A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization (1805.03671v2)

Abstract: A key legacy of the recently launched TESS mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. JWST, although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as ARIEL require the discovery and confirmation of several hundred additional sub-Jovian size planets (R_p < 10 R_Earth) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based ELTs will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al. (2015), we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of $\sim300$ high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.

• The paper introduces two novel metrics, TSM and ESM, to quantify the signal-to-noise ratio for exoplanet atmospheric observations.
• It applies these metrics to simulated TESS data, identifying about 300 high-quality candidates optimal for JWST and ARIEL follow-ups.
• The study emphasizes the need for precise radial velocity measurements to confirm planetary masses and refine atmospheric characterization.

A Framework for Prioritizing TESS Planetary Candidates for Atmospheric Characterization

This paper presents a systematic framework aimed at optimizing the selection of transiting exoplanets discovered by the Transiting Exoplanet Survey Satellite (TESS) for atmospheric characterization. The primary objective is to identify optimal candidates for follow-up atmospheric studies using the James Webb Space Telescope (JWST) and future astrobiological missions like ARIEL. The framework leverages two analytic metrics that quantify expected signal-to-noise ratios (S/N) for transmission and thermal emission spectroscopy, allowing for the prioritization of TESS candidates based on these criteria.

Key Aspects and Methodology

The researchers propose two novel metrics: the Transmission Spectroscopy Metric (TSM) and the Emission Spectroscopy Metric (ESM). These metrics are calculated for each TESS planet candidate to estimate their potential observational quality within the field of atmospheric studies.

• Transmission Spectroscopy Metric (TSM): This metric focuses on transmission spectroscopy and is proportional to factors influencing detectability such as the planetary radius and equilibrium temperature. The TSM is employed to assess the viability of transmission spectroscopy across several exoplanet categories, particularly targeting sub-Jovian planets as they hold prospects for higher S/N in atmospheric features due to their size and proximity to their host stars.
• Emission Spectroscopy Metric (ESM): This metric applies to assessing the emission spectroscopy's observational potential, especially for smaller, terrestrial-type planets. ESM computation considers variables like the planet’s brightness and temperature to evaluate their emission signal at mid-infrared wavelengths typical to JWST observations.

Results and Observations

Utilizing these metrics, the paper delineates certain threshold values critical for identifying candidates favorable for atmospheric characterization. Approximately 300 high-quality candidates emerged as potentially ideal targets from the simulated TESS data. The paper further highlights the statistical importance of extending the characterization efforts beyond giants, focusing on smaller, potentially habitable, rocky planets.

The metrics and selection criteria align flexibly across a range of planetary sizes, assimilating various uncertainties related to host star brightness and atmospheric composition. Importantly, the reliance on observational data from simulated TESS catalogs underscores their adaptability to different datasets, as evidenced by an investigation utilizing Barclay’s TESS catalog, which yielded a comparably consistent sample of prospective candidates.

Implications and Future Directions

The authors underscore the necessity for comprehensive follow-up, primarily through radial velocity (RV) measurements, to attain precise planetary masses—an essential component before proceeding with atmospheric studies. The larger ambition of constructing a statistically robust exoplanet atmospheric sample predicates on these precise initial measurements, crucial for deciphering trends like atmospheric metallicity distributions among species spanning the planetary size spectrum.

The future trajectory of such research is promising, particularly with advancements in scenarios involving RV instrumentation and methodologies, which hold potential to rapidly scale observations. The paper’s methodology and metrics set a course not just for JWST but also for subsequent platforms such as ARIEL and other ground-based telescopic efforts, indicating a future of intricate and enriching exoplanetary studies aimed at unraveling the complexities of atmospheric dynamics across varied exoplanet types.

In conclusion, this framework is pivotal for leveraging TESS's discoveries most effectively, enabling an era of insightful atmospheric characterization that can broaden our understanding of planetary systems and their potential for hosting life.