The Reflected Light of Exoplanets
When you imagine the Earth, perhaps something like the picture on the right pops into your head. A shining blue marble hanging in the inky darkness of space. Our world, highlighted in sunlight reflecting off its atmosphere, clouds and surface.
As the sun's light is reflected, it passes through parts of the planet's atmosphere where it gets imprinted with information about the atoms and molecules that make up the air. The total amount of light reflected depends on the properties of the atmosphere, clouds and surface. The total amount of reflection also changes with planetary phase with rainbows causing increased reflection at crescent phases and ocean glint at gibbous phases. Studying the reflected light of a planet can provide a wealth of information.
Hot Jupiters and Warm Neptunes
Measuring the reflected light of an exoplanet is not an easy task. Stars are much brighter than planets and can completely wash out their light. 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. Most of these aren't very reflective but there are a few exceptions. It's these exceptions that I observe and study to find out 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 neighbor, Proxima Centauri, hosts a planet that could be very much like our own; Proxima b. With upcoming Extremely Large Telescopes we may be to see for ourselves what this world is like. We would use 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. This technique is known as molecule mapping. Again, the noise can disguise the planet's spectrum so a cross correlation may be required to extract the information. We could use this technique on Proxima b using HARMONI on the ELT to learn more about its atmosphere including potentially detecting the presence of carbon dioxide and water.
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 – like molecule mapping but without the cross-correlation – meaning it can measure the brightness of a planet over its orbit. Over a planet’s orbit its phase (e.g. crescent, gibbous) changes. By studying the brightness of the planet at different phases we can detect scattering phenomena caused by oceans and rainbows on these worlds. This will tell us if the planet has an ocean and water cycle - key for life.
More Information
Want to learn more about how to detect rainbows on exoplanets? Check out Vaughan et al. 2023