How would you discover signs of life in a foreign world hundreds of light years from Earth?
One way would be to look for molecules in their atmosphere that indicate the presence of life. Sis, oxygen, methane, ozone, etc. removed. That would not prove that strangers live there, but it would certainly be interesting.
It is difficult to make such an observation, but nature delivers. As the planet’s orbit appears from Earth, we see the planet pass right in front of its star; what we call a transition. The fixed part of the planet blocks all the light, but the atmosphere of the planet lets some light through. This is important because molecules in that atmosphere absorb very specific light colors; this acts as a fingerprint allowing astronomers to identify the molecule.
Ozone, for example, absorbs light in the near ultraviolet, just bluer from 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 do not have telescopes that can do this with planets that do not yet revolve around stars, but we can test the idea much closer to home. Well, just above it.
If you stood on the moon, you would block the Earth from the sun. Some sunlight passes through the earth’s atmosphere, though … and this sounds familiar. We do not have a telescope on the moon that can do this, but that’s OK. The sunlight that passes through the Earth’s atmosphere hits the Moon and is then reflected back to us, so a telescope can be used here to look for it. Since we see in the UV, we need to be in space (because the Earth’s air absorbs it), so why not use a Hubble Space Telescope?
And that’s exactly what astronomers did*. They pointed the observatory to the moon on January 21, 2019 of the lunar flood, and (spoiler warning) they were finally able to discover ozone in the Earth’s atmosphere this way! But it was not easy.
For one thing, Hubble was not designed to track objects like the moon, so pointing the telescope was a pain … and even they were not 100% sure of where the scope was. The moon is also not a big reflector; it has darker and brighter features that make it difficult to compare one observation with another.
However, they were able to obtain the observations. What they did then was compare them to physical models of the Earth’s atmosphere. We know how the Earth’s air changes with altitude (such as pressure, density and temperature) as its composition (for example carbon dioxide and water vapor are lower in the atmosphere, while the ozone layer is much higher), so the way in which sunlight passing through the earth’s air can be modeled.
They created a model that includes things like O2 (the oxygen we inhale) and even the way in which the Earth’s air disperses light is called Rayleigh scattering (which makes solar stars red, which is also why lunar eclipses make the Moon red). They also made the models so that they could contain ozone absorption and also what it would look like if the earth had no ozone; in this 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 ozone, the spectrum would look a lot different. In other words, they saw the earth’s ozone absorbed sunlight reflected by the moon.
Remember, ozone is three oxygen atoms attached to each other, and is produced as molecular oxygen, O2 , is cut off by ultraviolet light from the sun, which then redirects ozone. O2 is in the air because of life; plants exhale it. That by detecting ozone the astronomers found evidence of life on Earth.
Intelligent life is, of course, another story.
This is an atmospheric observation. It shows that we can probably do this with exoplanets orbiting other stars. However, it is much more complicated (and, I will note, this kind of thing has been done before with both Hubble and ground-based telescopes, but not in the near-ultraviolet part of the spectrum where, in part, these observations were dien). We do not know how much ozone an alien world can have in its sky, if any, and it also depends on the type of star orbiting the planet and how closely it is orbiting it. There are many variables, however in principle it can at least be done. We now have proof of concept.
To do this really with other planets would mean using much larger telescopes – the light is fleeting – and it’s best if they’re in space, to avoid any problems involved with the air’s. earth that absorbs the colors you want to see from the target planet. Maybe in a decade or two such a thing will be possible.
After all, the first exoplanet was not even discovered until the early 1990s, and now less than 30 years later it is a thriving field of astronomy! What will we be able to achieve in the next 30 years if we look to other worlds?
*Specific STIS, the spectrograph of the space telescope, a camera I’ve been working on for several years, so I always love a chance to write about results!
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