High resolution Doppler spectroscopy of a transiting ultra-hot Jupiter at optical wavelengths

  • Stephanie Merritt

Student thesis: Doctoral ThesisDoctor of Philosophy


The discovery of exoplanets in the 90s has transformed our ability to study the formation, evolution, and structure of planetary systems. In particular, the huge, high-temperature exoplanets known as hot Jupiters are fascinating laboratories for extreme atmospheric physics, and their size, translucent atmospheres and proximity to their host stars makes them excellent targets for characterisation through transmission spectroscopy. Recently, ultra-hot Jupiters (UHJs) have emerged as a distinct class of their own, with their huge dayside temperatures (over 22,00 K) driving entirely different chemistry than their cooler cousins, differing from that predicted by earlier theory. This thesis presents an in-depth investigation at high resolution of the chemical constituents of the UHJ WASP-121b at optical wavelengths, using a single transit from VLT/UVES. Until recently, this was a wavelength regime little-explored in hot Jupiter atmospheres using cross-correlation techniques. Using the Doppler spectroscopy method, which takes advantage of the high radial velocity of the planet to disentangle its spectral lines from those of its host star, two investigations were performed.

The first study comprises a comprehensive search for the molecules TiO and VO, strong optical absorbers previously theorised to drive thermal inversions in hot Jupiters. No signs of either molecule could be found, though confirmation of the absence of VO awaits a more accurate linelist. These constraints provide evidence for recent theory, suggesting that TiO and VO are thermally dissociated in UHJs, and that the previously-observed thermal inversion in WASP-121b may be driven by atomic metals.

The second study presents the results of a broad search for neutral and ionised atomic species in the same dataset. Forty-three species were searched for over a grid of varying temperatures and cloud-deck pressures. Twenty potential signals were flagged and investigated in detail, with seven of these classified as detections after applying a set of detection criteria assessing different aspects of the signals in detail. This study underlines the huge potential in studying UHJs at optical wavelengths and high-resolution. In the future, these techniques will be applicable to observations from the next generation of high-resolution spectrographs, and may one day be used to characterise habitable planets.
Date of AwardDec 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy
SupervisorNeale Gibson (Supervisor), Christopher Watson (Supervisor) & Ernst de Mooij (Supervisor)


  • Astrophysics
  • exoplanet transits
  • exoplanets
  • spectroscopy
  • exoplanet atmospheres
  • high-resolution spectroscopy
  • astronomy
  • Doppler spectroscopy

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