelling spacecraft we have, the Parker Solar Probe (with a maximum speed of roughly 200 km/s), it would still take me over 6000 years to get there – much, much longer than the human lifespan.

So, how do astronomers find and characterize exoplanets that exist beyond our reach?

Finding exoplanets with signatures of life is partly having the right equipment and clever techniques, but also requires a bit of luck. We use light from these exosystems to research them; two detection methods dominate the search. One is the radial velocity (‘wobble’) method (or Doppler spectroscopy), in which the equal and opposite gravitational force exerted by orbiting planets on their host star causes the star to wobble, changing the colour of light (via the Doppler effect) we observe. Thus, a star’s wobble can tell us if an exostar has exoplanets, how many there are, and their size. The other is the transit method,  in which we record how much a star’s light dims when a planet passes (transits) the star in our line of sight and its periodicity.
 
Detecting evidence of life, intelligent or not, requires a different flavour of analysis. As light propagates through an exoplanet atmosphere, some wavelengths of light will be absorbed by chemicals and gases in the atmosphere (each gas absorbs light in a specific wavelength/colour). This leaves behind black gaps in the spectrum we detect, almost like a barcode. By scanning the atmospheric barcode of each exoplanet, we can determine whether it has the ingredients for life. Finding a barcode with indicators of methane and oxygen would be promising, but we also can’t rule out life that subsists on a different kind of biochemistry. (To learn more about light visit our earlier post – light)

So where are we in terms of answering the age-old question: “are we alone?” As of today (August 25, 2020), NASA has confirmed 4201 exoplanets and has 5481 awaiting confirmation; although, none of these bear the hallmarks of life. Despite this, I, an optimist, am still compelled to believe that extraterrestrial life probably exists out there. Given the almost incomprehensible size and age of the universe, it feels almost arrogant to assert that we are the only form of intelligence in an ever-expanding spacetime. It’s only a matter of time.