Everyone loves a rainbow, and now it is thought it might be possible to detect them on worlds far away from Earth. Sophia Vaughan, PhD student in Astrophysics, is working on measuring the light of exoplanets to determine what’s in their atmospheres and what it would be like to live there. Here she comments on her work with an international collaboration on how the search for exo-rainbows could soon reveal an Earth-like planet.
Finding life on another planet is not an easy challenge. One of the first steps is to find a planet which is habitable - one which is the right temperature for liquid water, and hence our type of life, to exist. The temperature of the planet tells us if it is in the 'habitable zone', that is, where liquid water could exist on its surface, but detecting this liquid water is very difficult. On Earth, water droplets form rainbows, and oceans sparkle in the Sun, could we use these to reveal liquid water on an alien world?
How can we see an exo-rainbow?
We don’t see exo-rainbows in the same way as rainbows on Earth – there are no beautiful bands of colour – rather, if there are rainbows, the exoplanet will get a little brighter when viewed at the right phase (just like the moon, exoplanets also have phases). We can see the glint of an ocean in a similar way. Rainbows affect the brightness of the planet between its half and full (gibbous) phases, while ocean glint changes the brightness only at crescent phases. If we can measure the brightness increase, especially in polarised light where the change in brightness is larger, we could detect the presence of liquid water on an alien world! By measuring how bright the planet gets, and the colour it's brightest in, we can also tell if the rainbow is formed from water droplets or something else like the cool liquid methane found on Titan.
Directly imaging exoplanets
Measuring the brightness of an Earth-like exoplanet is hard. The problem is that exoplanets are much fainter than the star which they are right beside. It is like trying to measure the brightness of a firefly as it flies around a lighthouse. Just as it is easier to see the firefly if you cover the lighthouse with your hand, it is easier to see the planet if you cover the star with what is known as a coronagraph. Astronomers haven't been able to make these brightness measurements for Earth-like exoplanets in the habitable zone yet – but that may be possible soon.
The future
NASA has recently begun planning the Habitable Worlds Observatory which is likely to launch in the 2040s. It is being designed to do lots of incredible science including taking pictures of star systems to search for planets like Earth. These pictures can be used to measure the brightness of the exoplanet.
In February this year, I led a team at a Lorentz Center workshop that studied how the size of the coronagraph used by the Habitable Worlds Observatory would affect the number of star systems we could search for exo-rainbows and ocean glints. The coronagraph size matters because the star and planet appear closer together on the sky at the phases where we could see the rainbows and ocean glint. If they are too close, it could put the planet behind the coronagraph and block it from our view. After an exciting week working with a diverse international team on this project, we found that for a precious handful of systems, these special rainbow and ocean glint phases would not be blocked and so would be visible to our cameras. This may allow astronomers to see liquid water for the first time on an exoplanet!
I presented our findings to NASA’s Science with the Habitable Worlds Observatory and Beyond conference which brought together scientists, technologists, and others to discuss the most compelling science drivers for HabWorlds and work towards developing the necessary technology to enable them. Combined with biosignature gases in the planet’s atmosphere, a signal of liquid water would be the strongest indication of a habitable world beyond our Solar system, and it is thrilling to know we are now working to make the telescope that will capture it.
Chasing rainbows and ocean glints: inner working angle constraints for the Habitable Worlds Observatory, S Vaughan et al, MNRAS, 31 July 2023