HADES RV Programme with HARPS-N at TNG

XVI. A super-Earth in the habitable zone of the GJ 3998 multi-planet system^★

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
³ Light Bridges S. L., 35004 Las Palmas de Gran Canaria, Spain
⁴ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
⁵ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁶ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), c/ Gran Capità 2–4, 08034 Barcelona, Spain
⁸ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁹ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁰ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy

^★★ Corresponding author; atanas.stefanov@iac.es

Received: 16 October 2024
Accepted: 30 January 2025

Abstract

The low masses of M dwarfs create attractive opportunities for exoplanet radial-velocity (RV) detections. These stars, however, exhibit strong stellar activity that may attenuate or mimic planetary signals. We present a velocimetric analysis of one such M dwarf, GJ 3998 (d = 18.2 pc), with two published short-period super-Earths: GJ 3998 b and GJ 3998 c. We use additional data from the HARPS-N spectrograph to confirm these two planets and to look for more. We carry out joint modelling of: (i) RV planetary signals, (ii) stellar rotation in RV and activity indicators through Gaussian processes, and (iii) long-term trends in RV and activity indicators. We constrain the rotational period of GJ 3998 to P_rot = 30.2 ± 0.3 d and discover long-term sinusoidal imprints in RV and the full width at half maximum with a period of P_cyc = 316₋₈⁺¹⁴ d. We confirm GJ 3998 b and GJ 3998 c, and detect a third planet: GJ 3998 d, whose signal had previously been attributed to stellar activity. GJ 3998 d has an orbital period of 41.78 ± 0.05 d, a minimum mass of 6.07_−0.96^+1.00 M_⊕, and a mean insolation flux of 1.2_−0.2^+0.3 Φ_⊕. This makes it one of the few known planets receiving an Earth-like insolation flux.