Surveying the atmospheres of TESS planets at high spectral resolution

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

The astonishing diversity of the thousands of exoplanets currently known naturally leads us to question the unicity of our planet and our solar system. Answering these questions ultimately requires probing the atmospheres of alien planets. Their chemical make up and physical parameters reveal clues about the true nature of an exoplanet, for instance whether it is gaseous or terrestrial. Atmospheres ultimately determine whether the surface conditions of a planet are suitable for life.

Thanks to recent technological advances in instrumentation and observational techniques, we are now ready to move from studies of very hot, gas giants (hot Jupiters), which are the easiest targets, to smaller and cooler planets. In this context, this project is focussing on a novel observational method based on very high resolution spectroscopy with ground-based telescopes. It aims at targeting the best exoplanets found by the upcoming TESS mission, which is just about to launch. It will look for new transiting exoplanets significantly smaller than previous discoveries, but still orbiting bright stars, so that following up part of these planets will become feasible. This is the first chance to fill the gap towards Earth-like planets, and what we will learn from TESS planets will be the foundation of our future hunt for signatures of life.

The technique that will be used in this project relies on comparing very high resolution spectra to models of planetary atmospheres through a process called cross correlation.
This essentially measures the match between the observed spectra and the theoretical model, similarly to matching a fingerprint. Since these planets have never been observed before at such high spectral resolution, it will also be necessary to compute a library of spectra, that will then be made available to the community.

Studying the atmosphere of TESS planets will reveal important clues about the transition between gaseous and rocky exoplanet, and the role of clouds in shaping their transmission spectrum.

Another important achievement of the project is the combination of observations from space and from the ground. So far these have always proceeded independently, however they contain highly complementary information that could be combined to improve our inference on the atmospheric properties. This project will show for the first time a systematic application of this new approach and set the foundation for a true synergy between the best observatories in space (HST and soon JWST) and on the ground (VLT and, in the next decade, the ELT).

Planned Impact

In line with the last Consolidating Grant of Warwick Astronomy, this project will have a strong Inspirational impact. The level of engagement of the general public with exoplanet science, and in particular the quest for an Earth analogue, has risen considerably in the past decade. Arguably this is due to the fact that exoplanet science is addressing questions of great philosophical appeal, such as the unicity of life in the universe.

To ensure that this project will contribute to Inspirational Impact, we will both use existing social media channels, such as Twitter, that is already extensively utilised by the department of Physics. In addition, we foresee the possibility of launching a website aimed at the general public where the aims of the project and its relevance for future knowledge is explained in simple terms. We will adopt the model of the ongoing "Pale Red Dot" campaign, where people engage with the public through a catchy and easy-to-follow website. We will also make sure to publish regular press releases highlighting the achievements of the project, and utilised the Traveling Planetarium initiative to enhance the exoplanet-related content.

More on the long term, we foresee some potential for technology transfer and economic impact. Artificial intelligence algorithms aimed at recognising patterns, currently in development for several commercial applications, could be used to extract the signal of exoplanet atmospheres from the data that will be produced through this project. This because time resolution makes the signal appear as a coherent trail of positive cross-correlation, which is a pattern. Tuning commercial algorithms on astrophysical data, which arguably requires better precision and accuracy, would both improve the quality of commercial application, with positive impact on the experience of customers, and speed up the analysis of scientific data.

Publications

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Title State-of-the art cross sections for modelling of exoplanet atmospheres 
Description We have gathered the most accurate and up-to-date lists of molecular lines suitable for exoplanet applications and converted them in tabulated cross sections, i.e. cross section as a function of planet temperature and pressure. There are computed at very high resolution, allowing the calculation of model spectra usable for cross correlation against observed spectra. 
Type Of Material Computer model/algorithm 
Year Produced 2020 
Provided To Others? Yes  
Impact It is challenging for groups inexperienced in the analysis or modelling of high resolution spectra to source and use accurate line lists. By pre-computing cross-sections for several key molecules and packaging them in a compressed and accessible format, we offered to the community a standardised database to compute their own spectra. This is obviously also key for our own group, as it allows the fast computation of spectra for a wide range of exoplanets. 
 
Description ExoMol - Testing line lists for exoplanet characterisation 
Organisation University College London
Country United Kingdom 
Sector Academic/University 
PI Contribution We provided the observational and modelling support to test line lists computed by ExoMol via modelling of exoplanet spectra and comparison to observations. This is the first time such comparison is made at high spectral resolution and the only way to validate the accuracy of their predictions.
Collaborator Contribution ExoMol has provided key guidelines to interpret the spectra of exoplanets at or above 1000K, pointing us to the most accurate line lists, giving us access to the latest work, and steering their efforts in order to prioritise those species that are most relevant to exoplanet atmospheres.
Impact The first paper is currently under review. We benchmark the state of the art in terms of line lists for 6 main species against real observed data of exoplanets. We inform on the accuracy of the line lists and highlight that some work still needs to be done on a key species - methane. This work is multidisciplinary in that it merges astrophysics, theoretical chemistry, and laboratory spectroscopy.
Start Year 2019
 
Description GAPS - Global Architecture of Planetary Systems 
Organisation Observatory of Turin
Country Italy 
Sector Academic/University 
PI Contribution In the pre-award phase, I have contributed to the training of researchers at the Astrophysical Observatory of Turin (OATo) in the analysis of high resolution spectra. In the post-award phase, PI Brogi continued to supervise the research activities related to the techniques and advised on the best target to include in the ongoing telescope survey. PDRA Sid Gandhi has provided modelling for the interpretation of data gathered within the consortium.
Collaborator Contribution PI Brogi has been granted proprietary access to all the data gathered in the context of the Large Program GAPS2, currently ongoing at the Telescopio Nazionale Galileo, in La Palma. This amounts to 34 nights per semester, or approximately 70 nights over the reporting period of this submission. About 50% of these nights were devoted to exoplanet science, and among the targets of the survey there are several Neptune-size, cooler exoplanets that are the specific subject of the current award. We note that the figure for in-kind contribution indicated above includes an operating cost of 15,000 euro per night for the TNG, scaled down from the figure for the UK-operated WHT (4-m class) on the same site, and 35 observing nights.
Impact There are 2 papers currently at the advanced stage of refereeing led by OATo (Guilluy et al.) and the University of Amsterdam (Pino et al.). In addition, there are two high-profile papers in preparation to be submitted to Nature. PI Brogi is writing one of them and will be second author, while PDRA Gandhi will be high-up on the author list due to his efforts with the modelling. These papers show the first evidence for a rich chemistry in both hot Jupiters and Neptune-size planets. For the latter, this is a clear signature of disequilibrium chemistry, previously theorised but never unambiguously proven.
Start Year 2016
 
Description Investigating unusual species in the atmospheres of Exoplanets - with Dr Amaury Triaud 
Organisation University of Birmingham
Country United Kingdom 
Sector Academic/University 
PI Contribution We offered the modelling background to calculate the spectra of some unusual species in the atmospheres of exoplanets, not yet investigated / found. These include Argon - abundant on Earth - and Phosphine. We also offer our data analysis expertise to process high-resolution spectra.
Collaborator Contribution The group of Dr Triaud has contributed intellectually with the original idea to look for these exotic species, and practically with sharing of proprietary data acquired over the last year. These data include spectra taken with state-of-the art facilities and instrumentation, such as CARMENES at CAHA.
Impact At the moment we are still in the preliminary phases in which we are exploring the feasibility of such observations and evaluating the implications for the physical and observable properties of exoplanets. This theme has strong connections with planetary science and in particular geology.
Start Year 2019