Radioactive Planet Formation

TL;DR

This paper investigates how cosmic ray-induced spallation near young stars can produce radioactive nuclei like $^{26}$Al, affecting planet formation, internal heating, and habitability, especially around red dwarf stars.

Contribution

It provides a model estimating radioactive abundances from cosmic rays during planet formation and explores their impact on planetary properties and habitability.

Findings

Radioactive abundances can be 10-20 times higher than early solar system levels.

Enhanced radioactivity leads to increased internal heating and volatile loss in forming planets.

Planets in the habitable zone of red dwarfs are exposed to maximum particle radiation.

Abstract

Young stellar objects are observed to have large X-ray fluxes and are thought to produce commensurate luminosities in energetic particles (cosmic rays). This particle radiation, in turn, can synthesize short-lived radioactive nuclei through spallation. With a focus on $^{26}$Al, this paper estimates the expected abundances of radioactive nulcei produced by spallation during the epoch of planet formation. In this model, cosmic rays are accelerated near the inner truncation radii of circumstellar disks, $r_{\scriptstyle X}\approx0.1$ AU, where intense magnetic activity takes place. For planets forming in this region, radioactive abundances can be enhanced over the values inferred for the early solar system (from meteoritic measurements) by factors of $\sim10-20$. These short-lived radioactive nuclei influence the process of planet formation and the properties of planets in several ways. The minimum size required for planetesimals to become fully molten decreases with increasing levels of radioactive enrichment, and such melting leads to loss of volatile components including water. Planets produced with an enhanced radioactive inventory have significant internal luminosity which can be comparable to that provided by the host star; this additional heating affects both atmospheric mass loss and chemical composition. Finally, the habitable zone of red dwarf stars is coincident with the magnetic reconnection region, so that planets forming at those locations will experience maximum exposure to particle radiation, and subsequent depletion of volatiles.