An Earth-sized Planet on the Verge of Tidal Disruption (2407.21167v1)

Abstract: TOI-6255~b (GJ 4256) is an Earth-sized planet (1.079$\pm0.065$ $R_\oplus$) with an orbital period of only 5.7 hours. With the newly commissioned Keck Planet Finder (KPF) and CARMENES spectrographs, we determined the planet's mass to be 1.44$\pm$0.14 $M_{\oplus}$. The planet is just outside the Roche limit, with $P_{\rm orb}/P_{\rm Roche}$ = 1.13 $\pm0.10$. The strong tidal force likely deforms the planet into a triaxial ellipsoid with a long axis that is $\sim$10\% longer than the short axis. Assuming a reduced stellar tidal quality factor $Q_\star^\prime \approx10^7$, we predict that tidal orbital decay will cause TOI-6255 to reach the Roche limit in roughly 400 Myr. Such tidal disruptions may produce the possible signatures of planet engulfment that have been on stars with anomalously high refractory elemental abundances compared to its conatal binary companion. TOI-6255 b is also a favorable target for searching for star-planet magnetic interactions, which might cause interior melting and hasten orbital decay. TOI-6255 b is a top target (Emission Spectroscopy Metric of about 24) for phase curve observations with the James Webb Space Telescope.

• The paper presents TOI-6255 b's unique orbital dynamics, identifying its proximity to the Roche limit and imminent tidal deformation.
• Methodologies include high-precision spectroscopy from KPF and CARMENES, yielding detailed measurements of the planet’s mass, radius, and density.
• Findings highlight implications for rapid orbital decay and star-planet interactions, broadening our understanding of ultra-short period exoplanets.

Insights into the Earth-Sized Planet TOI-6255 b and Its Orbital Dynamics

The paper of TOI-6255 b presented in this paper provides a meticulous analysis of a unique terrestrial exoplanet characterized by its extremely short orbital period and proximity to the Roche limit. This investigation involves observational data from both the Keck Planet Finder (KPF) and CARMENES spectrographs. This research offers significant insights into the planet's composition and the potential future evolution of its orbit. Crucially, it also contributes to a deeper understanding of star-planet interactions, tidal forces, and the planetary survivability of similar exoplanets which are positioned within the veritable shreds of their Roche limit.

Stellar and Planetary Parameters

TOI-6255 b, a planet about 1.079±0.065 R⊕ in size, orbits its host star every 5.7 hours. The host is a relatively low-mass star, measuring approximately 0.353 M⊙ with a radius of 0.370 R⊙; spectroscopic analysis indicates a metallicity slightly below solar. Additionally, radial velocity measurements have determined the planetary mass to be around 1.44±0.14 M⊕. Given these dimensions, the planet's density and structural composition are comparable with those of the rocky inner planets of our solar system, providing valuable data for extrapolating the likely composition of similar bodies.

Orbital Proximity and Tidal Dynamics

A salient feature of TOI-6255 b is its orbit, which is merely on the edge of the Roche limit for a rocky planet of its density. Located just outside this critical boundary, the planet exhibits a high likelihood of tidal deformation into a triaxial ellipsoid. Notably, this deformation heightens the probability of eventual orbital decay, potentially hastening in as few as 400 million years if the reduced stellar tidal quality factor Q' is approximately 107. The paper raises awareness about the phenomenon of planetary material transfer due to tidal forces, which can input refractory elements into the host star's atmosphere.

Implications of Tidal Distortion

The extreme tidal forces impacting TOI-6255 b result in a pronounced reshaping of the planet, predicted to be about 10% elongated along its longest axis. Such distortion, computationally modeled under the assumptions of Love number theory, potentially complicates simple interpretations of transit photometry and radial velocity data, suggesting that these measurements underpredict the true planetary dimensions. The impact of such tidal forces, in combination with possible magnetic interactions between the star and the planet, might influence observable atmospheric phenomena, in turn providing data on atmospheric composition and potential outgassing.

Future Observations and Theoretical Implications

TOI-6255 b emerges as a prime candidate for further exploration via emission and phase curve observations using the James Webb Space Telescope. With a strong Emission Spectroscopy Metric, the planet is well-suited to explore the surfaces and atmospheres of ultra-short period exoplanets, potentially unveiling previously unseen properties of exoplanetary mineralogy and atmospheric dynamics.

Conclusion

The research outlined in this paper highlights TOI-6255 b as an archetype of USPs whose fate is integrally tied to their host star's interplay of magnetic and tidal dynamics. The close-paper of TOI-6255 b promises to enhance our understanding of planetary evolution under extreme conditions, and demonstrates the intricate behavior of planets within short-period orbits. The paper helps to frame both the theoretical constructs and empirical methodologies necessary for examining similar exoplanetary bodies, laying groundwork for observations that could garner fresh insights into exoplanetary physics and chemistry.