Probing the Origin of Water in Planets within Habitable Zones by HWO

TL;DR

The paper tackles the origin of water on planets in habitable zones, a key factor in planetary habitability and the prevalence of life-supporting environments. It proposes the Habitable Worlds Observatory (HWO) as a means to test two competing hypotheses: whether water abundance in HZ planets correlates with the presence of outer planets or shows no correlation, within a dynamic formation framework that includes pebble drift and planetary migration. It outlines the physical parameters required to discriminate these hypotheses—such as sample size, precise mass-radius measurements, potential atmospheric water vapor detections, and outer-planet architectures—and specifies two observational channels: direct imaging and near-infrared spectroscopy, with synergy from ELT and multi-epoch orbital constraints. The findings establish a pathway for guiding HWO’s design and observational strategy to illuminate water delivery pathways and inform assessments of planetary habitability.

Abstract

How do habitable environments arise and evolve within the context of their planetary systems? This is one fundamental question, and it can be addressed partly by identifying how planets in habitable zones obtain water. Historically, astronomers considered that water was delivered to the Earth via dynamical shake-up by Jupiter, which took place during the formation and post-formation eras (e.g., $\lesssim 100$ Myr). This hypothesis has recently been challenged by a more dynamic view of planet formation; planet-forming materials move in protoplanetary disks via various physical processes such as pebble drift and planetary migration. \textit{Habitable Worlds Observatory} (HWO) will open a new window to address this important, but difficult question by discovering and characterizing Earth-like exoplanets around G-type stars. In this article, we consider two possible working hypotheses: (1) the abundance of water on planets in habitable zones has \textit{any} correlation with the presence of outer planets; and (2) the abundance of water on planets in habitable zones has \textit{no} correlation with the presence of outer planets. We discuss what physical parameters need to be measured to differentiate these two hypotheses and what observational capabilities are desired for HWO to reliably constrain these physical parameters.