Kavli Affiliate: George R. Ricker
| First 5 Authors: Elisa Goffo, Davide Gandolfi, Jo Ann Egger, Alexander J. Mustill, Simon H. Albrecht
| Summary:
GJ 367 is a bright (V $approx$ 10.2) M1 V star that has been recently found
to host a transiting ultra-short period sub-Earth on a 7.7 hr orbit. With the
aim of improving the planetary mass and radius and unveiling the inner
architecture of the system, we performed an intensive radial velocity follow-up
campaign with the HARPS spectrograph — collecting 371 high-precision
measurements over a baseline of nearly 3 years — and combined our Doppler
measurements with new TESS observations from sectors 35 and 36. We found that
GJ 367 b has a mass of $M_mathrm{b}$ = 0.633 $pm$ 0.050 M$_{oplus}$ and a
radius of $R_mathrm{b}$ = 0.699 $pm$ 0.024 R$_{oplus}$, corresponding to
precisions of 8% and 3.4%, respectively. This implies a planetary bulk density
of $rho_mathrm{b}$ = 10.2 $pm$ 1.3 g cm$^{-3}$, i.e., 85% higher than
Earth’s density. We revealed the presence of two additional non transiting
low-mass companions with orbital periods of $sim$11.5 and 34 days and minimum
masses of $M_mathrm{c}sin{i_mathrm{c}}$ = 4.13 $pm$ 0.36 M$_{oplus}$ and
$M_mathrm{d}sin{i_mathrm{d}}$ = 6.03 $pm$ 0.49 M$_{oplus}$, respectively,
which lie close to the 3:1 mean motion commensurability. GJ 367 b joins the
small class of high-density planets, namely the class of super-Mercuries, being
the densest ultra-short period small planet known to date. Thanks to our
precise mass and radius estimates, we explored the potential internal
composition and structure of GJ 367 b, and found that it is expected to have an
iron core with a mass fraction of 0.91$^{+0.07}_{-0.23}$. How this iron core is
formed and how such a high density is reached is still not clear, and we
discuss the possible pathways of formation of such a small ultra-dense planet.
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