A Mini-Neptune Orbiting the Metal-poor K Dwarf BD+29 2654

• Published in: The Astronomical journal 2023-08, Vol.166 (2), p.49
• Main Authors: Dai, Fei, Schlaufman, Kevin C., Reggiani, Henrique, Bouma, Luke, Howard, Andrew W., Chontos, Ashley, Pidhorodetska, Daria, Van Zandt, Judah, Akana Murphy, Joseph M., Rubenzahl, Ryan A., Polanski, Alex S., Lubin, Jack, Beard, Corey, Giacalone, Steven, Holcomb, Rae, Batalha, Natalie M., Crossfield, Ian, Dressing, Courtney, Fulton, Benjamin, Huber, Daniel, Isaacson, Howard, Kane, Stephen R., Petigura, Erik A., Robertson, Paul, Weiss, Lauren M., Belinski, Alexander A., Boyle, Andrew W., Burke, Christopher J., Castro-González, Amadeo, Ciardi, David R., Daylan, Tansu, Fukui, Akihiko, Gill, Holden, Guerrero, Natalia M., Hellier, Coel, Howell, Steve B., Lillo-Box, Jorge, Murgas, Felipe, Narita, Norio, Pallé, Enric, Rodriguez, David R., Savel, Arjun B., Shporer, Avi, Stassun, Keivan G., Striegel, Stephanie, Caldwell, Douglas A., Jenkins, Jon M., Ricker, George R., Seager, Sara, Vanderspek, Roland, Winn, Joshua N.
• Format: Article
• Language: English
• Subjects: Astronomy; Deposition; Exoplanets; Exosphere; Helium; Infrared photometry; James Webb Space Telescope; Jupiter; Metallicity; Mini Neptunes; Neptune; Ocean planets; Opacity; Parallax; Planet formation; Planetary cores; Planetary mass; Planets; Space telescopes; Spectroscopy
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/acdee8

We report the discovery and Doppler mass measurement of a 7.4 days 2.3 R ⊕ mini-Neptune around a metal-poor K dwarf BD+29 2654 (TOI-2018). Based on a high-resolution Keck/HIRES spectrum, the Gaia parallax, and multiwavelength photometry from the UV to the mid-infrared, we found that the host star has T eff = 4174 − 42 + 34 K, log g = 4.62 − 0.03 + 0.02 , [Fe/H] = − 0.58 ± 0.18, M * = 0.57 ± 0.02 M ⊙ , and R * = 0.62 ± 0.01 R ⊙ . Precise Doppler measurements with Keck/HIRES revealed a planetary mass of M p = 9.2 ± 2.1 M ⊕ for TOI-2018 b. TOI-2018 b has a mass and radius that are consistent with an Earthlike core, with a ∼1%-by-mass hydrogen/helium envelope or an ice–rock mixture. The mass of TOI-2018 b is close to the threshold for runaway accretion and hence giant planet formation. Such a threshold is predicted to be around 10 M ⊕ or lower for a low-metallicity (low-opacity) environment. If TOI-2018 b is a planetary core that failed to undergo runaway accretion, it may underline the reason why giant planets are rare around low-metallicity host stars (one possibility is their shorter disk lifetimes). With a K -band magnitude of 7.1, TOI-2018 b may be a suitable target for transmission spectroscopy with the James Webb Space Telescope. The system is also amenable to metastable Helium observation; the detection of a Helium exosphere would help distinguish between a H/He-enveloped planet and a water world.