Picking out the atmospheric constituents of a distant exoplanet requires the knowledge of a very simple natural phenomenon: Shadows.
You must've seen your shadow. In fact, when you were bored, you must've bent your fingers into different angles and thrown shadows of animal contours on your wall. Shadows happen because you're posing an obstruction in the path of light.
Photons that have arrived onto the wall's surface scatter off in different directions, and those that scatter into your eyeballs send a signal to your brain about the wall's colour and texture. Photons that never reached the wall in the first place draw a contrast by exposing the wall in the shadow-area to be dark.
Here, your hand was an opaque object. The wall was an opaque object. You could see them,. they obstructed your view. But what about things you can't see? Do they throw up shadows, too? Well, sort of.
Say you take a tank of pure molecular oxygen, maintained at atmospheric pressure and temperature (i.e., natural conditions), and place it against a wall. If you shined some light at it, would you see a shadow? Nope.
Herein lies the rub! It's easy to forget that there's no shadow only because our eyes can only perceive light in what's called the visible range: if the wavelength of light, or radiation, is between 400 and 700 nanometers (ns). Given the breadth of the electromagnetic spectrum, that's an awfully short range.
To overcome this, let's shine some light that's not just visible light but light that contains radiation of all wavelengths in the electromagnetic spectrum. Then, let's also set up equipment that's capable of "seeing" light at different wavelengths and not just in the visible range.
There’s more than what meets the eye!
Shine some light at the oxygen tank now and you'd see that all kinds of shadowing are at work! For one, there's going to be a prominent shadow around the ultraviolet (UV) band of the spectrum. There's also going to be a shadow toward the outer edge of the visible range and one in the near-infrared (near-IR) region.
While oxygen itself is transparent as a diffuse gas, these shadows are thrown because the molecules of oxygen in the tank are absorbing some light (like the wall scattered some light) at specific wavelengths while letting the light of other wavelengths pass.
Where does this absorbed light go? The answer lies with the molecular bonds, which are never perfectly still. Some bonds continuously vibrate at some frequency, some others rotate about their axes, and even some others do both.
When these bonds are irradiated with light, the photons knock into the electrons that comprise the bonds and excite them, make them more restless. This leads to more furtive vibrations and rotations.
However, there's a limit to how much the bonds may vibrate or rotate, and at some point may even snap and break off. Because of such limits, only some energy is removed from the knocking photons, creating a shadow at that corresponding wavelength.
Anyway, the shadows in the UV band are called the Hopfield bands (67-100 nm), the Schumann-Runge continuum (135-176 nm), and the Schumann-Runge bands (176-192.6 nm). In further fact, if light is shined at a "mass" of any gas anywhere in this universe, the presence of these shadows will indicate the gas is molecular oxygen.
Molecular oxygen's absorption spectrum
In other words, this is molecular oxygen's spectroscopic signature.
And because it's a signature, because it's so unique, it's used widely in chemistry and, of late, in astronomy to detect a sample's oxygen content.
The technique is not characteristic of only oxygen: All other elements in the period table have a unique spectroscopic signature. If the shadows appear in the patterns they're supposed to, then scientists can point their fingers and identify what's contained and, if they're careful enough, where.
It doesn't matter how big or small the sample is either. A small bunch of oxygen molecules, such as a million of them, should yield a partial, if not a complete, analytical picture for study. However, because of the small scale, more sensitive instruments will be required to study the outbound spectrum.
Sometimes, the spectrum can be inundated by background light. Imagine you're studying a small sample of oxygen molecules, and you're shining light at it that's incredibly blinding. Because of the relatively fewer number of oxygen molecules, the shadow is going to be extremely insignificant. The only way around will be to reduce the intensity of the light or use detectors with a very high spectral resolution.
Let there be... light?!
In space, there are copious and, fortunately, pertinent examples of this happening. When a planet moves in the foreground of a star, then the star's radiation works like the light we're shining, and the planet's atmosphere becomes the set of atoms and molecules that're going to shadow the radiation. Humans can study these "shadow-spectra" using space telescopes or powerful ground-based ones.
However, because the planet's likelier to be closer to its star than to Earth (if it's an exoplanet), it's going to be doused in starlight. I mean, knowing how big stars on average are and how big planets on average are, even a sufficiently separated star-and-planet system could have a planet that's a laughably small speck in the foreground of the star.
A shot of Venus (the bigger black dot) as it transited against the Sun on June 6, 2012. Photo: K. Ananthan
Imagine, then, the amount of radiation that must be bearing down upon the atoms and molecules in the planet's atmosphere. Forget the Hubble Space Telescope (HST) and even the powerful upcoming James Webb Space Telescope (JWST), we're going to need something that's either awfully futuristic or multiple transits of the planet-against-star to spectrally resolve the planet's atmosphere.
The latter technique's what is being currently employed. When a planet is spotted against a star, it's tracked by something like the HST for ten or tens of hours until it's atmosphere's shadows reveal its composition.
But what if the star was smaller? Two scientists have gone a step ahead with this and asked, "How about the light from white dwarfs?"
Let there be... less light!
When a star less than eight times the mass of the Sun runs out of hydrogen, it blows away its outer shell of gases in a supernova explosion while what’s left - the core - consists of carbon and oxygen as a result of helium-fusion. If the temperature of this core doesn’t manage to reach 1 billion kelvin (K), carbon fusion becomes ruled out and the core just sits in space where the star once was, slowly cooling off. This is called a white dwarf.
(In fact, like in J.R.R. Tolkien’s Middle Earth epic LotR, the only “inferior” class of dwarf remnants are called brown dwarfs. Of course, this could easily have to do with colour, but having recently watched ‘The Hobbit’, Gandalf the White and Radagast the Brown came to mind.)
In this Hertzsprung-Russell diagram, the absolute magnitude's on the y-axis and the type of radiation on the x. The region marked out as 'VII' is occupied by white dwarfs and that marked out as '0', by hypergiants. (Image: Wikimedia Commons)
A white dwarf gives off a much fainter glow than a star would - big or small. This is because almost all the light is just heat, and a white dwarf can take billions of years to cool off to become a black dwarf.
And if a planet passes in the foreground of a white dwarf, then it’s going to be bathed in less strong, but good enough, light. In fact, the two scientists, Dan Maoz and Abraham Loeb, have also calculated that oxygen-content in such a planet’s atmosphere can be figured out in in 5-6 hours - with about 160 two-minute trackings - if the telescope looking at them is the JWST.
That’s convenient, and gives the JWST yet another purpose in life. NASA plans to launch it in 2018.
(The calculations are fairly simple: Calculate the emission spectrum of the white dwarf, calculate the depth of the planet’s atmosphere and the planet’s size, find out how much of the white dwarf is occulted during each transit, and off you go!)
Dancing across a wall
So, why are they going after oxygen? As Maoz and Loeb write in their research paper: Oxygen is “The most prominent bio-marker in the present-day terrestrial atmosphere...”. A bio-marker is an indication of the possible presence of biological activities.
And atmospheric oxygen, which is molecular oxygen O2, is an important bio-marker because it is produced - on Earth - almost exclusively through photosynthesis. In fact, if life on Earth were to be wiped out, all the O2 would disappear in a million years.
So, as we reach out into the universe in search of other lifeforms, we’re still only using nature’s simplest techniques - albeit at advanced scales and tortuous precision - plus a few billion dollars. And the next time you make a shadow dance across a wall, think of what robot could already be doing it in outer space!