Thorium in solar twins: implications for habitability in rocky planets

TL;DR

This study measures thorium abundances in 53 solar twins to understand the energy budget for mantle convection in rocky planets, revealing super-solar Th levels that influence planetary habitability over Galactic history.

Contribution

It provides precise thorium abundance measurements in solar twins and links these to planetary mantle energy budgets and habitability implications.

Findings

Th abundance remains super-solar over Galactic evolution.

Large energy budget from Th decay supports mantle convection.

Th abundance correlates with Eu, not Nd, indicating nucleosynthesis pathways.

Abstract

We have investigated the thorium (Th) abundance in a sample of 53 thin disc solar twins covering a wide range of ages. These data provide constrains on the mantle energy budget of terrestrial planets that can be formed over the evolution of the Galaxy's thin disc. We have estimated Th abundances with an average precision of 0.025\,dex (in both [Th/H] and [Th/Fe]) through comprehensive spectral synthesis of a Th\,II line present at 4019.1290\,{\AA}, using very high resolution (R\,=\,115,000) high quality HARPS spectra obtained at the ESO La Silla Observatory. We have confirmed that there is a large energy budget from Th decay for maintaining mantle convection inside potential rocky planets around solar twins, from the Galactic thin disc formation until now, because the pristine [Th/H]$_{\rm ZAMS}$ is super-solar on average under a uniform dispersion of 0.056\,dex (varying from +0.037 up to +0.138\,dex based on linear fits against isochrone stellar age). Comparing to neodymium (Nd) and europium (Eu), two others neutron-capture elements, the stellar pristine abundance of Th follows Eu along the Galactic thin disc evolution, but it does not follow Nd, probably because neodymium has a significant contribution from the $s$-process (about 60\,per\,cent).