I study atmospheres and climates, both inside the Solar System and beyond. I am particularly interested in the connection between the study of Earth-like planets around other stars, and what these discoveries tell us about our own world.

One of the most fundamental questions for terrestrial exoplanets is whether they have atmospheres. If atmospheres are rare the galaxy must be littered with bare rocks like Mercury, whereas if atmospheres are common then we will be able to study dozens or more potentially Earth-like worlds within our lifetime.

In my PhD I used ideas from Earth’s atmosphere, including dimensional analysis, to show how thermal phase curves can be used to measure atmospheric thickness, and thus address this question. In my postdoc we then applied this idea to the exoplanet LHS 3844b, showing that it is likely a bare rock.

See Koll & Abbot (2015) and Kreidberg et al (2019).

A thermal phase curve.

Do rocky exoplanets have atmospheres?

Atmospheric heat engines and exoplanets

Meteorologists know that hurricanes resemble a heat engine, similar to the Carnot cycle. I showed that this simple idea carries over surprisingly well to the the atmospheric circulations of exoplanets (see right), where it allows us to predict both temperature structures and day-night heat transport of rocky exoplanets.

Even more, the same theory also captures the sonic wind speeds on hot Jupiters, and predicts how wind speeds on hot Jupiters should change as a function of planetary temperature. Thanks to a number of new ground-based instruments, we should be able to soon test this prediction!

See Koll & Abbot (2016) and Koll & Komacek (2018).

On a rocky exoplanet, the atmosphere’s heating and cooling (blue) balance friction on the turbulent dayside (red).

Why is Earth’s climate so linear?

Every physics student learns that the most basic model for radiation is a blackbody, and that a blackbody emits energy nonlinearly with temperature, σT4. However, actual measurements show that Earth looks nothing like a blackbody and that its outgoing longwave radiation (OLR) to space is instead basically linear in temperature (see left).

To understand why Earth’s OLR is linear, why it matters for global warming, and what it has to do with other planets, you can find the resolution to this puzzle in Koll & Cronin (2018).

Earth’s outgoing longwave radiation (OLR) looks nothing like the Stefan-Boltzmann law (red). Instead, OLR is roughly a linear function of temperature (blue).