A Mini-Neptune Orbiting the Metal-poor K Dwarf BD+29 2654
Fei Dai, Kevin C. Schlaufman, Henrique Reggiani, Luke G. Bouma, Andrew W. Howard, Ashley Chontos, Daria Pidhorodetska, Judah Van Zandt, Joseph M. Akana Murphy, Ryan A. Rubenzahl, Alex S. Polanski, Jack Lubin, Corey Beard, Steven Giacalone, Rae Holcomb, Natalie M. Batalha, Ian J. M. Crossfield, Courtney D. Dressing, Benjamin J. Fulton, Daniel Huber, Howard Isaacson, Stephen R. Kane, Erik A. Petigura, Paul Robertson, Lauren M. Weiss, Alexander A. Belinski, Andrew W. Boyle, Christopher J. Burke, A. Castro-González, David R. Ciardi, Tansu Daylan, Akihiko Fukui, Holden Gill, Natalia Guerrero, C. Hellier, Steve B. Howell, J. Lillo-Box, F. Murgas, Norio Narita, Ε. Πάλλη, David R. Rodriguez, Arjun B. Savel, Avi Shporer, Keivan G. Stassun, Stephanie Striegel, Douglas A. Caldwell, Jon M. Jenkins, G. Ricker, Sara Seager, R. Vanderspek, Joshua N. Winn
Abstract
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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>4174</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>42</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>34</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>g</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>4.62</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , [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.