Special Talk

• Special Talks

  Speaker: Stephen Schmidt (JHU)
  Title: K2-18 b is (Probably) Not a Hycean Biosphere
  Abstract: Sub-Neptunes are the most common type of planet in our galaxy. Interior structure models suggest that the coldest sub-Neptunes could host liquid water oceans underneath their hydrogen envelopes---sometimes called "hycean" planets. JWST transmission spectra of the ~ 250 K sub-Neptune K2-18 b were recently used to report detections of CH4 and CO2, alongside weaker evidence of (CH3)2S (dimethyl sulfide, or DMS). Atmospheric CO2 was interpreted as evidence for a liquid water ocean, while DMS was highlighted as a potential biomarker later corroborated by MIRI observations. However, these notable claims were derived using a single data reduction and retrieval modeling framework, which did not allow for standard robustness tests. Here we present a comprehensive reanalysis of K2-18 b's JWST NIRISS SOSS and NIRSpec G395H transmission spectra, including the first analysis of the second-order NIRISS SOSS data. We incorporate multiple well-tested data reduction pipelines and retrieval codes, spanning 60 different data treatments and over 250 atmospheric retrievals. We confirm the detection of CH4 (≈ 4σ), with a volume mixing ratio of log CH4 = -1.15 ± 0.5, but we find no statistically significant or reliable evidence for CO2 or DMS. Our best fit model fits the MIRI observations well, but there are inconsistencies with the MIRI best fit and the shorter wavelength data. Finally, we assess the retrieved atmospheric composition using photochemical-climate and interior models, demonstrating that our revised composition of K2-18 b can be explained by an oxygen-poor mini-Neptune without requiring a liquid water surface or life.

  Speaker: Emily Rickman (STScI)​​​​​​​
  Title: Characterizing Substellar Companions​​​​​​​
  Abstract: Understanding the atmospheric and orbital properties of giant planets and brown dwarfs is a major goal of modern astrophysics, but are often plagued by a lack of observational constraints. Measurements of companion brightness, atmospheric parameters, and dynamic environments require thermal emission measured through direct detection that historically has been difficult to do. High-contrast imaging has revealed a precious sample of benchmark brown dwarfs, analogous to giant exoplanets, that are ideal for comprehensively characterizing such properties through direct spectroscopy at an unprecedented level of resolution and sensitivity thanks to JWST and GRAVITY, without relying on a coronagraph, complementary to existing coronagraphic data. Through combining a diversity of exoplanet measurement and detection techniques, precise model-independent masses of companions have been determined through radial velocity data, as well as astrometric accelerations from ESA's Hipparcos and Gaia missions, and accurate astrometry from imaging and interferometry. To constrain the atmospheric properties of these companions, ground-based data is combined with longer wavelength space-based data that goes beyond directly imaging such companions, but comprehensively characterizing their atmospheric properties through direct spectroscopic observations in the high-contrast regime. I will discuss results from numerous observing campaigns leading to a catalog of well-characterized substellar companions including new JWST/NIRSpec IFU spectra from a mass- and age-calibrated sample of benchmark brown dwarf companions that span spectral types, that will set the standard for studying exoplanet atmospheres and identifying physical processes missing in giant planet evolutionary and spectral models.