Last Call for Life: Habitability of Terrestrial Planets Orbiting Red Giants and White Dwarfs

172 pagesAs a star evolves, the orbital distance where liquid water is possible on the surface of an Earth-like planet, the habitable zone, evolves as well. While stellar properties are relatively stable on the main sequence, post-main sequence evolution of a star involves significant changes in stellar temperature and radius, which is reflected in the changing irradiation at a specific orbital distance when the star becomes a red giant, and then later a white dwarf. To search planets in these systems for signs of life it is essential that we understand how stellar evolution influences atmospheric photochemistry along with detectable biosignatures. We use EXO-Prime, which consists of a 1D coupled climate/photochemistry and a line-by-line radiative transfer code, to model the atmospheres and spectra of habitable zone planets around red giants and white dwarfs, and assess the time dependency of detectable biosignatures

Is ozone a reliable proxy for molecular oxygen?:I. The O₂-O₃relationship for Earth-like atmospheres

Molecular oxygen (O2) paired with a reducing gas is regarded as a promising biosignature pair for the atmospheric characterization of terrestrial exoplanets. In circumstances when O2 may not be detectable in a planetary atmosphere (e.g., at mid-IR wavelengths) it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy to infer the presence of O2. However, O3 production has a nonlinear dependence on O2 and is strongly influenced by the UV spectrum of the host star. To evaluate the reliability of O3 as a proxy for O2, we used Atmos, a 1D coupled climate and photochemistry code, to study the O2–O3 relationship for “Earth-like” habitable zone planets around a variety of stellar hosts (G0V-M5V) and O2 abundances. Overall, we found that the O2–O3 relationship differed significantly with stellar hosts and resulted in different trends for hotter stars (G0V-K2V) versus cooler stars (K5V-M5V). Planets orbiting hotter host stars counter-intuitively experience an increase in O3 when O2 levels are initially decreased from 100% Earth’s present atmospheric level (PAL), with a maximum O3 abundance occurring at 25–55% PAL O2. As O2 abundance initially decreases, larger amounts of UV photons capable of O2 photolysis reach the lower (denser) regions of the atmosphere where O3 production is more efficient, thus resulting in these increased O3 levels. This effect does not occur for cooler host stars (K5V-M5V), since the weaker incident UV flux does not allow O3 formation to occur at dense enough regions of the atmosphere where the faster O3 production can outweigh a smaller source of O2 from which to create O3. Thus, planets experiencing higher amounts of incident UV possessed larger stratospheric temperature inversions, leading to shallower O3 features in planetary emission spectra. Overall it will be extremely difficult (or impossible) to infer precise O2 levels from an O3 measurement, however, with information about the UV spectrum of the host star and context clues, O3 will provide valuable information about potential surface habitability of an exoplanet

H₂-dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets

Following the discovery of the first exoplanet candidate transiting a white dwarf (WD), a "white dwarf opportunity" for characterizing the atmospheres of terrestrial exoplanets around WDs is emerging. Large planet-to-star size ratios and hence large transit depths make transiting WD exoplanets favorable targets for transmission spectroscopy - conclusive detection of spectral features on an Earth-like planet transiting a close-by WD can be achieved within a medium James Webb Space Telescope (JWST) program. Despite the apparently promising opportunity, however, the post-main sequence (MS) evolutionary history of a first-generation WD exoplanet has never been incorporated in atmospheric modeling. Furthermore, second-generation planets formed in WD debris disks have never been studied from a photochemical perspective. We demonstrate that transmission spectroscopy can identify a second-generation rocky WD exoplanet with a thick ($\sim1$ bar) H$_2$-dominated atmosphere. In addition, we can infer outgassing activities of a WD exoplanet based on its transmission spectra and test photochemical runaway by studying CH$_4$ buildup.Comment: 25 pages, 10 figures, 3 tables, accepted for publication in ApJ Letter

Modeling Atmospheric Lines by the Exoplanet Community (MALBEC) Version 1.0: A CUISINES Radiative Transfer Intercomparison Project

Abstract: Radiative transfer (RT) models are critical in the interpretation of exoplanetary spectra, in simulating exoplanet climates, and when designing the specifications of future flagship observatories. However, most models differ in methodologies and input data, which can lead to significantly different spectra. In this paper, we present the experimental protocol of the Modeling Atmospheric Lines By the Exoplanet Community (MALBEC) project. MALBEC is an exoplanet model intercomparison project that belongs to the Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies framework, which aims to provide the exoplanet community with a large and diverse set of comparison and validation of models. The proposed protocol tests include a large set of initial participating RT models, a broad range of atmospheres (from hot Jupiters to temperate terrestrials), and several observation geometries, which would allow us to quantify and compare the differences between different RT models used by the exoplanetary community. Two types of tests are proposed: transit spectroscopy and direct imaging modeling, with results from the proposed tests to be published in dedicated follow-up papers. To encourage the community to join this comparison effort and as an example, we present simulation results for one specific transit case (GJ-1214 b), in which we find notable differences in how the various codes handle the discretization of the atmospheres (e.g., sub-layering), the treatment of molecular opacities (e.g., correlated-k, line-by-line) and the default spectroscopic repositories generally used by each model (e.g., HITRAN, HITEMP, ExoMol)

Large Interferometer For Exoplanets (LIFE): I. Improved exoplanet detection yield estimates for a large mid-infrared space-interferometer mission

One of the long-term goals of exoplanet science is the atmospheric characterization of dozens of small exoplanets in order to understand their diversity and search for habitable worlds and potential biosignatures. Achieving this goal requires a space mission of sufficient scale. We seek to quantify the exoplanet detection performance of a space-based mid-infrared nulling interferometer that measures the thermal emission of exoplanets. For this, we have developed an instrument simulator that considers all major astrophysical noise sources and coupled it with Monte Carlo simulations of a synthetic exoplanet population around main-sequence stars within 20 pc. This allows us to quantify the number (and types) of exoplanets that our mission concept could detect over a certain time period. Two different scenarios to distribute the observing time among the stellar targets are discussed and different apertures sizes and wavelength ranges are considered. Within a 2.5-year initial search phase, an interferometer consisting of four 2 m apertures covering a wavelength range between 4 and 18.5 μm could detect up to ~550 exoplanets with radii between 0.5 and 6 R⊕ with an integrated SNR≥7. At least ~160 of the detected exoplanets have radii ≤1.5 R⊕. Depending on the observing scenario, ~25-45 rocky exoplanets (objects with radii between 0.5 and 1.5 ⊕) orbiting within the empirical habitable zone (eHZ) of their host stars are among the detections. With four times 3.5 m aperture size, the total number of detections can increase to up to ~770, including ~60-80 rocky, eHZ planets. With four times 1 m aperture size, the maximum detection yield is ~315 exoplanets, including ≤20 rocky, eHZ planets. In terms of predicted detection yield, such a mission can compete with large single-aperture reflected light missions