Kavli Affiliate: Sara Seager
| First 5 Authors: Zifan Lin, Saverio Cambioni, Sara Seager, ,
| Summary:
Some exoplanets have much higher densities than expected from stellar
abundances of planet-forming elements. There are two theories – metal-rich
formation hypothesis and naked core hypothesis – that explain how formation and
evolution can alter the compositions and structures of rocky planets to diverge
from their primordial building blocks. Here, we revisit the naked core
hypothesis, which states that high-density planets are remnant cores of giant
planets that remain in a fossil-compressed state, even after envelope loss.
Using a planetary interior model and assuming energy-limited atmospheric
escape, we show that a large fraction, if not all, of the iron-silicate core of
a giant planet is molten during the planet’s early evolution. Upon envelope
loss, molten part of the planets can rapidly rebound due to low viscosity,
resulting in a decrease in radius by at most 0.06%, if they had hydrogen/helium
envelopes, or by at most 7%, if they had H$_2$O envelopes, compared to
self-compressed counterparts with the same core mass fraction. Based on our
findings, we reject the hypothesis that all high-density exoplanets are naked
cores with Kolmogorov-Smirnov p-value $ll$ 0.05 for both envelope
compositions. We find that some high-density exoplanets can still possibly be
naked cores, but the probabilities are lower than $sim$1/2 and $sim$1/3 for
the ice giant and gas giant scenario, respectively, in 95% of the cases. We
conclude that most high-density exoplanets are unlikely to be remnant
giant-planets cores.
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