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My research focusses on measuring the atmospheres of extrasolar planets (“exoplanets”) in order to determine why exoplanets are so diverse and in such stark contrast to the planets in our Solar System (read on to see how diverse they are!).

To measure an exoplanet atmosphere’s composition, I use telescopes to observe the amount of light blocked by the exoplanet as it transits in front of its background star along our line of sight. At specific wavelengths, chemicals within the exoplanet’s atmosphere will block more of the background star’s light than at other wavelengths. Therefore, by measuring the amount of light blocked by the planet as a function of wavelength (a technique called transmission spectroscopy) I can determine which chemicals are present within the exoplanet’s atmosphere.

A list of my publications can be found here.

The population of exoplanets and those that I am studying

In the plot below, I show the population of transiting exoplanets (in grey) on a plot of orbital period against planetary size. This demonstrates how the vast majority of transiting exoplanets have shorter orbital periods than Mercury (!) and have sizes between the Earth and Neptune. Despite these being the most common type of exoplanet, there is no equivalent in our Solar System. There is also a population of Jupiter-sized exoplanets with orbital periods of around 1 day. These “hot Jupiters” have significantly challenged our understanding of gas giant formation.

I am interested in understanding exoplanets across all sizes from Earth-sized to Jupiter-sized. The specific planets that I am studying and the observatories/methods I am using are highlighted on the plot. I go into more detail about these at the bottom of this page.

JWST

I am the co-PI of an upcoming JWST Cycle 2 programme (PID: 3838) that will obtain near-IR transmission spectra of a sample of 6 hot Jupiters, 3 whose orbital planes are aligned with their host stars’ spin axes, and 3 whose orbital planes are misaligned with their host stars’ spin axes. It is widely believed that aligned hot Jupiters migrated through a protoplanetary disc to end up on their close-in orbits, while misaligned hot Jupiters migrated after the dispersal of the protoplanetary disc, via dynamical interactions with additional bodies in the system (see Dawson and Johnson, 2018 for an excellent overview of the proposed origins of hot Jupiters). My programme will test whether the chemical composition of hot Jupiters depends upon how a planet migrated and will offer novel tests of planet formation models.

Beyond my Cycle 2 programme, I am involved in several Cycle 1 programmes that are using JWST to measure the atmospheres of exoplanets, including a number of Earth-sized exoplanets. I was also a key contributor to the exciting first results from the JWST Transiting Exoplanet Early Release Science program, including the first detection of carbon dioxide in an exoplanet atmosphere and an exquisite spectrum of a hot Jupiter that revealed a plethora of chemical species.

My involvement in JWST-related science has primarily been via the development of my own independent pipeline, Tiberius, that can extract exoplanet atmospheric spectra from raw JWST data files.

HST

In addition to JWST, I have also analysed data from Hubble including the first atmospheric spectrum of the extremely long period (1.5 years!) and low-density (styrofoam-like) exoplanet HIP 41378f. For this exoplanet we found our results were consistent with high-altitude hazes, a heavy atmosphere, or even circumplanetary rings!

LRG-BEASTS

I am the Principal Investigator of the Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy (LRG-BEASTS, “large beasts”). This has been a very successful programme and has acquired optical data of several tens of exoplanets. You can find more detail about LRG-BEASTS here.

Helium escape

Capitalising on the recent first detection of helium in an exoplanet atmosphere, I have been a pioneer of observing exoplanetary helium with ground-based telescopes in order to measure the loss of exoplanet atmospheres. My success has enabled me to start my own programme on the 10 metre Keck telescope to perform this science for additional exoplanets.

High-resolution optical studies

High-resolution observations of sodium and potassium provide high altitude information that complements low-resolution, lower altitude, studies of exoplanet atmospheres such as those performed by LRG-BEASTS. I am the Principal Investigator of a programme using the high-resolution spectrograph MIKE on the 6.5 metre Magellan telescope in Chile. With these data, I will be able to determine how an exoplanet’s atmospheric chemistry and physics varies with altitude in the planet’s atmosphere.

ACCESS

Similar to LRG-BEASTS, ACCESS (PIs: López-Morales, Apai, Osip) is obtaining optical data of hot Jupiters but using the 6.5 metre Magellan telescopes. As a co-investigator of ACCESS, I have been involved in several studies analysing data from this programme.