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Most of the currently known planets are small worlds with radii between thatof the Earth and that of Neptune. The characterization of planets in thisregime shows a large diversity in compositions and system architectures, withdistributions hinting at a multitude of formation and evolution scenarios.Using photometry from the K2 satellite and radial velocities measured with theHARPS and CORALIE spectrographs, we searched for planets around the bright andslightly evolved Sun-like star HD 137496. We precisely estimated the stellarparameters, $M_*$ = 1.035 +/- 0.022 $M_\odot$, $R_*$ = 1.587 +/- 0.028$R_\odot$, $T_\text{eff}$ = 5799 +/- 61 K, together with the chemicalcomposition of the slightly evolved star. We detect two planets orbiting HD137496. The inner planet, HD 137496 b, is a super-Mercury (an Earth-sizedplanet with the density of Mercury) with a mass of $M_b$ = 4.04 +/- 0.55$M_\oplus$, a radius of $R_b = 1.31_{-0.05}^{+0.06} R_\oplus,$ and a density of$\rho_b = 10.49_{-1.82}^{+2.08}$ $\mathrm{g cm^{-3}}$. With an interiormodeling analysis, we find that the planet is composed mainly of iron, with thecore representing over 70% of the planet's mass ($M_{core}/M_{total} =0.73^{+0.11}_{-0.12}$). The outer planet, HD 137496 c, is an eccentric ($e$ =0.477 +/- 0.004), long period ($P$ = $479.9_{-1.1}^{+1.0}$ days) giant planet($M_c\sin i_c$ = 7.66 +/- 0.11 $M_{Jup}$) for which we do not detect a transit.HD 137496 b is one of the few super-Mercuries detected to date. The accuratecharacterization reported here enhances its role as a key target to betterunderstand the formation and evolution of planetary systems. The detection ofan eccentric long period giant companion also reinforces the link between thepresence of small transiting inner planets and long period gas giants.

The HD 137496 system: A dense, hot super-Mercury and a cold Jupiter