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How would you detect signs of life on an alien world hundreds of light years from Earth?
One way would be to look for molecules in its atmosphere that indicate the presence of life. Say, oxygen, methane, ozone, and so on. That wouldn’t prove that aliens live there, but it would certainly be interesting.
It’s difficult to make such an observation, but Nature provides. If the planet’s orbit appears edge-on from Earth we see the planet passing directly in front of its star; what we call a transit. The solid part of the planet blocks all the light, but the atmosphere of the planet allows some light through. That’s important, because molecules in that atmosphere absorb very specific colors of light; this acts like a fingerprint allowing astronomers to ID the molecule.
Ozone, for example, absorbs light in the near-ultraviolet, just bluer of what our eyes can see (and protects us from even shorter wavelength UV which can damage our skin and eyes), as well as light in the blue-green part of the spectrum, too.
We don’t have telescopes that can do quite this with planets orbiting distant stars just yet, but we can test the idea much closer to home. Well, just above it.
During a lunar eclipse, if you were standing on the Moon you’d see the Earth blocking the Sun. Some sunlight passes through Earth’s atmosphere, though… and this is sounding familiar. We don’t have a telescope on the Moon that can do this, but that’s OK. The sunlight passing through Earth’s atmosphere hits the Moon and then is reflected back to us, so a telescope back here can be used to look for it. Since we’re looking in the UV we need to be in space (because the Earth’s air absorbs it), so why not use Hubble Space Telescope?
And that’s just what astronomers did*. They point the observatory at the Moon during the 21 January 2019 lunar eclipse, and (spoiler alert) in the end they were able to detect ozone in Earth’s atmosphere this way! But it wasn’t easy.
For one thing, Hubble isn’t designed to track objects like the Moon, so pointing the telescope was a pain… and even then they weren’t 100% sure where the ‘scope was looking. Also, the Moon isn’t a great reflector; it has darker and lighter features that make it hard to compare one observation to another.
Still, they were able to get the observations. What they did then was compare them to physical models of the Earth’s atmosphere. We know the way the Earth’s air changes with height (like the pressure, density, and temperature) as well as the composition (for example, carbon dioxide and water vapor are lower in the atmosphere, while the ozone layer is much higher up), so the way sunlight passes through Earth’s air can be modeled.
They created a model that includes things like O2 (the oxygen we breathe) and even the way the Earth’s air scatters light, called Rayleigh scattering (which makes sunsets red, and is also why lunar eclipses turn the Moon red). They also made the models such that they could include ozone absorption as well as what it would look like if the Earth had no ozone; that way they can see if their observations matched.
What they found is pretty good: A drop in light with wavelengths longer than about 4500 Angstroms (in the blue part of the spectrum), as well as one with wavelengths shorter than about 3300 Angstroms, in the UV. These are exactly the places where ozone absorbs light, and the model clearly shows that if you leave out ozone the spectrum would look much different. In other words, they saw the Earth’s ozone absorbing sunlight reflected by the Moon.
Mind you, ozone is three oxygen atoms bound together, and is produced when molecular oxygen, O2 , is zapped by ultraviolet light from the Sun, which then rearranges itself ozone. O2 is in the air due to life; plants exhale it. So by detecting ozone the astronomers found evidence of life on Earth.
Intelligent life of course, is a different story.
This is a nifty observation. It shows that we might be able to do this with exoplanets orbiting other stars. It’s a lot more complicated, though (and, I’ll note, this sort of thing has been done before both with Hubble and with ground-based telescopes, but not in the near-ultraviolet part of the spectrum where, in part, these observations were done). We don’t know how much ozone an alien world might have in its air, if any, and it also depends on the kind of star the planet orbits and how closely it orbits. There are a lot of variables, but in principle it can be done, at least. We now have a proof of concept.
To do this for real with other planets would mean using much bigger telescopes — the light is faint — and it’s best if they’re in space, to avoid all the problems involved with Earth’s air absorbing the colors you want to see from the target planet. Perhaps in a decade or two such a thing will be possible.
After all, the first exoplanet wasn’t even found until the early 1990s, and now less than 30 years later it’s a flourishing field of astronomy! What else will be able to accomplish in the next 30 years as we gaze out at other worlds?
*Specifically STIS, the Space Telescope Imaging Spectrograph, a camera I worked on for several years, so I always love a chance to write about results from it!