K. Hesse, I. Mireles, F. Bouchy, D. Dragomir, S. Ulmer-Moll, N. L. Eisner, K. G. Stassun, S. N. Quinn, H. P. Osborn, S. G. Sousa, C. N. Watkins, K. A. Collins, E. M. Bryant, J. M. Irwin, C. Hellier, M. C. Johnson, C. Ziegler, S. B. Howell, D. R. Anderson, D. Bayliss, A. Bieryla, C. Briceño, R. P. Butler, D. Charbonneau, R. Cloutier, J. D. Crane, J. Dittmann, J. D. Eastman, S. A. Freigeiro, B. J. Fulton, S. Gill, M. N. Günther, H. Im, J. M. Jenkins, M. Kunimoto, B. Lavie, M. Lendl, M. B. Lund, A. W. Mann, B. Nicholson, D. Osip, M. Paegert, N. C. Santos, R. P. Schwarz, S. Seager, S. A. Shectman, J. Teske, J. D. Twicken, S. Udry, V. Van Eylen, J. Vinés, S. X. Wang, P. J. Wheatley, J. N. Winn, E. E. Zuidema

Abstract
We report the discovery and confirmation of two planets orbiting the metal-poor Sun-like star HD 35843 (TOI 4189). HD 35843 c is a temperate sub-Neptune transiting planet with an orbital period of 46.96 days that was first identified by Planet Hunters TESS. We combine data from TESS and follow-up observations to rule out false-positive scenarios and validate the planet. We then use ESPRESSO radial velocities (RVs) to confirm the planetary nature and characterize the planet's mass and orbit. Further analysis of these RVs reveals the presence of an additional planet, HD 35843 b, with a period of 9.90 days and a minimum mass of 5.84 ± 0.84 M_⊕. For HD 35843 c, a joint photometric and spectroscopic analysis yields a radius of 2.54 ± 0.08 R_⊕, a mass of 11.32 ± 1.60 M_⊕, and an orbital eccentricity of e = 0.15 ± 0.07. With a bulk density of 3.80 ± 0.70 g cm⁻³, the planet might be rocky with a substantial H₂ atmosphere or it might be a "water world." With an equilibrium temperature of ∼480 K, HD 35843 c is among the coolest ∼5% of planets discovered by TESS. Combined with the host star's relative brightness (V = 9.4), HD 35843 c is a promising target for atmospheric characterization that will probe this sparse population of temperate sub-Neptunes.

Keywords
Exoplanet astronomy / Exoplanet detection methods / Planet hosting stars / Transit photometry / Radial velocity / High resolution spectroscopy / Exoplanets / Exoplanet systems / Earth and Planetary Astrophysics

The Astronomical Journal
Volume 170, Number 108, Page 25
2025 August

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