Company for the Ultra-high Density, Ultra-short Period Sub-Earth GJ 367 b : Discovery of Two Additional Low-mass Planets at 11.5 and 34 Days

GJ 367 is a bright (V approximate to 10.2) M1 V star that has been recently found to host a transiting ultra-short period subEarth 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 yr-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-b = 0.633 +/- 0.050M. and a radius of R-b = 0.699 +/- 0.024 R-circle plus, corresponding to precisions of 8% and 3.4%, respectively. This implies a planetary bulk density of rho(b) = 10.2 +/- 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 similar to 11.5 and 34 days and minimum masses of M-c sin i(c) = 4.13 +/- 0.36 M-circle plus and M-d sin i(d) = 6.03 +/- 0.49M(circle plus), respectively, which lie close to the 3:1 mean motion commensurability. GJ 367 b joins the small class of highdensity 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(-0.23)(+0.07). 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.