Most stars form in massive clusters bathed in harsh ultraviolet radiation, yet almost everything we know about planet formation comes from studying a handful of nearby, sheltered discs that are the exception, not the rule. This matters because the ultraviolet environment drives "external photoevaporation" — winds that strip gas and dust from protoplanetary discs, slashing their mass, radius, and lifetime. Without accounting for this, our picture of how most planets form is fundamentally incomplete. The project builds a theoretical framework linking star formation in giant molecular clouds down to the chemistry of dispersing discs, using 3D radiation hydrodynamic models to predict wind structure and mass-loss rates across different UV environments. It will also model, for the first time, how external winds interact with internal winds driven by the host star. This is fundamental science with no immediate practical application. But understanding the most common planet formation pathway shapes where and how we search for exoplanets, and could eventually explain why some stellar systems produce rocky worlds while others yield gas giants — knowledge that quietly underpins humanity’s place in the cosmos.
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Our understanding of planet formation is heavily influenced by observations of protoplanetary discs within 150pc of the Sun. Their proximity makes them easiest to detect and resolve, however, these discs are actually atypical. They are in sparse stellar clusters without massive stars, and so the radiation environment is weak. Most stars form in massive clusters, with OB stars that emit copious UV radiation. This drives "external photoevaporation" (EP) winds from discs, resulting in a rapid reduction of their mass, radius and lifetime. The radiation environment could hence control the resulting exoplanets in stellar clusters and this is being missed in our focus on nearby discs. However, understanding the role of UV environment requires a huge chain of physics and astrophysics, from giant molecular clouds and star formation, down to the radiation hydrodynamic chemical models of dispersing discs. We cannot understand typical planet formation without the necessary theoretical framework for EP, verified by observations. This proposal will establish that theoretical framework. We will develop new state of the art 3D radiation hydrodynamic and photodissociation region chemistry calculations to determine the structure and mass loss rate of EP winds in a range of UV environments. Understanding EP in isolation is just part of the story, with internal winds driven by the host star. We will develop the first models of the interplay between internal and external winds to understand the combined mass loss and observational implications. The connection to star formation is also vital. Stars form over time in clusters and begin their lives embedded, which can shield discs from EP. We will determine how giant molecular clouds collapse and stellar feedback in the cloud sets the role of EP. Our theoretical models will provide predictions to drive observational programs. This work is necessary for understanding the most common disc evolution and hence planet formation scenarios.
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