Figure 1 – Three broadband transit light curves corrected for slit losses. The effects of limb darkening may be seen by comparing the blue spectrum (top) to the red spectrum (bottom).

Detecting Exoplanet Atmospheres From the Ground


The Big Picture

The field of exoplanet research is expanding rapidly. We are able to identify hundreds of exoplanets and estimate their orbital parameters, masses, and radii (see But the next step lies in characterizing their atmospheres – in observing atmospheric temperatures, chemical compositions, albedo (how reflective the atmosphere is), dynamics, and structure.

We expect a huge diversity of exoplanet atmospheres, as the formation and composition of an atmosphere depends on a planet’s evolutionary history, its present distance from the host star, and where in the disk it formed. Hot Jupiters are believed to have primitive atmospheres in that little evolution has taken place and they contain roughly the same atmospheric gases as they did at their formation. By studying such atmospheres we will probe these planet’s formation history in a way that has not been done before.

How do we detect an exoplanet’s atmosphere? When the exoplanet passes in front of its host star, the exoplanet’s atmosphere will absorb the host star’s light at specific wavelengths. These absorption lines correspond to specific energy transitions within the elements in the exoplanet’s atmosphere. They will therefore tell us the exoplanet atomsphere’s composition. Comparing the two spectra gives the planet’s transmission spectrum (see Nathan’s astrobite and Caroline’s astrobite), where the specific absorption line is deeper during transit than out of transit.

It’s not easy to obtain the exoplanet’s transmission spectrum. Until recently, it was only feasible using space-based telescopes. On the ground, the earth’s atmosphere is a huge hindrance because it has many of the spectral features we might look for in the spectrum of an exoplanet. The earth’s atmosphere is also turbulent, meaning that the exact shape of its spectral features may change with the weather. One technique to avoid confusing changes in the earth’s atmosphere with changes in the atmosphere of an exoplanet is to simultaneously observe a comparison star while observing the transit. At the end of the day you can divide the comparison star’s spectrum by the host star’s spectrum, deleting any atmospheric effects, and reducing the noise to Poisson noise only.

Read more: Detecting Exoplanet Atmospheres From the Ground | astrobites.

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