In the past few decades, the number of planets discovered beyond our Solar System has grown into the thousands. At present, 4,389 exoplanets have been confirmed in 3,260 systems, with another 5,941 candidates awaiting confirmation. Thanks to numerous follow-up observations and studies, scientists have learned a great deal about the types of planets that exist in our Universe, how planets form, and how they evolve.

A key consideration in all of this is how planets become (and remain) habitable over time. In general, astrobiologists have operated under the assumption that habitability comes down to where a planet orbits within a system – within its parent star’s habitable zone (HZ). However, new research by a team from Rice University, indicates that where a planet forms in its respective star system could be just as important.

The study, which was recently published in Nature Geoscience, was led by Rice graduate student Damanveer Grewal, who was joined by multiple colleagues from the Department of Earth, Environmental, and Planetary Sciences at Rice University (including Rajdeep Dasgupta, the Maurice Ewing Professor of Earth Systems Science at Rice). Together, they looked beyond the Goldilocks Zone of stars to consider how factors involved in planetary formation would ultimately affect habitability.

A study by Rice University scientists shows that where a planet forms in a star system will play a vital role in its habitability. Credit: Rice University/Amrita P. Vyas

Basically, a star’s HZ (or Goldilocks Zone) refers to the region where an orbiting planet will experience conditions warm enough to support liquid water on its surface and a rich atmosphere – the key ingredients for life. But after taking into account the elements that go into planetary formation, Grewal and his colleagues concluded that the amount of volatile elements a planet captures and retains during formation will also determine if it becomes habitable.

Central to this is the time it takes for material to accrete from a circumsolar disk into a protoplanet and the time the protoplanet takes to differentiate into its distinct layers (a metallic core, a silicate mantle and crust, and an atmospheric envelope). The balance between these two processes is critical in determining what volatile elements a rocky planet will retain, particularly nitrogen, carbon, and water, that give rise to life.

Using Dasgupta’s high-pressure lab at Rice, the research team used nitrogen as a proxy for volatiles and simulated how protoplanets undergo differentiation. What they found was that during this process, most of a protoplanet’s nitrogen is lost from the mantle and escapes into the atmosphere. From there, the nitrogen is lost to space as the protoplanet either cools or collides with other celestial objects during the next stage of its growth.

However, if the metallic core retains enough nitrogen, it could still be significant enough that over time, it will help form an “Earth-like” atmosphere later on (where it will play an important role as a buffer gas). From this, the researchers were able to model the thermodynamic and how it affects the distribution of nitrogen between a protoplanet’s atmosphere, molten silica layers, and core.

Artist’s impression of the range of habitable zones for different types of stars. Credit:
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