One of the big questions we are trying to answer in exoplanet astronomy is: Just how common are habitable planets? We already know that exoplanets themselves are extremely common: Astronomers tell us there are on average 1.6 planets for every star in our galaxy, so there are more planets than stars out there.
That by itself is pretty amazing, but what we really want to know is, where’s the life? How common are planets that could potentially support life?
The answer so far has eluded us, and the news reports and studies coming out are a bit contradictory. For example, as this recording is made, in July, 2023, over the course of just three days two studies were announced. One is titled:
“Potentially Habitable Worlds are 100x times more common than We Thought” and a few day later: “Latest James Webb Space Telescope images hint that habitable planets are not common”
From these headlines, it’s pretty clear that we are still sorting out how common habitability is, which is understandable because we’ve only just started studying exoplanets.
What are astronomers looking for and how are they doing it?
Astronomers are using every tool at their disposal to understand the habitability question of exoplanets. Space Telescopes like Webb, Spitzer, Hubble, Kepler, TESS, and many others are setting their sites on finding and confirming exoplanets, arranging them by size and whether they exist in a habitable zone of their host star.
Ground-based telescopes are measuring the wobble in the spectra of stars to determine planet masses and orbital periods.
But the real breakthrough will be measuring exoplanets directly and observing their disks when possible. Right now, only the James Webb Space Telescope stands equipped to make those measurements. Its mirror has the resolution necessary to see the exoplanets but more importantly it has a spectrograph that can measure the starlight as it travels through any atmosphere that might exist and give hints as to what it’s made of.
The main things JWST can show us with respect to the habitability of a given exoplanet are: whether it has an atmosphere, if it has one, what are some of the major chemical components of that atmosphere, including water, and crucially: are there any biosignatures in the atmosphere?
Biosignatures are those elements that are produced by life and accumulate in an atmosphere to detectable levels. What are they? Astronomers are taking the approach that since we know what compounds life on Earth produces, let’s look for those first. What might the spectrum of Earth look like if it was transplanted to another, slightly different, Earth-like planet?
Things like an oxygen-rich atmosphere, certain methane compounds, ammonia and carbon monoxide are good things to see for a start. The trick for JWST will be getting enough orbits to make good measurements. Since time is precious on JWST, this means that looking for planets that go around their stars quickly, and a primary candidate is the fascinating system of TRAPPIST-1. Home to several worlds that could harbor life, are close to their star and orbit quicky.
So, let’s not be too hard on astronomers this early in the search. Right now our observations are preliminary and rough. As they improve, our understanding of whether habitable planets are common or not will only get better.