Kavli Affiliate: Christopher J. Burke
| First 5 Authors: Fei Dai, Kevin C. Schlaufman, Henrique Reggiani, Luke Bouma, Andrew W. Howard
| Summary:
We report the discovery and Doppler mass measurement of a 7.4-day
2.3-$R_oplus$ 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
multi-wavelength photometry from the ultraviolet to the mid-infrared, we found
that the host star has $T_{text{eff}}=4174^{+34}_{-42}$ K,
$log{g}=4.62^{+0.02}_{-0.03}$, $[text{Fe/H}]=-0.58pm0.18$,
$M_{ast}=0.57pm0.02~M_{odot}$, and $R_{ast}=0.62pm0.01~R_{odot}$. Precise
Doppler measurements with Keck/HIRES revealed a planetary mass of
$M_{text{p}}=9.2pm2.1~M_{oplus}$ for TOI-2018 b. TOI-2018 b has a mass and
radius that are consistent with an Earth-like core with a $sim1%$-by-mass
hydrogen/helium envelope, or an ice-rock mixture. The mass of TOI-2018 b is
close to the threshold for run-away accretion and hence giant planet formation.
Such a threshold is predicted to be around 10$M_oplus$ or lower for a
low-metallicity (low-opacity) environment. If TOI-2018 b is a planetary core
that failed to undergo run-away 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.
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