Nikku Madhusudhan. 2019. Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects. Annual Review Astronomy and Astrophysics. 57:617-663. https://doi.org/10.1146/annurev-astro-081817-051846
As exoplanets orbit their parent star, they may pass – or transit – in front of that star as seen from Earth. As it transits, the exoplanet blocks out a fraction of the star’s light. This small change in the star’s light can be observed to detect exoplanets and tell us more about their orbits.
During a transit event, some of the light from the host star passes through the exoplanet’s atmosphere before reaching the telescope, and is attenuated at different levels at different wavelengths depending on the chemicals in the atmosphere. The transmission spectrum is the resulting attenuation as a function of wavelength. This contains information about the planet’s atmospheric properties, such as composition, temperature structure, and presence of clouds/haze.
Robust detections of exoplanetary atmospheric spectra can be made using:
- transit spectroscopy
- direct imaging
- doppler spectroscopy
Transit Spectroscopy
The transit method allows for three configurations to explore a planet’s atmosphere:
- a transmission spectrum when the planet transits in front of the star (primary eclipse)
- an emission spectrum when the planet transits behind the star (secondary eclipse)
- a phase curve as the planet orbits between the primary and secondary eclipse
During the primary eclipse, the star light along the line of sight passes through the atmosphere at the day-night terminator of the planet.
- in-transit spectrum = light observed from the star when the planet passes directly in front of it
- out-of-transit spectrum = baseline light of the star taken before or after the eclipse
Transmission spectrum = difference between in-transit and out-of-transit, normalized by the out-of-transit spectrum
“the transmission spectrum is effectively a measure of extinction due to planetary atmosphere at its day-night terminator region. However, the secondary eclipse spectrum measures the emergent spectrum from the dayside atmosphere of the planet.”