The Dynamical Environment within the Habitable Zone of the Gaia-4 and Gaia-5 Planetary Systems

TL;DR

This work analyzes Gaia-4 and Gaia-5 to quantify how a massive, eccentric giant shapes the dynamical environment of their Habitable Zones. By combining Gaia astrometry with radial velocities, the authors determine true companion masses ($11.80\,M_J$ and $20.87\,M_J$) and perform $10^7$-year injection simulations of Earth-mass planets using the Mercury integrator, revealing that Gaia-4b leaves portions of the HZ unstable while Gaia-5b largely destroys stable HZ orbits. They employ the angular momentum deficit as a diagnostic of past scattering and document the prevalence of “wrecking-ball” giants beyond the OHZ, with implications for planet formation history and direct-imaging target selection. The results underscore the value of astrometry–RV synergy for robust dynamical inferences and provide practical guidance for planning future imaging surveys aimed at detecting potentially habitable worlds.

Abstract

Exoplanetary systems exhibit a broad range of architectures which, in turn, enable a variety of dynamical environments. Many of the known planetary systems do not transit the host star, and so we measure the minimum masses of their planets, making it difficult to fully assess the dynamical environment within the system. Astrometry can resolve the mass ambiguity and thus allow a more complete dynamical analysis of systems to be conducted. Gaia-4 and Gaia-5 are two such systems, whose study with radial velocities and data from the Gaia mission revealed that each star harbors a massive planet on a highly eccentric orbit. In this work, we provide the results of a dynamical analysis of each system, including calculations of the Habitable Zone (HZ), from which we show that the presence of the known companions largely exclude the presence of planets within the HZ. We discuss the diagnostics of potential past planet-planet scattering events, and the occurrence of similar systems whereby a giant planet on an eccentric orbit can substantially disrupt orbital integrity of terrestrial planets. These "wrecking ball" systems have an impact on the target selection for planned direct imaging missions that seek to identify potentially habitable environments.

Figures (5)

• Figure 1: System architectures and HZ boundaries for the Gaia-4 (left) and Gaia-5 (right) systems, showing the orbit of the planet in each case. The HZ regions are shown in green, where light green and dark green indicate the CHZ and OHZ, respectively. The scale of the figure panels are 2.5 AU and 1.5 AU along each side for Gaia-4 and Gaia-5, respectively.
• Figure 2: Percentage of the simulation that the injected Earth-mass planet survived as a function of semi-major axis for the Gaia-4 (top panel) and Gaia-5 (bottom panel) systems. As for Figure \ref{['fig:hz']}, the CHZ is shown in light green and the OHZ is shown in dark green. The periastron passage of Gaia-5b is indicated by the vertical dotted line.
• Figure 3: The mass and semi-major axis of a possible lost planet in a circular orbit whose angular momentum equals the angular momentum deficit (AMD) for each of the considered systems: Gaia-4 (solid line) and Gaia-5 (dashed line). The dots indicate the planet mass and semi-major axis of the known companions.
• Figure 4: The eccentricity distribution of giant planets ($M_p > 0.1$$M_J$) whose semi-major axis lies beyond the OHZ outer boundary (left panel). Both the shade and the size of the plotted data are logarithmically proportional to the planet mass, where dark green indicates a low mass and light green indicates a high mass. The right panel shows a subset (the wrecking ball planets) whose orbits either pass through the HZ or whose periastron distance lies within 20% of the outer OHZ boundary.
• Figure 5: Angular separation of Gaia-4b (solid line) and Gaia-5b (dashed line) from their respective host stars as a function of their orbital phase.