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TOI-733 b: a planet in the small-planet radius valley orbiting a Sun-like star (2304.06655v2)

Published 13 Apr 2023 in astro-ph.EP

Abstract: We report the discovery of a hot ($T_{\rm eq}$ $\approx$ 1055 K) planet in the small planet radius valley transiting the Sun-like star TOI-733, as part of the KESPRINT follow-up program of TESS planets carried out with the HARPS spectrograph. TESS photometry from sectors 9 and 36 yields an orbital period of $P_{\rm orb}$ = $4.884765 { - 2.4e-5 } ^ { + 1.9e-5 }$ days and a radius of $R{\mathrm{p}}$ = $1.992 { - 0.090 } ^ { + 0.085 }$ $R{\oplus}$. Multi-dimensional Gaussian process modelling of the radial velocity measurements from HARPS and activity indicators, gives a semi-amplitude of $K$ = $2.23 \pm 0.26 $ m s${-1}$, translating into a planet mass of $M_{\mathrm{p}}$ = $5.72 { - 0.68 } ^ { + 0.70 }$ $M{\oplus}$. These parameters imply that the planet is of moderate density ($\rho_\mathrm{p}$ = $3.98 _{ - 0.66 } ^ { + 0.77 }$ g cm${-3}$) and place it in the transition region between rocky and volatile-rich planets with H/He-dominated envelopes on the mass-radius diagram. Combining these with stellar parameters and abundances, we calculate planet interior and atmosphere models, which in turn suggest that TOI-733 b has a volatile-enriched, most likely secondary outer envelope, and may represent a highly irradiated ocean world - one of only a few such planets around G-type stars that are well-characterised.

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References (2)

Summary

  • The paper details the discovery and in-depth characterization of TOI-733 b, pinpointing it within the small-planet radius valley.
  • It employs TESS photometry and HARPS spectroscopy to determine a precise 4.885-day orbital period and a high equilibrium temperature of approximately 1055 K.
  • The findings reveal that TOI-733 b is a transitional exoplanet with a possible secondary atmosphere or water-rich envelope, providing key insights into planetary evolution mechanisms.

Planet TOI-733~b in the Small-Planet Radius Valley

The paper discusses the discovery and detailed characterization of an exoplanet, TOI-733~b, which orbits a Sun-like star. This planet is noted for its location within the small-planet radius valley, a range in the mass-radius space that distinguishes super-Earths from mini-Neptunes due to a scarcity of planets with radii around 1.8 Earth radii. Through photometric and spectroscopic observations from TESS and HARPS, TOI-733~b was found to have notable characteristics that further our understanding of planet formation and evolution processes.

The central findings for TOI-733~b include:

  • A high equilibrium temperature of approximately 1055 K.
  • An orbital period derived from TESS data to be about 4.885 days.
  • Its radius and mass place it at a transitional region, suggesting a composition that lies between rocky and volatile-rich planets.

TOI-733~b's density suggests it may possess a secondary atmosphere or a water-rich envelope, possibly classifying it as a highly irradiated ocean world. This is supported by interior and atmosphere models suggesting a volatile-rich composition.

Observational and Analytical Methods

The discovery utilized TESS observations and was confirmed via the KESPRINT follow-up program using the HARPS spectrograph. TESS data from two sectors provided insight into the planet's transit characteristics, while high-precision radial velocity measurements from HARPS were critical in determining the planet’s mass and orbit with more exacting precision. The use of Gaussian process detrending methods effectively addressed stellar variability issues in the light curves.

Context and Implications

Being one of the few well-characterized planets around a G-type star in this parameter space, TOI-733~b serves as a critical data point for testing theories related to the small-planet radius gap. Theoretical frameworks such as atmospheric photoevaporation and core-powered mass loss have been proposed to explain this gap, yet empirical validation remains partial. With planets like TOI-733~b, researchers hope to better delineate mechanisms of planetary formation and atmospheric evolution.

Given its mass and radius, TOI-733~b contributes to the ongoing discourse on whether similar planets are remnants of originally larger, gas-rich planets or have retained a significant amount of water from formation. Current models predict planets in this radius valley to reflect diverse origins—some having initially substantial gaseous envelopes now largely lost to hydrodynamic escape, while others might have commenced as water worlds beyond the snow line.

Prospects for Future Research

The characteristics of TOI-733~b make it a suitable candidate for future atmospheric studies with larger telescopes expected in the coming decades, despite current limitations due to observational constraints. Examination of its atmospheric composition via transmission spectroscopy might elucidate the nature of its envelope, offering clues about its evolutionary history and providing empirical data to test the water-world hypothesis.

Finally, TOI-733~b adds a significant constraint to the population studies of exoplanets, thereby contributing substantially to the field's understanding of small-planet formation mechanisms in G-type star systems.

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