The Reflected Light of Exoplanets
When we think of the Earth, we don't think of it in terms of the heat it emits, nor do we think about the transparency of its atmosphere. We think of a gleaming blue marble gently illuminated by the light of the sun - we see our world in reflected light.
Exoplanets, planets that orbit stars other than the sun, also reflect light. By measuring their reflected light as a function of wavelength, we can measure properties such as the chemical composition of their atmospheres or the type of surface/clouds they have. If we measure the reflected light as a function of planetary phase (planets also have phases like the moon) we could also measure certain 'scattering phenomena', like rainbows and ocean glints, which can tell us if there is liquid water on the planet!
Hot Jupiters and Warm Neptunes
A lot can be learned by measuring the reflected light of an exoplanet, but it is not an easy task. Stars are much brighter than planets and can completely wash out their light so; to learn more, we have to separate out the star light from the planet’s light.
The method I primarily use for this task is called High Resolution Cross-Correlation Spectroscopy (HRCCS). Since the planet is moving much faster than the star or the Earth (our own planet’s atmosphere imprints things on the planet’s spectrum too) its spectrum is Doppler shifting a lot more. At high spectral resolution – where individual spectral lines are resolved – we can use this to separate out the planet's spectrum. Unfortunately, we cannot measure the spectrum perfectly, there will always be noise. This noise can completely disguise the weak planet’s spectrum. To counter this, we use a cross-correlation which can provide information on its spectrum even though we can't see it directly.
Today we can use this technique on the closest-in and largest planets and while most of these aren't very reflective, there are a few exceptions. It's these exceptions that I observe to discover what makes these worlds so shiny when others like them are not.
Towards Rocky Worlds
While giant planets are fascinating in their own way, the Trekkie in me wants to seek out new life and new civilizations - to look for worlds like our own. This may not be so far beyond the horizon. Our nearest neighbour, Proxima Centauri, hosts a planet that could be very much like our own; Proxima b. With upcoming Extremely Large Telescopes we may be able to see for ourselves what this world is like. For this we would use molecule mapping, a technique similar to HRCCS, only this time, instead of using the Doppler shift of the planet, we will use the spatial separation between it and its star to isolate the planet’s spectrum. Again, the noise can disguise the planet's spectrum so a cross correlation may be required to extract the information. With HARMONI on the ELT, we could use this technique to learn more about Proxima b’s atmosphere including potentially detecting the presence of methane.
If the planet can instead be directly imaged (the cross-correlation isn't needed), then we can learn some additional interesting things about the planet. The Habitable Worlds Observatory will be characterizing planets using direct imaging and could measure the brightness of a planet at different phases during its orbit. These measurements can be used to detect scattering phenomena caused by oceans and rainbows on these worlds. This will tell us if the planet has an ocean and water cycle which is key for life as we know it.
More Information
Want to learn more about how to detect rainbows on exoplanets? Check out Vaughan et al. 2023
Or how to characterise Proxima b with HARMONI on the ELT? Check out Vaughan et al. 2024