TOI-1728b: The Habitable-zone Planet Finder confirms a warm super Neptune orbiting an M dwarf host (2006.14546v1)

Abstract: We confirm the planetary nature of TOI-1728b using a combination of ground-based photometry, near-infrared Doppler velocimetry and spectroscopy with the Habitable-zone Planet Finder.TOI-1728 is an old, inactive M0 star with \teff{} $= 3980^{+31}_{-32}$ K, which hosts a transiting super Neptune at an orbital period of $\sim$ 3.49 days. Joint fitting of the radial velocities and TESS and ground-based transits yields a planetary radius of $5.05_{-0.17}^{+0.16}$ R${\oplus}$, mass $26.78{-5.13}^{+5.43}$ M${\oplus}$ and eccentricity $0.057{-0.039}^{+0.054}$. We estimate the stellar properties, and perform a search for He 10830 \AA absorption during the transit of this planet and claim a null detection with an upper limit of 1.1$\%$ with 90\% confidence. A deeper level of He 10830 \AA ~ absorption has been detected in the planet atmosphere of GJ 3470b, a comparable gaseous planet. TOI-1728b is the largest super Neptune -- the intermediate subclass of planets between Neptune and the more massive gas-giant planets -- discovered around an M dwarf. With its relatively large mass and radius, TOI-1728 represents a valuable datapoint in the M-dwarf exoplanet mass-radius diagram, bridging the gap between the lighter Neptune-sized planets and the heavier Jovian planets known to orbit M-dwarfs. With a low bulk density of $1.14_{-0.24}^{+0.26}$ g/cm$^3$, and orbiting a bright host star (J $\sim 9.6$, V $\sim 12.4$), TOI-1728b is also a promising candidate for transmission spectroscopy both from the ground and from space, which can be used to constrain planet formation and evolutionary models.

• The paper confirms TOI-1728b as a warm super Neptune orbiting an M dwarf, bridging the gap between Neptune-sized and Jovian planets.
• The study employed TESS photometry, ground-based follow-up, and HPF near-infrared Doppler velocimetry to robustly detect the planet.
• Key findings include a low orbital eccentricity and a null helium absorption detection, refining theoretical models of planet formation.

Overview of "TOI-1728b: The Habitable-zone Planet Finder Confirms a Warm Super Neptune Orbiting an M Dwarf Host"

The paper by Kanodia et al. presents the discovery and confirmation of a transiting super Neptune, TOI-1728b. This planet orbits an M0-type star and has been confirmed through observations combining ground-based photometry, near-infrared Doppler velocimetry, and spectroscopy using the Habitable-zone Planet Finder (HPF). The paper contributes significantly to the growing repository of exoplanets orbiting M dwarf stars, which are of particular interest for their potential in hosting habitable worlds.

Key Findings

The system is centered around TOI-1728, an inactive M0 star with a measured temperature of 3980 K. The planet, TOI-1728b, is found to be a super Neptune with a radius of approximately $5.05_{-0.17}^{+0.16} R_{\oplus}$, a mass of $26.78_{-5.13}^{+5.43} M_{\oplus}$, and a low orbital eccentricity of approximately 0.057. It orbits its star every 3.49 days, which places it in a regime characterized by a moderate insolation level, classifying it as a warm Neptune.

Methodology and Results

The confirmation of TOI-1728b's planetary nature was achieved using TESS photometry and ground-based follow-up, alongside near-infrared RV data collected with HPF. The radial velocity semi-amplitude measured is $15.12_{-2.87}^{+3.04}$ m/s, which suggests a robust detection of the planetary signal. The analysis required accounting for potential systematics in the TESS and ground-based photometry through Gaussian Process regression, which restored a high level of confidence in the transit parameters.

Additionally, a search for helium absorption at 10830 \AA ~ during transits yielded a null detection, with an upper limit set at 1.1% with 90% confidence. This outcome aligns with observations from other similar exoplanets, indicating variability in helium signatures even among comparable objects.

Theoretical and Practical Implications

The position of TOI-1728b in the mass-radius diagram notably bridges the gap between the known population of Neptune-sized planets and the gaseous Jovian planets orbiting M-dwarfs. Its discovery aids in testing and refining theoretical models of planet formation and evolution, particularly those that focus on the influence of host mass and metallicity on planet characteristics.

The relatively low density ($1.14_{-0.24}^{+0.26}$ g/cm$^3$) and proximity to a bright host star (J ~ 9.6) make TOI-1728b a prime candidate for transmission spectroscopy. Such investigations could provide insights into its atmospheric composition, thereby enabling comparisons with atmospheres of other known sub-Jovian planets.

Future Directions

TOI-1728b sets a precedent for further discoveries of sub-Neptunes in habitable zones around M-dwarfs, many of which may undergo similar atmospheric and compositional studies with facilities like the James Webb Space Telescope. This could potentially reveal deep insights into atmospheric dynamics and elemental abundances, facilitating a deeper understanding of their formation histories and the environmental conditions on these planets.

Conclusively, TOI-1728b presents a compelling subject for ongoing and future exoplanetary research, contributing to both observational strategies and theoretical frameworks that aim to elucidate the nature and distribution of planets in the galaxy.