Preferred mechanism of planet formation

Determine whether core accretion or gravitational instability is the preferred mechanism of planet formation in young planetary systems, focusing on establishing which scenario best explains observational evidence from directly imaged planets and protoplanets.

Background

The chapter contrasts the two leading theories for giant planet formation: core accretion and gravitational instability. It highlights how direct imaging uniquely probes young, wide-separation planets and protoplanets, providing spectra and system architecture that inform formation scenarios. Despite advances, the author notes that deciding which mechanism predominates across planetary systems remains unresolved.

Direct imaging captures photons from young and actively forming planets, and observations of systems like HR 8799, β Pictoris, and PDS 70 reveal diverse environments and dynamical histories. These data are central to testing formation models but have not yet yielded a definitive conclusion about the dominant mechanism.

References

Direct imaging provides then a whole picture of a newly formed system, giving the opportunity to investigate which is the preferred mechanism of planet formation, a debate that still remains open.

— Direct imaging of exoplanets  (2404.05797 - Zurlo, 2024) in Section 7 (Conclusions and future perspectives)

Another possibility is that the observed GEMS are the result of disk fragmentation (A. P. Boss 2006; A. P. Boss & S. Kanodia 2023), although the efficiency of this mechanism around low-mass stars remains unconstrained.

— The metallicity of giant exoplanets around low-mass stars and how Ariel can help  (2609.04944 - Helled et al., 4 Sep 2026) in Section 1, page 2

How these substellar companions form and how their formation pathways differ from exoplanets is still a major question in astronomy.

— The Dyn-Atmo Survey: High-contrast Imaging Spectroscopy of the Substellar Companion HD 13724 B with the JWST NIRSpec IFU  (2609.11827 - Hoch et al., 10 Sep 2026) in Section 6.3, “Formation”