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

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.

Abstract

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 Teff=4174−42+34 K, logg=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 10M ⊕ 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.

• Journal Article Type: Article
• Acceptance Date: Jun 13, 2023
• Online Publication Date: Jul 7, 2023
• Publication Date: Aug 1, 2023
• Deposit Date: Jul 10, 2023
• Publicly Available Date: Jul 10, 2023
• Journal: The Astronomical Journal
• Print ISSN: 0004-6256
• Electronic ISSN: 1538-3881
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 166
• Issue: 2
• Article Number: 49
• DOI: https://doi.org/10.3847/1538-3881/acdee8
• Keywords: Exoplanets, Ocean planets, Mini Neptunes