My research is currently focused on analysing the atmospheres of transiting exoplanets using space-based telescopes such as Hubble and JWST.
I am the co-PI of the JWST GO program 4105, which aims to explore the chemical processes in the exo-Neptune HAT-P-11b. I am also involved in several JWST observer collaborations, including the Transiting Exoplanet Community Early Release Science Program, the JWST Telescope Science Team GTO Program and the large GO program COMPASS (Compositions of Mini-Planet Atmospheres for Statistical Study).
To analyse JWST Time Series Observations, I maintain ExoTiC-JEDI as part of the ExoTiC suite of Exoplanet Timeseries Characterisation Software and Programs produced and developed by the exoplanet atmospheres team at the University of Bristol. JEDI is an end-to-end reduction and analysis pipeline designed for NIRSpec observations, and is under active development.
With HST, I am a co-I of the large treasury program HUSTLE (Hubble UV-optical Survey of Transiting Legacy Exoplanets, 17183) and am also a member of the STARGATE Collaboration.
To analyse JWST Time Series Observations, I maintain ExoTiC-JEDI as part of the ExoTiC suite of Exoplanet Timeseries Characterisation Software and Programs produced and developed by the exoplanet atmospheres team at the University of Bristol. JEDI is an end-to-end reduction and analysis pipeline designed for NIRSpec observations, and is under active development.
With HST, I am a co-I of the large treasury program HUSTLE (Hubble UV-optical Survey of Transiting Legacy Exoplanets, 17183) and am also a member of the STARGATE Collaboration.
WASP-39b
Early Release Science with JWST
The white light (a) and spectroscopic (b) transit of WASP-39b as observed by G395H, and the exquisite precisions achieved (c)
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The Transiting Exoplanet Community Early Release Science Program is a JWST Early Release Science (ERS) program that aims to provide the exoplanet community with publicly-available datasets and analysis toolkits as soon as possible after the telescope's launch. Through these observations, the program will help to understand the spectrophotometric precision achievable, and develop best practices for transiting exoplanet studies with each JWST instrument.
In the Panchromatic Transmission Working Group, the hot Saturn-mass exoplanet WASP-39b was studied with four key instrument modes aboard JWST to produce a comprehensive and continuous transmission spectrum from 0.6–5 μm. With one of these modes, NIRSpec G395H, we examined how best to handle mirror tilt events, which can cause a jump in flux during an observation (left), and set out some best practices for producing clean stellar spectra (Alderson et al. 2023). |
Our 3–5 μm transmission spectrum detected significant absorption from CO2 (28.5σ) and H2O (21.5σ), and identified SO2 as the mystery source of absorption at 4.1 μm seen in previous ERS work. Best-fit atmospheric models range between 3 and 10x solar metallicity, with sub-solar to solar C/O ratios. These results showcase NIRSpec G395H as an excellent mode for studying transiting exoplanet atmospheres for years to come.
WASP-17b
The first project of my PhD revolved around WASP-17b, a hot Jupiter which has frequently featured in comparative exoplanet studies, and will observed by multiple JWST modes in GTO program 1353. However, the space-based observations upon which these studies critically rely occurred before many of the now widely used high precision observing modes and analysis techniques had been developed.
In Alderson et al. (2022), I combined analysis of new high precision HST WFC3 IR observations, along with a consistent reanalysis of the previous Hubble and Spitzer data (Sing et al. 2016) to present an updated 0.3–5 μm transmission spectrum. By taking advantage of recent advances in exoplanet data reduction and analysis techniques, the new transmission spectrum achieves a markedly improved average precision of 132 ppm. Through comprehensive atmospheric retrieval modelling we detected H2O absorption and found evidence of CO2 absorption. These retrievals also uncovered complexities that highlight both the importance of using robust statistics for model selection and the need for high precision panchromatic observations.
In Alderson et al. (2022), I combined analysis of new high precision HST WFC3 IR observations, along with a consistent reanalysis of the previous Hubble and Spitzer data (Sing et al. 2016) to present an updated 0.3–5 μm transmission spectrum. By taking advantage of recent advances in exoplanet data reduction and analysis techniques, the new transmission spectrum achieves a markedly improved average precision of 132 ppm. Through comprehensive atmospheric retrieval modelling we detected H2O absorption and found evidence of CO2 absorption. These retrievals also uncovered complexities that highlight both the importance of using robust statistics for model selection and the need for high precision panchromatic observations.
WASP-17b Poster as presented at Exoplanets IV in May 2022
WASP-21b
Prior to starting my PhD, I spent a year in research at the Center for Astrophysics | Harvard-Smithsonian under the supervision of Dr. James Kirk, working as a part of the LRG-BEASTS project.
The goal of the Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy (LRG-BEASTS, "large beasts") is to provide a large sample of optical transmission spectra of hot Jupiter atmospheres, in order to determine the processes driving aerosol formation and to provide the optical data necessary to place tight constraints on abundances derived from IR data.
I combined three transits of the hot-Saturn WASP-21b to obtain the optical transmission spectrum from 4636-9000Å (Alderson et al., 2020) at an average precision of 197ppm, less than one atmospheric scale height. We were able to detect sodium absorption at greater than 4 sigma confidence, in addition to a steep scattering slope, similar to that of the atmosphere of HD189733b. I worked to rule out stellar activity as the cause of this slope, with atmospheric aerosols being the most likely cause. We found no evidence of potassium absorption, adding WASP-21b to the growing list of planets with only one of the alkali metals present in their atmosphere.
The goal of the Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy (LRG-BEASTS, "large beasts") is to provide a large sample of optical transmission spectra of hot Jupiter atmospheres, in order to determine the processes driving aerosol formation and to provide the optical data necessary to place tight constraints on abundances derived from IR data.
I combined three transits of the hot-Saturn WASP-21b to obtain the optical transmission spectrum from 4636-9000Å (Alderson et al., 2020) at an average precision of 197ppm, less than one atmospheric scale height. We were able to detect sodium absorption at greater than 4 sigma confidence, in addition to a steep scattering slope, similar to that of the atmosphere of HD189733b. I worked to rule out stellar activity as the cause of this slope, with atmospheric aerosols being the most likely cause. We found no evidence of potassium absorption, adding WASP-21b to the growing list of planets with only one of the alkali metals present in their atmosphere.
LRG-BEASTS optical transmission spectrum of WASP-21b with the best fitting atmospheric retrieval model (Alderson et. al 2020)
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HEADER IMAGE CREDIT: NASA, ESA, G. BACON
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