We present a reanalysis of the K2-106 transiting planetary system, with a focus on the composition of K2-106b, an
ultra-short-period, super-Mercury candidate. We globally model existing photometric and radial velocity data and
derive a planetary mass and radius for K2-106b of Mp = 8.53 ± 1.02 M⊕ and = -
+ Rp 1.71 0.057 RÅ
0.069 , which leads to a
density of r = -
+ 9.4 p 1.5
1.6 g cm−3
, a significantly lower value than previously reported in the literature. We use planet
interior models that assume a two-layer planet comprised of a liquid, pure Fe core and an iron-free, MgSiO3
mantle, and we determine that the range of the core mass fractions are consistent with the observed mass and
radius. We use existing high-resolution spectra of the host star to derive the Fe/Mg/Si abundances ([Fe/
H] = −0.03 ± 0.01, [Mg/H] = 0.04 ± 0.02, [Si/H] = 0.03 ± 0.06) to infer the composition of K2-106b. We find
that K2-106b has a density and core mass fraction ( -
+ 44 %15
12 ) consistent with that of Earth (CMF⊕ = 32%).
Furthermore, its composition is consistent with what is expected, assuming that it reflects the relative refractory
abundances of its host star. K2-106b is therefore unlikely to be a super-Mercury, as has been suggested in previous
literature.
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On the Probability That a Rocky Planet’s Composition Reflects Its Host Star
The bulk density of a planet, as measured by mass and radius, is a result of planet structure and composition. Relative proportions of iron core, rocky mantle, and gaseous envelopes are degenerate for a given density. This degeneracy is reduced for rocky planets without significant gaseous envelopes when the structure is assumed to be a differentiated iron core and rocky mantle, in which the core mass fraction (CMF) is a first-order description of a planet’s bulk composition. A rocky planet’s CMF may be derived both from bulk density and by assuming the planet reflects the host star’s major rock-building elemental abundances (Fe, Mg, and Si). Contrasting CMF measures, therefore, shed light on the outcome diversity of planet formation from processes including mantle stripping, out-gassing, and/or late-stage volatile delivery. We present a statistically rigorous analysis of the consistency of these two CMF measures accounting for observational uncertainties of planet mass and radius and host-star chemical abundances. We find that these two measures are unlikely to be resolvable as statistically different unless the bulk density CMF is at least 40% greater than or 50% less than the CMF as inferred from the host star. Applied to 11 probable rocky exoplanets, Kepler-107 c has a CMF as inferred from bulk density that is significantly greater than the inferred CMF from its host star (2
- Award ID(s):
- 1724693
- NSF-PAR ID:
- 10484948
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Planetary Science Journal
- Volume:
- 2
- Issue:
- 3
- ISSN:
- 2632-3338
- Format(s):
- Medium: X Size: Article No. 113
- Size(s):
- ["Article No. 113"]
- Sponsoring Org:
- National Science Foundation
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