Periodic Reporting for period 4 - exoZoo (High definition and time-resolved studies of exoplanet atmospheres: a new window on the extreme diversity of the exoplanet zoo)
Reporting period: 2023-07-01 to 2024-12-31
As we mark 30 years since the first confirmation of planets orbiting stars beyond our Solar system, it is clear that even after 5000+ discoveries of these exoplanets, we have barely scratched the surface of the seemingly infinite forms that other worlds may take. Each new planet discovery and the follow up study of its atmosphere reveals a rich, eclectic zoo of conditions; from the blistering heat of hot Jupiters that rain gemstones from their skies, to the enigmatic mini-Neptunes shrouded in hazes and clouds, and to the rocky planets whose diversity abounds even within our own Solar system, be it the sulphuric clouds of Venus, the arid deserts of Mars, or our cosy home on Earth. It remains an unsolved problem as to how a universal star-planet formation process could create and evolve such differing worlds, and how many of these result in conditions similar to our Earth and may ultimately be capable of supporting life. However, we live at a special time where the technological power to measure the properties needed to address these questions about our origins is imminent, with the Extremely Large Telescopes (ELTs) due online at the end of the 2020s. This is the driving force behind this ERC project (exoZoo). Its overall objectives are to open three new observational avenues that will ultimately leverage these powerful machines to study exoplanet atmospheres in exquisite detail, connecting their properties to their formation and their appearance, and searching for signs of life, the so-called biosignatures, including oxygen and other gases in their atmospheres. These three new avenues include: i) multi-resolution, combining ground-based high resolution spectroscopy with space based observations to place tight constraints on the composition and structure of exoplanet atmospheres, ii) reflected light, to access wavelengths containing key biosignature gases, and iii) variability, using cutting-edge optical components to isolate and monitor light from exoplanets directly in order to map out their atmospheric features as they rotate in and out of view (exocartography). We use several world leading workhorse instruments on large telescopes as well as new innovative instrumentation for these avenues, including the high resolution infrared spectrographs ARIES/MMT and the upcoming CRIRES+/VLT, as well as ESO’s HARPS and ESPRESSO visible light high resolution spectrographs, and state-of-the-art liquid crystal components, the vector Apodizing Phase Plates (vAPPs), at Magellan and the Large Binocular Telescope to achieve our goals.
Amidst the backdrop of the global coronavirus pandemic, our team has continued to push forward observational techniques to study exoplanet atmospheres.
In multi resolution studies, we have made substantial progress in analysing ultra hot Jupiters in our large MMT Exoplanet Atmosphere SURvEy (MEASURE). These data help pinpoint the transition of giant exoplanets into ultra hot Jupiters, where water dissociation and thermal inversions give rise to extreme climates. Our custom-built pipeline takes in raw data, then reduces and assesses it in a Bayesian likelihood framework that includes 3D atmospheric models with rotation and hotspots. We also assess the impact of non local thermal equilibrium effects on exoplanet spectra that are now evident at high resolution, as we have helped show for neutral oxygen lines for the first time in hot Jupiters. We have further contributed to showing the importance of accurate cross sections in high resolution model spectra, and contributed to the first detection of hydroxyl radical emission, which highlights water dissociation in ultra hot Jupiters.
In reflected light spectroscopy, we have demonstrated that even 3.5m telescopes are sensitive to contrast ratios of 10ppm, which would be sufficient to see reflective cloud layers gaseous exoplanets. We further highlighted the need to use models templates to measure chemical abundances (e.g. oxygen) in the future. We have also helped show in simulations that high resolution spectroscopy can still measure atmospheric composition in the presence of hazes and clouds, unlike muted lower resolution spectroscopy from space. We have performed simulations for next generation instruments to assess their capability to perform biosignature surveys, including METIS/ELT, MICADO/ELT, HARMONI/ELT, the GMT and TMT, and the next great observatories in ESA Voyage 2050 and NASA’s decadal review. Our end-to-end HARMONI/ELT simulation shows potential for robust detection of Earth-like atmospheres in reflected light around the nearest rocky exoplanet, Proxima b, but that it requires modified focal plane masks.
Using vAPP imaging, we show that these innovative optical components can null the glare of exoplanet host stars to enable detection of their companions, while also providing a simultaneous reference source. The goal is to use this reference to remove non astrophysical variations and reveal the light curve of the planet itself. We implemented a new flipped differential imaging technique, highlighting its detection potential in well behaved optical systems. With vAPP spectroscopy at LBT, we are working to show that via differential spectrophotometry, akin to space-based observations, we can reach light curve uncertainties at the 4% level, which would be sufficient for seeing the largest exoplanet storms rotate in and out of view.
In multi resolution studies, we have made substantial progress in analysing ultra hot Jupiters in our large MMT Exoplanet Atmosphere SURvEy (MEASURE). These data help pinpoint the transition of giant exoplanets into ultra hot Jupiters, where water dissociation and thermal inversions give rise to extreme climates. Our custom-built pipeline takes in raw data, then reduces and assesses it in a Bayesian likelihood framework that includes 3D atmospheric models with rotation and hotspots. We also assess the impact of non local thermal equilibrium effects on exoplanet spectra that are now evident at high resolution, as we have helped show for neutral oxygen lines for the first time in hot Jupiters. We have further contributed to showing the importance of accurate cross sections in high resolution model spectra, and contributed to the first detection of hydroxyl radical emission, which highlights water dissociation in ultra hot Jupiters.
In reflected light spectroscopy, we have demonstrated that even 3.5m telescopes are sensitive to contrast ratios of 10ppm, which would be sufficient to see reflective cloud layers gaseous exoplanets. We further highlighted the need to use models templates to measure chemical abundances (e.g. oxygen) in the future. We have also helped show in simulations that high resolution spectroscopy can still measure atmospheric composition in the presence of hazes and clouds, unlike muted lower resolution spectroscopy from space. We have performed simulations for next generation instruments to assess their capability to perform biosignature surveys, including METIS/ELT, MICADO/ELT, HARMONI/ELT, the GMT and TMT, and the next great observatories in ESA Voyage 2050 and NASA’s decadal review. Our end-to-end HARMONI/ELT simulation shows potential for robust detection of Earth-like atmospheres in reflected light around the nearest rocky exoplanet, Proxima b, but that it requires modified focal plane masks.
Using vAPP imaging, we show that these innovative optical components can null the glare of exoplanet host stars to enable detection of their companions, while also providing a simultaneous reference source. The goal is to use this reference to remove non astrophysical variations and reveal the light curve of the planet itself. We implemented a new flipped differential imaging technique, highlighting its detection potential in well behaved optical systems. With vAPP spectroscopy at LBT, we are working to show that via differential spectrophotometry, akin to space-based observations, we can reach light curve uncertainties at the 4% level, which would be sufficient for seeing the largest exoplanet storms rotate in and out of view.
Each avenue within the project has already lead to advances in the state of the art. With MEASURE, we have shown that by pushing beyond the standard assessment of high resolution spectroscopy with just cross correlation, and instead use a Bayesian likelihood framework, that its additional scaling parameter allows us to infer more about the exoplanet atmosphere. In particular, we have been able to show that with high resolution 3D model spectra, we find consistency for eastward offset thermal hotspots in contrast to westward offsets in optical light curves. This is access via the line contrast ratio which is a key parameter in the high resolution spectroscopy technique. We also achieved all this at resolution of just R~15,000 due to the fast orbital speed of the planet. It shows that in some cases, e.g. very close in habitable zones, that spectral resolution could be traded for photon collecting power in fainter systems.
In reflected light, we have showed however that for fast orbiting systems, where the star and planet are not fully locked (i.e. the planet orbits faster than the star rotates), that its reflected light spectrum can be significantly broadened. This reduces the number of lines that can be detected with the high resolution spectroscopy technique, and should be accounted for when determining observing time, and interpreting contrast ratios.
We are developing a novel method, based on exoplanet transmission spectroscopy, to extract precision light curves, potentially across multiple wavelengths, of exoplanets directly. This is enabled by the R~70 integral field spectrograph of LBT which we use in combination with the vAPP. It takes a classic method of differential spectrophotometry, but applies it in the new regime, after the star has been heavily nulled at the planet position. With just one night of good data, our preliminary analysis already reached 4% precision. Further observations and analysis may ultimately take us to the sub 1% precision needed to use this method to track weather systems and even exomoon transits across the planetary disk.
As we await the full on sky operation of CRIRES+, which we will use to study the thermal atmospheric properties of smaller, cooler planets such as mini Neptunes, we anticipate new ESPRESSO observations for systems expected to be highly reflective e.g. low surface gravity mini Neptunes which allow lofted bright silicate clouds. Accessing these reflected spectra will open a rich new avenue of study. We further anticipate more vAPP spectroscopy to push our capabilities to study exoplanet variability that can make use the great spatial resolution of large ground based telescopes.
In reflected light, we have showed however that for fast orbiting systems, where the star and planet are not fully locked (i.e. the planet orbits faster than the star rotates), that its reflected light spectrum can be significantly broadened. This reduces the number of lines that can be detected with the high resolution spectroscopy technique, and should be accounted for when determining observing time, and interpreting contrast ratios.
We are developing a novel method, based on exoplanet transmission spectroscopy, to extract precision light curves, potentially across multiple wavelengths, of exoplanets directly. This is enabled by the R~70 integral field spectrograph of LBT which we use in combination with the vAPP. It takes a classic method of differential spectrophotometry, but applies it in the new regime, after the star has been heavily nulled at the planet position. With just one night of good data, our preliminary analysis already reached 4% precision. Further observations and analysis may ultimately take us to the sub 1% precision needed to use this method to track weather systems and even exomoon transits across the planetary disk.
As we await the full on sky operation of CRIRES+, which we will use to study the thermal atmospheric properties of smaller, cooler planets such as mini Neptunes, we anticipate new ESPRESSO observations for systems expected to be highly reflective e.g. low surface gravity mini Neptunes which allow lofted bright silicate clouds. Accessing these reflected spectra will open a rich new avenue of study. We further anticipate more vAPP spectroscopy to push our capabilities to study exoplanet variability that can make use the great spatial resolution of large ground based telescopes.