NOIRLab Flash Talk: Sukrit Ranjan and Antranik Sefilian

Sukrit Ranjan, (UA/LPL) 

Model Intercomparisons Falsify O2 False Positives and Strengthen O2 as an Exoplanet Biosignature Gas

 Current and upcoming facilities (James Webb Space Telescope, JWST; Extremely Large Telescopes, ELTs; Habitable Worlds Observatory, HWO) aim to characterize habitable exoplanets in search of signs of life (“biosignatures”; Seager et al. 2014). The canonical biosignature, motivated by the Solar System (Sagan et al. 1993), is atmospheric O2. However, scenarios have been proposed whereby abiotic mechanisms like photochemistry can generate abundant O2 on exoplanets (“false positive scenarios”; Meadows et al. 2018), questioning the value of O2 as an exoplanet biosignature gas. Particularly pernicious is the “M-dwarf scenario”, wherein the high FUV/NUV emission ratio of M-dwarfs may drive abundant abiotic O2 accumulation on the only class of temperate planet accessible to near-term atmospheric characterization with JWST (Domagal-Goldman et al. 2014, Harman et al. 2015, 2018, Hu et al. 2020). Here, we re-examine this scenario using a model intercomparison. We show that model artifacts drive the M-dwarf scenario, which, when rectified, eliminate this false positive scenario (Ranjan et al. 2023). Coupled with our earlier work using new UV cross-section measurements to rule out the “low outgassing” O2 false positive scenario (Ranjan et al. 2020), 2 of the 3 proposed photochemical O2 false positives have been eliminated, dramatically strengthening the case for oxygen as an exoplanet biosignature gas. Additionally, our modelling confirms earlier suggestions that CO is a high-reliability discriminant for abiotic O2 production (Schwieterman et al. 2016). Crucially, our findings replicate across multiple independently developed photochemical models, making them highly robust. Overall, our work strengthens the biosignature gas paradigm for exoplanet life search in general and O2 as a biosignature gas in particular, with significant implications for ongoing JWST observations of temperate terrestrial planets like TRAPPIST-1e (Allen et al. 2024), in-development ELT instruments like the Fabry-Perot Instrument for Oxygen Searches in Exoplanet Atmospheres (FIOS; Rukdee et al. 2019), and design of the upcoming HWO (Clery et al. 2023).

Antranik Sefilian, (UA/Steward) 

Planet-induced Stirring of Debris Disks: The Role of Disk Self-gravity 

Debris disks, analogous to our Solar System’s asteroid and Kuiper belts, are ubiquitous around main-sequence stars. Since dust grains are short-lived, their sustained presence in debris disks is thought to require dynamical excitation, i.e., “stirring”, of a massive reservoir of large planetesimals, resulting in a collisional cascade that continually supplies fresh dust. A commonly considered stirring mechanism is that due to interactions with yet-unseen planets. Such models, however, ignore the self-gravitational effects of the disks themselves, assuming debris disks are massless. This talk presents novel findings exploring scenarios where the disk’s (self-)gravitational effects hinder planet-induced stirring. I will present results showing that the disk gravity can strongly suppress planetesimal eccentricities and collisional velocities throughout the disk, sometimes by more than an order of magnitude when compared to massless disk models. These findings not only have significant implications for planetary inferences within debris-bearing systems, but also provide valuable constraints on the total masses of debris disks.

The seminar will also be broadcast on Zoom:

Zoom:  https://noirlab-edu.zoom.us/j/95755933488?pwd=Nj6GWmbx2gPRrqxTTR4IRlUWdOxTbS.1 

Or, go to https://noirlab-edu.zoom.us/join and enter meeting ID: 957 5593 3488 and password: 563463