Focusing on MKG stars, Pascucci and colleagues suggested that most common planet around a lower mass stars is small in mass. They also suggested that the radius of planet depends on the stellar mass. Meaning, it increases with the stellar mass. Now, Lozovsky and colleagues explored why more massive stars tend to host larger planets. Using exoplanetary data of planets with measured masses and radius, they explored 3 possible explanations for the identified correlation between the planetary radius and stellar mass.
- Case-1: Planets are larger due to thermal inflation: since more massive stars are more luminous, larger stellar irradiation could inflate the planetary radius.
- Case-2: Planets around more massive stars are more massive and are in consequence larger for any given composition.
- Case-3: Planets around more massive stars tend to be more volatile-rich and are therefore larger in size.
They first confirmed that planets surrounding larger stars tend to be larger in mass and radius.
Later, they showed that, larger radii of planets surrounding more massive stars (like G and K) cannot be explained by inflation due to higher irradiation of the star, nor by a higher planetary mass (i.e. Case 1 and 2): the effect of both properties on the radius was found to be significantly smaller than the observed difference. In other words, they showed that the difference in planetary mass, which is given by RV/TTV data, is insufficient to explain the observed difference in planetary radii, and the planetary temperature is even found to anticorrelate with the radius.
Finally, it has been shown that the larger planetary radii can be explained by a larger fraction of volatile material (H-He atmospheres) among planets surrounding more massive stars (i.e. by Case-3). This is because planets forming around more massive stars tend to accrete H-He atmospheres more efficiently. This can be a result of more massive protoplanetary disks that lead to faster core growth; allowing the cores to accrete substantial gaseous envelopes before the gas disk dissipates.
“Thus, we argue that planets around G- and K-stars are different by formation.”, they wrote. “It is desirable to have further constraints available to test our findings. Further information on the planetary composition would come from JWST and Ariel missions with constraints on atmospheric composition, which will greatly shed more light into the diversity of planetary volatile envelopes, their formation efficiencies and evolutions.” they concluded.
Featured image credit: Getty Images
Reference: Michael Lozovsky, Ravit Helled, Illaria Pascucci, Caroline Dorn, Julia Venturini, Robert Feldmann, “Why do more massive stars host larger planets?”, Arxiv, 2021.
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