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.
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.
Organisations
People |
ORCID iD |
Nikku Madhusudhan (Primary Supervisor) | |
Ryan Macdonald (Student) |
Publications
Helling C
(2019)
Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b I. Cloud and chemistry mapping
in Astronomy & Astrophysics
Kilpatrick B
(2018)
Community Targets of JWST's Early Release Science Program: Evaluation of WASP-63b
in The Astronomical Journal
MacDonald R
(2017)
Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres
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
MacDonald R
(2018)
Exploring H 2 O Prominence in Reflection Spectra of Cool Giant Planets
in The Astrophysical Journal
MacDonald R
(2017)
Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres
in The Astrophysical Journal Letters
Molaverdikhani Karan
(2020)
Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b II. Mapping the effects of gas kinetics
in arXiv e-prints
Pinhas A
(2019)
H2O abundances and cloud properties in ten hot giant exoplanets
Pinhas A
(2019)
H2O abundances and cloud properties in ten hot giant exoplanets
in Monthly Notices of the Royal Astronomical Society
Ryan J. MacDonald
(2019)
A Metal-Enriched Atmosphere for the Exo-Neptune HAT-P-26b
in Zenodo
Sedaghati E
(2017)
Detection of titanium oxide in the atmosphere of a hot Jupiter.
in Nature
Sedaghati E
(2017)
Detection of titanium oxide in the atmosphere of a hot Jupiter.
Studentship Projects
Project Reference | Relationship | Related To | Start | End | Student Name |
---|---|---|---|---|---|
ST/N503988/1 | 30/09/2015 | 30/03/2021 | |||
1638355 | Studentship | ST/N503988/1 | 30/09/2015 | 31/03/2019 | Ryan Macdonald |
Description | Kavli Summer Program in Astrophysics 2016 |
Amount | $1,200 (USD) |
Organisation | The Kavli Foundation |
Sector | Private |
Country | United States |
Start | 05/2016 |
End | 08/2016 |
Description | Royal Astronomical Society Travel Grant |
Amount | £325 (GBP) |
Organisation | Royal Astronomical Society |
Sector | Academic/University |
Country | United Kingdom |
Start | 11/2018 |
End | 07/2019 |
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/3470301#.Xlx2L6hKibg |
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 |