NASA held a press conference this week to announce the discovery of 7 earth-sized planets orbiting a star named TRAPPIST-1 39 lightyears away. After much hype and speculation on the internet, many were expecting the announcement to affirm the existence of extra-terrestrial life, and though a lot of people are excited at the prospects of these new planets, questions with regards to whether they sustain life remain unanswered.
What we do know is that 3 of the 7 planets in the system fall within what astronomers call the “habitable zone” – the distance from the sun that is optimal for sustaining the proper temperature needed to nuture the basic requirements for harbouring life. Remember Goldilocks’ paradigm? Not too hot, not too cold. Although this news is exciting, what are the next steps researchers must take to determine if extraterrestrial life exists on these planets?
Part of the answer lies in the method used by NASA researchers to find the planets in the first place. The planets were found using the “transit method” of detection- which basically means observing a star through a powerful telescope and recording the “dips” in the amount of light produced by the star. These “dips” occur as a result of the planets passing directly in front of the star on their orbits, blocking out a portion of the light emitted by the star. Ryan Cloutier, a graduate student affiliated with the Institute for Research on Exoplanets, explains the process:
“As a function of time, you measure [a star’s] brightness. When the planet is not transiting, you see constant brightness. When you have a transit, you lose a little light, so you have this dip [in the measurement of light] which tells you the size of the planet.”
The same concept can be applied to detecting the atmosphere of a planet – using different “filters” on the lens of the telescope, researchers measure the object at different wavelengths of light.
“Detecting a planet is hard, detecting an atmosphere on a planet is REALLY hard.” Says Cloutier.
According to Dr. Nicholas Cowen of McGill University’s Space Institute, “If [exoplanets] do have atmospheres, we’ll want to know what they’re made of. That’s the realm of the James Webb Space Telescope. For exoplanets, we’re looking for atmospheres that are out of whack. For example, Earth has lots of oxygen and some methane, which couldn’t happen without life.”
What can observing an atmosphere tell us about the molecules on the planet? Ryan Cloutier explains; “If you look at [one particular wavelength] you get a little dip… when you look at [the planet on the other wavelength] suddenly the planet looks bigger, because its atmosphere is doing some absorbing [of certain wavelengths of light.]”
Studies on molecules have revealed that certain types of molecules have different “absorption signatures” – they trap a specific wavelength of light: “If [a planet] also has an atmosphere – the opacity – the amount of light that gets through, [sic] isn’t the same at all wavelengths like it is for a rock- it changes. Our atmosphere does a really good job at scattering blue light… but another wavelength of light- for example, red light, doesn’t get absorbed at this wavelength.” This means researchers don’t need to actually penetrate the outer atmospheres of the planet to be able to have a sense of what molecules are present- water, carbon, and other molecules which are the basis of life here on earth can be detected through careful observation and calculations. Comparing differences in the size of the dip on the graph across different wavelengths will reveal which molecules are present on the planet and which molecules are not.
“Assuming you’re looking for life that lives on the surface [of the planet] there are signatures of a biosphere,” says Cloutier. To elaborate on Dr. Cowen’s remarks about life, Cloutier explains;
“You don’t expect molecules like oxygen and ozone to be in the atmosphere unless there’s a biosphere. The only reason we have those molecules in our atmosphere is because we have a biosphere – trees taking in carbon dioxide and ‘breathing out’ oxygen- that’s the only way we can get oxygen in our atmosphere.”
Once researchers detect (and confirm) a molecule like oxygen or ozone exists in an exoplanet’s atmosphere, there would be a strong reason to believe that the molecules were being produced by a plant-like organism like a tree, for example (meaning – potential life).