Water on Exoplanets: How Rocky Planets Could Acquire Oceans Through Magma Ocean Chemistry

Geology Bites host Oliver Strimpel speaks with Anat Shahar of the Carnegie Institution for Science about how rocky planets might acquire water during their early formation, particularly through magma oceans surrounded by hydrogen-rich atmospheres. The discussion covers why liquid water, radiation shielding, and nutrient recycling matter for habitability, how water can form without external sources, and what this implies for detecting life beyond Earth. The conversation also touches on atmospheric observations, biosignatures, and the challenges of ruling out abiotic sources when interpreting potential signs of life.

Key takeaways include the idea that water production could be a natural outcome of planetary formation on planets above a certain mass, the role of plate tectonics and volcanism in nutrient cycling and climate stability, and the importance of careful interpretation of biosignatures and water signals in exoplanet atmospheres.

Introduction and scope

The podcast features Oliver Strimpel and Anat Shahar discussing how habitability on rocky exoplanets can arise from processes during planet formation, with a focus on water delivery. Shahar explains that an Earth-like planet would need liquid water, protection from radiation, and nutrient recycling to sustain life over geological timescales. She emphasizes that Earth’s example is our guide, but warns that life could emerge under a broader set of conditions on other worlds.

Water as a fundamental prerequisite

Water is highlighted not only as a solvent for nutrients but also as a critical factor in supporting cellular processes. While Earth’s life is water-dependent, the conversation acknowledges the Earth-centric bias and notes that water’s role in exoplanet habitability remains central to the search for life.

“Water is essential because it can bring nutrients into a cell and remove waste, and no life has yet been found without liquid water.” - Anat Shahar

Shielding and stability

Protection from stellar radiation is discussed in terms of magnetic fields and atmospheric thickness. Shahar describes alternative shielding mechanisms, such as Venus-like thick atmospheres or subsurface life, and notes that these protections help maintain habitable conditions on planetary surfaces over long timescales.

“There are multiple shielding strategies besides a magnetic field, including very thick atmospheres or deep subsurface environments.” - Anat Shahar

Recycling nutrients and climate stability

The necessity of nutrient recycling is linked to surface replenishment, ecological balance, and climate regulation through plate tectonics or volcanism. The host and guest discuss how tectonic and volcanic processes contribute to long-term climate stability, enabling life to persist on geologic timescales.

“Plate tectonics and volcanism together help recycle nutrients and stabilize climate over billions of years.” - Anat Shahar

Water formation during planet formation

Shahar presents both theoretical and experimental work showing that water can form on rocky planets during the magma ocean stage when surrounded by a hydrogen-rich atmosphere. Laboratory experiments using high-pressure, high-temperature conditions reveal two pathways for water production: reduction of iron oxides and dissolution of hydrogen into the silicate magma ocean, suggesting water generation could be a natural outcome of planetary formation.

“Water formation is a natural consequence of planetary formation in a magma ocean with a hydrogen-rich gas.” - Anat Shahar

Planet size, hydrogen retention, and water fate

Experiments indicate that planets as small as roughly 0.4 Earth masses could form water via these processes, with larger planets producing more water due to higher pressures and longer molten states. Hydrogen escape is discussed as a challenge, mitigated by thicker atmospheres on bigger planets, allowing water-formation reactions to proceed for longer periods.

Detection and biosignatures

The podcast turns to how we might detect life and water on exoplanets. Magnetic fields, atmospheric signatures, and biosignatures are examined, along with the risk of abiotic false positives. Water vapor detections via planetary transits and spectroscopy (for example, with the James Webb Space Telescope) are highlighted as encouraging signs but not definitive proof of life.

“Biosignatures can be created abiotically, so we must rule out false positives before claiming life.” - Anat Shahar

Future directions and other elements

Shahar notes the need to study how carbon, nitrogen, oxygen, and other key elements move through exoplanet interiors and atmospheres under exoplanetary conditions. Her team plans to extend their experiments to these elements, explaining that understanding their behavior will illuminate the broader chemical evolution of rocky planets and habitability potential.

Conclusion

The conversation closes with an emphasis on water’s potential ubiquity on certain rocky exoplanets and the importance of continued observations, experimental work, and careful interpretation of atmospheric data to search for life beyond Earth.