Mass-radius relations of rocky exoplanets and the B1-B2 mantle phase transition of ferropericlase (Mg, Fe)O

Kurzfassung

We calculate mass-radius relations based on structural models of low-mass solid planets of less than ten Earth masses using equations of state valid in the high-pressure limit for the radial density distribution. To partly overcome inherent non-uniqueness problems, we additionally calculate the fluid Love numbers kn,f for spherical harmonic degrees 2 <= n < 4 [1], which are a measure for central mass concentration and, therefore, relevant for determinations of rotationally and tidally induced planetary shape parameters and light curve variations [2] and transit timing variations [3]. We assume super-Earth planets that are subdivided into an iron core and overlain by an MgO-rich mantle and investigate the sensitivity of kn,f to the structural phase transition from the cubic B1 (NaCl-type) structure to the B2 (CsCl-type) phase of ferropericlase (Mg,Fe)O at mantle pressures above 500 GPa [4]. The preliminary calculations of the resultant mass-radius relations suggest that this structural phase transformation would be most relevant for relatively massive rocky exoplanets with small iron cores due to the small density contrast between the B1 structure and the B2 phase. Whereas planets with radii above 1.45 times that of the Earth are likely to harbor the B2-MgO mantle phase, the existence of the B2-MgO mantle phase for planets as massive as 3 Earth masses is only possible if their central iron cores were sufficiently small. In turn, more massive planets could possess larger iron cores and still maintain the B2-MgO phase in their mantles [5]. References: [1] Padovan et al., this meeting. [2] Hellard et al., this meeting. [3] Csizmadia et al., this meeting. [4] Dubrovinsky, this meeting. [5] This work is supported by the DFG within the FOR 2440 Matter under Planetary Interior Conditions - High Pressure, Planetary, and Plasma Physics.