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| Paper: |
Identifying Rocky Planets and Water Worlds Among Sub-Neptune-sized Exoplanets with the Habitable Worlds Observatory |
| Monograph: |
10, HWO25 Proceedings Part I: Community Science Case Development Documents |
| Page: |
429 |
| Authors: |
Renyu Hu; Michiel Min; Max Millar-Blanchaer; Jacob Lustig-Yaeger; Tyler Robinson; Jennifer Burt; Athena Coustenis; Mario Damiano; Chuanfei Dong; Courtney Dressing; Luca Fossati; Stephen Kane; Soumil Kelkar; Tim Lichtenberg; Jean-Baptiste Ruffio; Dibyendu Sur; Armen Tokadjian; Martin Turbet |
| DOI: |
10.26624/PVXA1224 |
| Abstract: |
Astronomers are debating whether the plentiful “sub-Neptune” exoplanets—worlds a bit larger than Earth
but smaller than Neptune—are predominantly rocky planets, water-rich “ocean worlds,” or gas-enshrouded
mini-Neptunes. This question is crucial because such sub-Neptune-sized planets are among the most common
in our galaxy, yet we have no analog in our own solar system, making them a key to understanding planet
formation and diversity. It also directly impacts the search for habitable worlds: larger-than-Earth planets
with solid surfaces or oceans could support life, whereas gas-rich mini-Neptunes likely cannot. However,
distinguishing these types using only a planet’s mass and radius is very challenging, because different
compositions can produce similar densities, leaving a world’s nature ambiguous with current data. The
proposed Habitable Worlds Observatory (HWO), a future NASA flagship telescope, offers a solution. HWO
could directly image and spectroscopically analyze starlight reflected from 50∼100 sub-Neptunes around
nearby stars, aiming to reveal their atmospheric compositions and potential surfaces. Using visible and
near-infrared spectroscopy along with sensitive polarimetry, HWO would detect atmospheric gases (such as
water vapor, methane, and carbon dioxide) and search for telltale surface signatures, including rock absorption
features and the characteristic reflectivity patterns of oceans. By analyzing these signals, we could determine
whether sub-Neptunes are large rocky planets or water worlds rather than gas-dominated mini-Neptunes.
Crucially, expanding the search beyond Earth-sized planets to include these abundant sub-Neptunes may
uncover entirely new classes of potentially habitable worlds, directly advancing HWO’s mission to identify
and characterize planets that could support life. |
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