Probing the Limits of Habitability: A Catalog of Rocky Exoplanets in the Habitable Zone

TL;DR

The study builds a catalog of rocky exoplanets in the Habitable Zone (HZ) by integrating Gaia DR3 stellar parameters with the NASA Exoplanet Archive (NEA), updating stellar temperatures and radii to refine HZ boundaries. It distinguishes earthy-like planets (rocky) using a radius cutoff of $R<2\,R_\oplus$ (or $M<5\,M_\oplus$ for non-transiting cases) and computes both empirical HZ limits and a narrower 3D-HZ, including time-averaged irradiation for eccentric orbits. The authors identify 45 rocky worlds in the empirical HZ and 24 in the 3D-HZ (27 and 15 transiting, respectively), and provide a comprehensive catalog with transmission spectroscopy metric (TSM), maximum angular separation $\theta$, and star–planet contrast to prioritize observations for JWST, ELTs, HWO, and LIFE. They analyze the impact of Gaia DR3 versus NEA data on HZ membership, document the oldest known HZ rocky planets, and discuss uncertainties and stellar activity that can influence habitability assessments. The work offers a practical framework and ready-to-use target lists to guide current and future observational campaigns and mission design aimed at probing the limits of exoplanet surface habitability.

Abstract

While most of the 6000 discovered exoplanets are highly unlike the Earth, the first rocky worlds in the Habitable Zone (HZ) provide intriguing targets for the search for life in the cosmos. As detections increase, it is critical to test the empirical HZ as well as its limits using known exoplanets. However, there is not yet a list of rocky worlds that observers can use to test the limits of surface habitability. We analysed data from Gaia DR3 and the NASA Exoplanet Archive (NEA) of all known exoplanets, identifying future targets to test limits of habitability through i) orbits near the edges of the HZ, ii) similar irradiation environments to modern Earth, and iii) large eccentricities. We prioritize targets for transmission observations, light curve measurements, and direct imaging, identify the oldest HZ rocky worlds based on the NEA and complementary literature data, and provide theoretical limits for the empirical HZ and a 3D-HZ for each system. Our analysis shows 45 rocky worlds in the empirical HZ and 24 in a narrower 3D-HZ. For context, we compare their demographics to those of the full catalog of exoplanets in the NEA. The resulting list of rocky exoplanet targets in the HZ will allow observers to shape and optimize search strategies with space- and ground-based telescopes -- such as the James Webb Space Telescope (JWST), Extremely Large Telescope (ELT), Habitable Worlds Observatory (HWO), and LIFE -- and design new observing strategies and instruments to explore these worlds, addressing the question of the limits of exoplanet surface habitability.

Figures (5)

• Figure 1: Color-magnitude diagram of all exoplanet host stars (top) and rocky HZ exoplanet host stars (bottom) compared to all stars with Gaia DR3 data within 100 pc.
• Figure 2: Comparison of the subset of rocky HZ exoplanets (blue) to all known exoplanets (grey): mass vs. radius (top), eccentricity vs. incident flux (middle), minimum mass vs. incident flux (bottom left), and radius vs. incident flux (right). HZ planets are shown in blue, planets in the 3D HZ limits in dark blue, and all other known exoplanets in grey. Rocky planets are solid-color, while non-rocky planets are semi-transparent. Planets with only upper mass limits are not shown in the mass-radius plot.
• Figure 3: All known exoplanets shown in terms of their host star's temperature and the incident stellar flux they receive. Transiting rocky exoplanets are shown as circles, and planets where only minimum mass is known as diamonds. Exoplanets in the HZ are shown in blue, with exoplanets $\leq 2\,R_{\oplus}$ and $\leq 5\,M_{\oplus}$ shown as smaller symbols in solid colors. Empirical HZ limits and the 3D inner HZ limit are shown as dashed lines. A solid light grey line indicates similar irradiation to modern Earth for different stellar temperatures.
• Figure 4: Apparent angular separation of rocky exoplanets in the HZ plotted versus contrast ratio between host star and planet (top) and versus transmission spectroscopy metric (TSM) (bottom). Rocky planets in the HZ with nominal values are plotted as solid-color circles, and rocky planets in the HZ including measurement uncertainties (in irradiation and planet size) are plotted as triangles. Other HZ planets are larger, semi-transparent circles. A TSM value of 10 is shown as a green dashed line.
• Figure 5: Tabulated age estimates for rocky HZ planets vs. stellar irradiation up to 2 times modern Earth's. Dot sizes correspond to planet radii or minimum masses and colours to host star $\bm{T}_\text{eff}$. Planets that require flux uncertainties to be in the HZ are included, but planets that could be rocky based on size uncertainty are excluded. Earth, Venus, and Mars are included for reference.