July 21, 2022

At only four light-years away, Proxima Centauri b is our closest known exoplanet neighbor.  Proxima b is a super Earth exoplanet that orbits a M-type star. Its mass is 1.27 Earths, it takes 11.2 days to complete one orbit of its star, and is 0.0485 AU from its star. Its discovery was announced in 2016.  In the habitable zone of its star, Proxima Centauri, Proxima b encounters bouts of extreme ultraviolet radiation hundreds of times greater than Earth does from the Sun. That radiation generates enough energy to strip away not just the lightest molecules — hydrogen — but also, over time, heavier elements such as oxygen and nitrogen.

Just because Proxima b’s orbit is in the habitable zone, which is the distance from its host star where liquid water could pool on a planet’s surface, doesn’t mean it’s habitable. It doesn’t take into account, for example, whether water actually exists on the planet, or whether an atmosphere could survive at that orbit. Atmospheres are also essential for life as we know it: Having the right atmosphere allows for climate regulation, the maintenance of a water-friendly surface pressure, shielding from hazardous space weather, and the housing of life’s chemical building blocks.

One of the main uses of the James Webb Space Telescope will be to study the atmospheres of exoplanets, to search for the building blocks of life elsewhere in the universe. But Webb is an infrared telescope. How is this good for studying exoplanets?

One method Webb will use for studying exoplanets is the transit method, which means it will look for dimming of the light from a star as its planet passes between us and the star. (Astronomers call this a "transit".) Collaboration with ground-based telescopes can help us measure the mass of the planets, via the radial velocity technique (i.e., measuring the stellar wobble produced by the gravitational tug of a planet), and then Webb will do spectroscopy of the planet's atmosphere.

Webb will also carry coronagraphs to enable direct imaging of exoplanets near bright stars. The image of an exoplanet would just be a spot, not a grand panorama, but by studying that spot, we can learn a great deal about it. That includes its color, differences between winter and summer, vegetation, rotation, weather...How is this done? The answer again is spectroscopy.

Spectroscopy is simply the science of measuring the intensity of light at different wavelengths. The graphical representations of these measurements are called spectra, and they are the key to unlocking the composition of exoplanet atmospheres. When a planet passes in front of a star, the starlight passes through the planet's atmosphere. If, for example, the planet has sodium in its atmosphere, the spectrum of the star, added to that of the planet, will have what we call an "absorption line" in the place in the spectra where would expect to see sodium (see graphic below). This is because different elements and molecules absorb light at characteristic energies; and this is how we know where in a spectrum we might expect to see the signature of sodium (or methane or water) if it is present.  Why is an infrared telescope key to characterizing the atmospheres of these exoplanets? The benefit of making infrared observations is that it is at infrared wavelengths that molecules in the atmospheres of exoplanets have the largest number of spectral features. The ultimate goal, of course, is to find a planet with a similar atmosphere to that of Earth.

Credit

NASA Visualization Technology Applications and Development (VTAD)

Source - NASA