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A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization
Eliza M. -R. Kempton, Jacob L. Bean, Dana R. Louie, Drake Deming, Daniel D. B. Koll, Megan Mansfield, Jessie L. Christiansen, Mercedes Lopez-Morales, Mark R. Swain, Robert T. Zellem, Sarah Ballard, Thomas Barclay, Joanna K. Barstow, Natasha E. Batalha, Thomas G. Beatty, Zach Berta-Thompson, Jayne Birkby, Lars A. Buchhave, David Charbonneau, Nicolas B. Cowan, Ian Crossfield, Miguel de Val-Borro, Rene Doyon, Diana Dragomir, Eric Gaidos, Kevin Heng, Renyu Hu, Stephen R. Kane, Laura Kreidberg, Matthias Mallonn, Caroline V. Morley, Norio Narita, Valerio Nascimbeni, Enric Palle, Elisa V. Quintana, Emily Rauscher, Sara Seager, Evgenya L. Shkolnik, David K. Sing, Alessandro Sozzetti, Keivan G. Stassun, Jeff A. Valenti, Carolina von Essen
TL;DR
Atmospheric missions need many well-characterized transiting planets, while TESS candidates require efficient prioritization and resource-intensive mass follow-up. The paper introduces analytic transmission and emission metrics, applies thresholds to simulated TESS yields, and identifies approximately 300 high-quality targets, while noting scope and modeling limitations.
Problem
Atmospheric characterization requires hundreds of additional small transiting planets, but TESS candidates must be rapidly prioritized and confirmed with resource-intensive RV mass measurements.
Method
The paper develops analytic transmission and thermal-emission metrics and applies threshold criteria to simulated TESS detections, using expected spectroscopy signal-to-noise.
Results
Approximately 300 high-quality atmospheric-characterization targets are identified across planet-size bins, extending to Earth-size potentially habitable worlds.
Takeaways & Limitations
The proposed metrics and thresholds are intended to prioritize TESS candidates for rapid RV follow-up and atmospheric investigations with JWST, ARIEL, and ELTs.
Takeaways & Limitations
The emission analysis focuses on terrestrial planets because a full-scale MIRI secondary-eclipse S/N estimate for the Sullivan et al. catalog is beyond the paper’s scope.
Abstract
from arXiv · showhide
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.
1. INTRODUCTION
TESS is expected to supply bright, small-host transiting planets well suited to atmospheric characterization, but substantial follow-up—especially RV mass measurement—is needed quickly. The paper proposes threshold criteria based on simulated TESS yields to prioritize candidates for this follow-up.
- TESS is designed to detect many hundreds of sub-Jovian planets that are better atmospheric-characterization targets than Kepler discoveries.
- JWST and ELTs are expanding atmospheric-observation capabilities, and TESS planets are intended to help realize that science program.
- Few known planets smaller than 10 R⊕ are currently suitable for transmission spectroscopy, while future atmosphere missions require hundreds spanning a broad mass and radius range.
- TESS candidates require host-star characterization, improved ephemerides, planetary validation or confirmation, and planet-mass measurement before atmospheric observations.
- Precise RV masses are resource-intensive but important for interpreting transmission spectra and predicting secondary-eclipse times, motivating rapid prioritization.
- The paper develops threshold criteria from the simulated Sullivan et al. (2015) TESS catalog to identify candidates most amenable to atmospheric characterization and rapid RV follow-up.
2. SAMPLE SELECTION
The study constructs transmission and emission samples from simulated TESS detections, selecting planets by size, temperateness, and predicted atmospheric-observation signal-to-noise. It emphasizes terrestrial emission targets while acknowledging instrument and scope limitations.
- SAMPLE SELECTION: The analysis considers two transmission samples—broad planet sizes and small temperate planets—and one terrestrial emission-spectroscopy sample.
- 2.1. Statistical Sample: A statistical transmission sample is built from four planet-size bins in the Sullivan et al. (2015) simulated TESS catalog.
- 2.1. Statistical Sample: Transmission targets are initially ranked using predicted JWST/NIRISS transmission spectroscopy S/N, with NIRISS chosen because simulations already exist and it provides high information per observing time.
- 2.1. Statistical Sample: 287 planets with Rp < 10 R⊕ are selected by taking the top 100 from each sub-Neptune bin, 50 sub-Jovians, and 37 terrestrial planets.
- 2.2. Small Temperate Sample: The small temperate sample contains 60 planets with 0.2–2.0 S⊕ insolation and Rp < 2.0 R⊕, followed by down-selection using transmission detectability.
- 2.3. Emission Sample: The emission sample estimates terrestrial-planet secondary-eclipse S/N by scaling to a well-studied example planet, with the ESM using 7.5 µm and host-star K-band flux.
- 2.3. Emission Sample: The analysis focuses on terrestrial emission spectroscopy because a full MIRI catalog calculation for larger planets is beyond the paper’s scope.
3. ANALYSIS
The analysis defines analytic transmission and emission metrics that approximate atmospheric-observation S/N and calibrates them against detailed models and simulations. It also states the assumptions and limitations underlying these metrics.
- 3.1. Transmission Metric: The transmission spectroscopy metric estimates relative S/N from planet radius, atmospheric scale height, host-star radius, and stellar brightness under cloud-free atmospheres.Its normalization is chosen to match Louie et al. (2018) JWST/NIRISS 10-hour simulations.
- 3.1. Transmission Metric: The transmission metric omits mean molecular weight, implicitly assigning planets within each size bin the same atmospheric composition.The adopted compositions differ by size: μ = 18 for Rp < 1.5 R⊕ and μ = 2.3 for Rp > 1.5 R⊕.
- 3.2. Emission Metric: The emission spectroscopy metric estimates the S/N of a JWST secondary-eclipse detection near 7.5 µm using planetary and stellar blackbody emission, dayside temperature, and K-band brightness.The metric is applied to terrestrial-composition planets and uses the MIRI LRS bandpass.
- 3.2. Emission Metric: The planetary blackbody assumption limits the emission metric because molecular absorption can change mid-infrared emitted flux by an order of magnitude.The authors describe the metric as a relative broadband observability estimate rather than a detailed atmospheric-spectrum prediction.
- 3.2. Emission Metric: The dayside temperature used by the emission metric is set to Tday = 1.10 × Teq to account for secondary-eclipse observations of planetary daysides.The correction factor is theoretically derived from atmospheric energy-transport calculations.
4. RESULTS
The transmission spectroscopy metric tracks simulated JWST/NIRISS signal-to-noise for most targets, supporting threshold-based prioritization, while bright-star duty cycles and uncertain planet populations limit interpretation. An emission metric similarly identifies promising small planets, but its predictions are affected by detector and stellar-brightness constraints.
- Statistical Sample: For targets with mJ > 9, the analytic transmission metric tracks Louie et al. (2018) NIRISS S/N with little scatter.Targets with mJ < 9 show a different relationship because the metric does not capture JWST observational duty-cycle reductions.
- Statistical Sample: Bright-star duty-cycle reductions are not corrected in the general transmission metric, although multiplying by the square root of the duty cycle can account for them.The authors retain the uncorrected metric because bright host stars also offer observational benefits.
- Statistical Sample: The proposed cutoffs prioritize the top quintile of terrestrial planets, the top 100 planets in larger bins, and the top 50 sub-Jovian planets.The sub-Jovian recommendation reflects a sharp TSM decline and the relative scarcity of such targets at TESS-sensitive orbital periods.
- Small Temperate Sample: The analytic TSM performs well for small temperate planets except for the small number orbiting stars brighter than mJ = 9.The analysis evaluates both empirical mass-radius masses and Earth-like compositions with Zeng et al. (2016) masses.
- Small Temperate Sample: A TSM value of ∼10 is proposed as a starting threshold for potentially habitable planets identified by TESS.Candidates substantially above this value should receive high-priority atmospheric follow-up, while thresholds should be reevaluated after the first year of actual TESS yields.
- Emission Sample: The emission sample uses GJ 1132b’s ESM = 7.5 as the cutoff for selecting top small-planet JWST emission targets.All 20 identified planets have higher equilibrium temperatures than GJ 1132b, but the ESM can overestimate real S/N for very bright or faint host stars.
5. DISCUSSION AND CONCLUSION
The paper proposes analytic TSM and ESM thresholds to prioritize TESS atmospheric-characterization targets and advance them for RV follow-up. Applying these criteria yields roughly 300 transmission targets, while predictions remain sensitive to catalog assumptions and observational challenges.
- Selection criteria: TSM > 10 for Rp < 1.5 R⊕ and TSM > 90 for 1.5 < Rp < 10 R⊕ should select high-quality transmission targets, with TSM = 10 for putative habitable-zone planets.For terrestrial-planet emission spectroscopy, the recommended threshold is ESM = 7.5.
- Predicted samples: ∼300 new ideal transmission-spectroscopy targets are expected from TESS after applying the proposed metric cuts.An independent Barclay et al. (2018) catalog produces a similar statistical sample of 250–300 planets, but with an altered radius distribution.
- Selection criteria: The proposed TSM and ESM thresholds provide a primary way to identify TESS candidates for JWST atmospheric characterization and can also guide targets for ARIEL, ELTs, and K2.Ground-based thresholds may need revision because those observations are more sensitive to host-star brightness.
- Caveats: The metrics and predicted S/N depend on assumptions about atmospheric mean molecular weight and clouds, while higher molecular weights or aerosols can reduce expected transmission S/N.Additional prioritization should consider RV amplitude, stellar activity, false positives, and JWST observability.
- RV follow-up: The predicted best-target RV semi-amplitudes mostly range from 1–10 m s−1, but small temperate planets remain challenging because of small signals and faint host stars.Measuring masses for ∼300 best targets is estimated to require approximately 400 observing nights.