Observational signatures
Several measurable features point to planet engulfment when stars evolve off the main sequence. The most commonly reported indicator is lithium enhancement in the stellar atmosphere, because lithium is normally destroyed in stellar interiors but can be supplied by a disrupted planet or produced through engulfment-triggered internal mixing. G. Israelian Instituto de Astrofísica de Canarias reported anomalous lithium isotopes in the planet-hosting star HD 82943, a finding widely discussed as direct evidence of recent accretion of planetary material. Infrared excess and transient dust emission around evolved stars are additional direct signatures: disrupted planetary bodies can produce warm circumstellar dust detectable with mid-infrared photometry and spectroscopy. K. Adamów Nicolaus Copernicus Astronomical Center has studied populations of Li-rich giants and noted correlations between lithium abundance and other unusual properties consistent with external pollution.
Other signatures include stellar spin-up, where the deposition of orbital angular momentum from an infalling planet increases surface rotation and magnetic activity, and chemical pollution in refractory elements that preferentially reside in rocky planets. High-resolution spectroscopy revealing enhanced refractory-to-volatile element ratios can therefore flag past engulfment. Asteroseismic changes and unexpected chromospheric emission can also occur when engulfment perturbs stellar convection zones. Not every lithium-rich giant or fast rotator is an engulfment case; internal stellar processes can mimic some of these signals.
Causes and consequences
Engulfment typically results from tidal orbital decay as a star expands on the red giant branch or asymptotic giant branch, bringing close-in planets inside the stellar envelope. The physical consequences include localized heating, mixing that can dredge up fresh material, and angular momentum transfer that alters rotational and magnetic properties. Theoretically, these outcomes were anticipated in models that explore interactions between stars and substellar companions; Mario Livio Space Telescope Science Institute has written extensively on the astrophysical implications of such accretion events.
Observational consequences matter for broader astrophysical inference. Chemical pollution can bias stellar metallicity estimates used in galactic archaeology and exoplanet demographics, and increased mass loss following engulfment can modify late-stage stellar evolution and circumstellar environments. In a human and cultural context, understanding engulfment informs predictions for the Solar System’s long-term fate and the survival prospects of exoplanetary systems across different stellar populations and galactic environments. Detecting a combination of lithium enrichment, infrared dust signatures, rotation anomalies, and refractory element excesses together gives the most robust observational case for recent planet engulfment.