Combined Exoplanet Mass and Atmospheric Characterization for Accelerated Exoplanetology

TL;DR

The paper addresses the bottleneck of obtaining precise planetary masses before atmospheric studies by proposing a TS-based mass retrieval workflow that leverages small JWST atmospheric exploration programs. It combines synthetic spectra, instrument-performance tests, and retrieval analyses to show that mass constraints with $RMU$ as low as $1$–$2\%$ are achievable for high-$TSM$ targets using configurations like $NIRSPEC/PRISM$, $NIRSPEC/G395H$, and $NIRISS/SOSS$, with $RMU$ scaling approximately linearly with the transmission-spectroscopy metric $TSM$ down to $TSM\sim75$–$100$ and tolerable opacity-perturbation effects. The approach suggests that about $23\%$ of planets with existing mass constraints could reach similar or better precision via small JWST programs ($\leq2$ hours in transit), potentially increasing the sample by up to $20\%$ and enabling focused study of Neptune-sized, young, and hot-star exoplanets, while also informing future directly-imaged planets through emission-spectrum mass encoding. This strategy promises faster discovery-to-atmosphere pipelines, reduced reliance on RV facilities, and enhanced scientific returns across multiple exoplanetary populations and observational regimes.

Abstract

Today's most detailed characterization of exoplanet atmospheres is accessible via transit spectroscopy (TS). Detecting transiting exoplanets only yields their size, and it is thus standard to measure a planet's mass before moving towards their atmospheric characterization, or even the publication of their discovery. This framework, however, can act as a bottleneck for high-throughput exoplanetology. Here, we review existing applications of an alternative approach deriving exoplanet masses in small JWST atmospheric exploration programs and quantify the potential of its systematic application. We find that for $\sim$20\% of transiting exoplanets with existing mass constraints, a small JWST exploration program could yield the planetary mass with a similar -- or better -- precision. Such results suggest that proceeding directly with atmospheric exploration programs for favorable exoplanets (i.e., with a transmission spectroscopy metric, TSM, $\geq$100) could substantially reduce the time from detection to exoplanet atmospheric study and further support JWST's scientific output over its lifetime while saving up to 20\% of resources on radial-velocity (RV) facilities. Furthermore, it can substantially increase the sample of characterized planets of three distinct subpopulations (Neptune-sized, young, and hot-star exoplanets), each providing specific insights into formation and evolution processes. As the field of exoplanets increasingly turns to directly imaged planets, mastering the determination of planetary masses from atmospheric spectra will become essential.

Figures (5)

• Figure 1: Flowchart of the exoplanet detection-to-characterization framework. Current one-size-fits-all framework is shown in black, while the suggested upgrade is shown in blue. When applied to newly-detected exoplanets with TSM$\geq100$ (transmission-spectroscopy metric, Ref. Kempton2018), it speeds up the framework by months to years for tens of exoplanets while saving $\sim20\%$ of radial-velocity resources yearly (step 2 bis). For a portion of these planets---primarily rocky worlds, a degeneracy between the planetary mass and the mean molecular weight ($\mu$) will require independent radial-velocity measurements to yield the mass and identify the primary atmospheric compound (step 4 b).
• Figure 2: Exoplanet mass measurements via transmission spectroscopy.a. Synthetic transmission spectrum of WASP-193 b-like planet when observed by various instruments aboard JWST. All instruments except MIRI allow constraining the mass with precision better than 10%, with better than 1% precision obtained with NIRSPEC/PRISM. The inset figure shows the posterior distribution of the mass obtained during the fit, where the black line shows the true mass used to create the synthetic spectrum. b. Same as left with a focus on testing the mass measurements sensitivity to imperfections of opacity models. Combined, these results show that mass measurements can be reliably performed despite stellar contamination, hazes, and/or imperfect opacity models for high-TSM atmospheres.
• Figure 3: Application domain for mass measurements via small JWST atmospheric exploration programs. Size of known exoplanets with mass measurements (relative mass uncertainty, RMU, as color) versus their estimated TSM---data from Ref.exoplanet_eu. Planets with mass informed by transmission spectroscopy are labeled (e.g., V1298 TauBarat2024Barat2025 and HIP 67522 bThao2024). For planets with TSM$\geq$100, direct mass measurements via atmospheric exploration is recommended---with a caveat for the rare terrestrial worlds with TSM$\geq$100 for which a degeneracy between the atmospheric mean molecular weight ($\mu$) and the planetary mass ($M$) may require independent mass insights from RV. While 734 exoplanets have 5$\sigma+$ mass constraints (i.e., RMU$<20\%$), 171 of them (23$\%$) reach a similar or better precision (RMU$\leq10\%$) with a small JWST exploration program ($\leq$2hrs of in-transit data). That number reaches 284 when accounting for all known exoplanets, which is equivalent to a third of the current mass-constrained population.
• Figure 4: The exoplanet population ripe for mass measurements via transmission spectroscopy.a. Size of known exoplanets with RMU $\leq10\%$ (or expected to be such with TS) versus their estimated TSM with equilibrium temperature as color---data from Ref.exoplanet_eu. Planets with RV-based mass-measurements shown as stars, for which TS is expected to perform better than RV as empty circles, for which TS-based mass is within reach while no RV-based mass exist as filled circles---see \ref{['tab:Table1']}. b. Same sample as a. but showing properties of the exoplanets' host stars. c. Histogram of exoplanet size for the sample with existing RV-based masses and the sample without mass but within reach of TS. d. Same as c. but for the exoplanets' host-star age. e. Same as c. but for the exoplanets' host-star effective temperature. f. Same as c. but for the exoplanets' host-star V magnitude. It shows that three distinct subpopulations may particularly benefit from this new framework, thereby consolidating the insights to be gained via their study into formation and evolution processes.
• Figure 5: Towards mass measurements from emission spectroscopy. Synthetic emission spectra of a Jupiter-sized planet for 3 different masses (0.1, 1, and 10 M$_{\rm Jup}$), all other properties unchanged. It highlights how the mass of a planet shapes its emission spectrum---esp. the amplitude of absorption features---via the density gradient along the light path; the lower the planetary mass, the larger atmospheric scale height, the larger absorption features, and the shallower the pressure levels probed. As size-brightness degeneracies will exist, the bottom panel presents the difference between the normalized flux ($\Delta\hat{F_\nu}$). Information about a planet's mass is thus recorded in its emission spectrum, which will be of relevance for future directly-imaged planets---most of which will be inaccessible with RV facilities.