Hic Sunt Dracones: Uncovering Dynamical Perturbers Within the Habitable Zone

Abstract

The continuing exploration of neighboring planetary systems is providing deeper insights into the relative prevalence of various system architectures, particularly with respect to the solar system. However, a full assessment of the dynamical feasibility of possible terrestrial planets within the Habitable Zones (HZ) of nearby stars requires detailed knowledge of the masses and orbital solutions of any known planets within these systems. Moreover, the presence of as yet undetected planets in or near the HZ will be crucial for providing a robust target list for future direct imaging surveys. In this work, we quantify the distribution of uncertainties on planetary masses and semi-major axes for 1062 confirmed planets, finding median uncertainties of 11.1% and 2.2%, respectively. We show the dependence of these uncertainties on stellar mass and orbital period, and discuss the effects of these uncertainties on dynamical analyses and the locations of mean motion resonance. We also calculate the expected radial velocity (RV) semi-amplitude for a Neptune-mass planet in the middle of the HZ for each of the proposed Habitable Worlds Observatory target stars. We find that for more than half of these stars, the RV semi-amplitude is less than 1.5 m/s, rendering them unlikely to be detected in archival RV data sets and highlighting the need for further observations to understand the dynamical viability of the HZ for these systems. We provide specific recommendations regarding stellar characterization and RV survey strategies that work toward the detection of presently unseen perturbers within the HZ.

Figures (3)

• Figure 1: Percentage uncertainties for the planetary masses (left column) and semi-major axes (right column) of our confirmed planet list as a function of their respective parameters (top row; panels A and B), stellar mass uncertainties (middle row; panels C and D), and orbital period uncertainties (bottom row; panels E and F). Planets detected only in RV surveys are denoted as cyan circles, while those detected in both RV and transit surveys are denoted as maroon diamonds. The power law relations between stellar and planetary masses and the semi-major axis, orbital period, and stellar mass (see Equations \ref{['eqn:mp']} and \ref{['eqn:a']}) are evident as diagonal crowdings of points that set the lower limit in the planet mass and semi-major axis uncertainties. For stars with mass uncertainties above $\sim$20%, the stellar mass uncertainty dominates the planet mass determination, rather than the precision of the RV semi-amplitude measurement. Orbital period uncertainties, however, play only a minor role in the planet mass uncertainties. When calculating the semi-major axis, orbital period uncertainties beyond $\sim$10% become the dominant source of error but the stellar mass uncertainty does not produce a distinct boundary identifying when it begins dominating the semi-major axis determination.
• Figure 2: Percentage uncertainties for the planetary masses (left column) and semi-major axes (right column) of our confirmed planet list as a function of host star brightness. Planets detected only in RV surveys are denoted as cyan circles, while those detected in both RV and transit surveys are denoted as maroon diamonds. The impact of transit surveys, which can more efficiently detect planets around fainter stars, is notable but neither planetary uncertainty shows a significant trend with host star magnitude.
• Figure 3: The RV semi-amplitude of a Neptune-mass planet in the middle of the OHZ for each of the stars on the mamajek2024 HWO target list, shown as a function of stellar effective temperature (left panel) and as a histogram (right panel). The vertical dashed line indicates the Kraft break at T$_{\rm{eff}}$ = 6250 K, beyond which precision RV measurements become increasingly challenging due to the rapid rotation of the stars and the associated broadening of the stars' absorption lines. The majority these hypothetical HZ Neptunes would have RV semi-amplitudes below 1.5m s$^{-1}$ making them challenging detections, especially at the 100+ day orbital periods of these HZs. Long term, dedicated EPRV survey efforts would be required to detect these potential perturbers and determine what portions of each star's HZ remains dynamically viable for Earth analog planets. For the hottest stars, similar surveys via precision astrometry could provide complementary insights into the star's HZ inhabitants.