Inverse Modelling Methods for the Characterisation of Exoplanetary Atmospheres

Lead Research Organisation: University of Cambridge
Department Name: Institute of Astronomy

Abstract

Since the launch of the Kepler Space Telescope in 2009, results now suggest that 1 in 5 Sun-like stars may have habitable planets (Petigura et al., 2013), which potentially holds startling implications for the field of Astrobiology and the search for life elsewhere in the Universe.

In quick succession over the past 20 years, we have gone from planetary detection, to measuring bulk properties (such as density), to, most recently, characterisation of the atmospheres of a selection of these planets. Such atmospheric studies are vital to the long term goal of assessing habitability, as even within our own solar system the case studies of Mars and Venus demonstrate how otherwise 'habitable' planets can prove inhospitable due to their atmospheric makeup. We now face a continual drive to lower mass worlds, bringing the prospect of probing the atmosphere of a true Earth-analogue tantalisingly close to fruition.

One of the issues that has plagued many past attempts to truly characterise exoplanet atmospheres is poorly understood degeneracies. For instance, when we observe the light from a transiting planet-star system, certain spectral features that have been interpreted as absorption due to a given molecule can also be explained by a different atmospheric temperature structure. In the past, single models were run until one fitting the data was found, without a throughout investigation of other combinations of atmospheric parameters that could also explain the data. To circumvent this, so called 'atmospheric retrieval' models have been developed (originally by my supervisor, Dr. Madhusudhan), which run millions of models to fully explore all the possible explanations for the observed behaviour. These take in observational data and use it to infer the properties of the atmosphere (often called 'inverse modelling').

My current research focuses on theoretical modelling of exoplanet transmission spectra using the aforementioned methodology of atmospheric retrieval. My work involves investigating new physical phenomena that can be included in such models, such that a greater information content can be extracted from existing exoplanet transmission spectra. This will prove increasingly vital in the upcoming era of high-precision exoplanet spectroscopy (for instance, from the James Webb Space Telescope), as the quality of data will be sufficient to characterise such worlds in unprecedented detail. This, however, requires extensive theoretical exploration of previously neglected physical effects to ensure the maximum extraction possibility of potential discoveries.

Publications

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MacDonald R (2017) HD 209458b in new light: evidence of nitrogen chemistry, patchy clouds and sub-solar water in Monthly Notices of the Royal Astronomical Society

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MacDonald R (2018) Exploring H 2 O Prominence in Reflection Spectra of Cool Giant Planets in The Astrophysical Journal

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MacDonald R (2017) Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres in The Astrophysical Journal

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Pinhas A (2019) H2O abundances and cloud properties in ten hot giant exoplanets in Monthly Notices of the Royal Astronomical Society

Studentship Projects

Project Reference Relationship Related To Start End Student Name
ST/N503988/1 01/10/2015 30/09/2020
1638355 Studentship ST/N503988/1 01/10/2015 31/03/2019 Ryan Macdonald
 
Description Kavli Summer Program in Astrophysics 2016
Amount $1,200 (USD)
Organisation Peking University 
Department Kavli Institute for Astronomy and Astrophysics
Sector Academic/University
Country China
Start 06/2016 
End 08/2016
 
Title Reflection Spectra Repository for Cool Giant Planets 
Description A collection of >50,000 reflection spectra models of cold giant planets provided for the exoplanet atmospheres modelling community. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact This model grid is seeing usage by other researchers. Informing science case studies for NASA's upcoming WFIRST mission. 
URL https://zenodo.org/record/1210305#.XIhTtij7Sbg
 
Description Naked Scientists Podcast radio interview 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I was interviewed via audio recording for the 'Naked Scientists Podcast' discussing the results of our recent Nature paper 'Detection of titanium oxide in the atmosphere of a hot Jupiter'. This interview was broadcast on BBC Radio 5 Live nationally and internationally on ABC Radio (Australia).
Year(s) Of Engagement Activity 2017
URL https://www.thenakedscientists.com/articles/interviews/exoplanet-atmosphere-explored-astronomers
 
Description New Scientist feature article: 'Weather forecasts from alien worlds' 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I was invited to write a feature length article for the New Scientist about exoplanet weather after meeting one of their science editors at a conference. This article was published in print and online, explaining discoveries from my field of research to a general public audience.
Year(s) Of Engagement Activity 2018
URL https://www.newscientist.com/article/mg24032032-000-weather-forecasts-from-alien-worlds-are-in-and-i...
 
Description Podcast interview on my research 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact An hour long interview with the online science podcast 'The Unseen Podcast' centred on my paper: 'HD209458b in New Light: Evidence of Nitrogen Chemistry, Patchy Clouds and Sub-Solar Water'.
Year(s) Of Engagement Activity 2017
URL http://www.unseenpodcast.com/2017/06/exoplanet-atmospheres.html
 
Description Press release: first detection of titanium oxide in an exoplanet atmosphere 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact The European Southern Observatory issued a joint press release with the University of Cambridge following the publication of our paper 'Detection of titanium oxide in the atmosphere of a hot Jupiter'. The main result, the detection of titanium oxide, was discovered by a computer code I wrote as part of my STFC PhD programme. The press release resulted in global media coverage in multiple languages, reaching a wide international audience.
Year(s) Of Engagement Activity 2017
URL https://www.eso.org/public/unitedkingdom/news/eso1729/
 
Description Public talks on my research 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact I have delivered a talk entitled 'The Inferno World with Titanium Skies' based on our Nature paper 'Detection of titanium oxide in the atmosphere of a hot Jupiter' on four occasions: a public talk at the Institute of Astronomy in Cambridge (200+ attendees); at the Papworth Astronomy Club in Cambridgeshire (~40 attendees); at the Long Eaton Astronomical Society in Derbyshire (~30 attendees); and at Newstead Wood School in Greater London (50+ attendees). There were extensive question and answer sessions after each talk, with the talk at Newstead Wood School in particular serving to promote the study of science at Oxbridge.
Year(s) Of Engagement Activity 2017,2018
URL http://www.ast.cam.ac.uk/public/talks/5264