Mass determination of the ultra-short-period planet LHS 3844 b. First K-band radial velocity measurements with CRIRES+

E. Nagel; J. Köhler; M. Zechmeister; A. D. Rains; U. Seemann; A. Hatzes; A. Reiners; N. Piskunov; L. Boldt-Christmas; P. Bristow; P. Chaturvedi; D. Cont; S. Czesla; R. J. Dorn; E. Guenther; Y. Jung; O. Kochukhov; F. Lesjak; F. Lucertini; T. Marquart; L. Nortmann; M. Rengel; F. Rodler; J. V. Smoker

arXiv · 2603.28238

Mass determination of the ultra-short-period planet LHS 3844 b. First K-band radial velocity measurements with CRIRES+

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Abstract

We present the first planet mass measurement obtained with CRIRES+ radial velocity (RV) observations using the K-band gas cell. Our target, LHS 3844 b (TOI-136), is a transiting super-Earth with radius $R_b=1.286^{+0.043}_{-0.044}R_\oplus$ and an orbital period of $P_b = 0.462929709^{+0.000000044}_{-0.000000042}$d, placing it in the class of ultra-short-period (USP) planets. The host star LHS 3844 is an old ($7.8\pm1.6$Gyr), slowly rotating ($P_{rot} = 130.0^{+16.9}_{-13.4}$d) M5.0 dwarf with $M_\star = 0.151\pm0.014M_\odot$ at a distance of 15pc (V=15.2mag, K=9.2mag). Combining our CRIRES+ RVs with archival ESPRESSO spectra, and confirming the signal in each dataset independently, we detected periodic RV variations with a semi-amplitude $K_b=6.95^{+0.55}_{-0.60}$m/s, implying a planetary mass of $m_b = 2.37\pm0.25M_\oplus$ and a bulk density of $ρ_b = 6.15^{+0.60}_{-0.61}$gcm$^{-3}$, consistent with a predominantly rocky composition. We further found excess RV variability that may be attributed to stellar jitter or to an additional planetary signal, for which we identified a tentative super-Earth candidate with a period of $\approx6.88$d. Owing to its proximity to its M-dwarf host, LHS 3844 b experiences intense irradiation and is unlikely to retain a substantial H/He envelope. Interior modeling places an upper limit on the iron-core mass fraction, which is consistent with an Earth-like rocky composition. With an emission spectroscopy metric of 28, LHS 3844 b is a prime JWST target for atmospheric and surface characterization and the most promising surface-characterization target known. Phase-curve spectroscopy may reveal its surface mineralogy and enable the first robust detection of exoplanet surface spectral features. Our results demonstrate that near-infrared RVs obtained with CRIRES+ enable robust mass measurements of super-Earths orbiting late M dwarfs.

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E. Nagel, J. Köhler, M. Zechmeister, A. D. Rains, U. Seemann, A. Hatzes, A. Reiners, N. Piskunov, L. Boldt-Christmas, P. Bristow, P. Chaturvedi, D. Cont, S. Czesla, R. J. Dorn, E. Guenther, Y. Jung, O. Kochukhov, F. Lesjak, F. Lucertini, T. Marquart, L. Nortmann, M. Rengel, F. Rodler, J. V. Smoker. 2026-03-30. Mass determination of the ultra-short-period planet LHS 3844 b. First K-band radial velocity measurements with CRIRES+. https://arxiv.org/abs/2603.28238

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