TOI-3757 b: A low density gas giant orbiting a solar-metallicity M dwarf

Abstract

We present the discovery of a new Jovian-sized planet, TOI-3757 b, the lowest density planet orbiting an M dwarf (M0V). It orbits a solar-metallicity M dwarf discovered using TESS photometry and confirmed with precise radial velocities (RV) from HPF and NEID. With a planetary radius of $12.0_{-0.5}^{+0.4}$ $R_{\oplus }$ and mass of $85.3_{-8.7}^{+8.8}$ $M_{\oplus }$, not only does this object add to the small sample of gas giants ($\sim 10$) around M dwarfs, but also, its low density ($\rho =$ $0.27_{-0.04}^{+0.05}$ ${\text{g cm}}^{-3}$) provides an opportunity to test theories of planet formation. We present two hypotheses to explain its low density; first, we posit that the low metallicity of its stellar host ($\sim$ 0.3 dex lower than the median metallicity of M dwarfs hosting gas giants) could have played a role in the delayed formation of a solid core massive enough to initiate runaway accretion. Second, using the eccentricity estimate of $0.14\pm 0.06$ we determine it is also plausible for tidal heating to at least partially be responsible for inflating the radius of TOI-3757b b. The low density and large scale height of TOI-3757 b makes it an excellent target for transmission spectroscopy studies of atmospheric escape and composition (TSM $\sim$ 190). We use HPF to perform transmission spectroscopy of TOI-3757 b using the helium 10830 Å~ line. Doing this, we place an upper limit of 6.9 \% (with 90\% confidence) on the maximum depth of the absorption from the metastable transition of He at $\sim$ 10830 Å, which can help constraint the atmospheric mass loss rate in this energy limited regime.

AJ. arXiv admin note: text overlap with arXiv:2107.13670

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