Density separates the rocky and water-rich exoplanets that orbit red dwarf stars

Density measurements of small exoplanets reveal three distinct populations – gassy, rocky and water-rich planets – that can form around a red dwarf star, according to a new study. Small planets are common around red dwarf stars (M dwarfs), and many of those that transect their host star constitute the bulk of known exoplanets ideal for atmospheric characterization via transmission spectroscopy and represent some of the best places to search for signatures of life. However, whether the small worlds surrounding M dwarfs are potentially habitable remains unclear, partly due to our lack of understanding of their composition. Moreover, studying these distant planets from Earth is challenging because of the faint red light their stars emit. In general, the radii of small exoplanets are known to have a bimodal distribution, which has been interpreted as two separate rocky and gassy populations, planets with a thin or thick hydrogen-helium atmosphere, respectively. Using radius and mass measurements from 34 newly detected planets found around closer M dwarf stars from the Transiting Exoplanet Survey Satellite (TESS), Rafael Luque and Enric Pallé show that the planets’ densities provide a cleaner distribution of planet types. Differing from previous understanding, Luque and Pallé discovered that the planets orbiting M dwarfs fall into three different density categories – rocky, gassy and watery exoplanets. According to the findings, the third population matches the density predictions from a planetary model of 50% rock and 50% water. The authors suggest that these water-rich planets likely form with ice and rock far away from the host star before migrating into a closer orbit with the host star. “Although the presence of watery small exoplanets is particularly enticing, all three types of planets around red dwarfs could present potentially habitable conditions for life,” writes Johanna Teske in a related Perspective. “Leaving aside this possibility for discovering alien life-forms, measuring the compositional diversity of planets around red dwarf stars – the most common type of star in the Milky Way – is important for piecing together the complex puzzle of small planets’ formation and evolution.”