TOI-4860 b, a short-period giant planet transiting an M3.5 dwarf (2308.01454v2)

Abstract: We report the discovery and characterisation of a giant transiting planet orbiting a nearby M3.5V dwarf (d = 80.4 pc, $G$ = 15.1 mag, $K$=11.2 mag, R$\star$ = 0.358 $\pm$ 0.015 R$\odot$, M$\star$ = 0.340 $\pm$ 0.009 M$\odot$). Using the photometric time series from TESS sectors 10, 36, 46, and 63 and near-infrared spectrophotometry from ExTrA, we measured a planetary radius of 0.77 $\pm$ 0.03 R$J$ and an orbital period of 1.52 days. With high-resolution spectroscopy taken by the CFHT/SPIRou and ESO/ESPRESSO spectrographs, we refined the host star parameters ([Fe/H] = 0.27 $\pm$ 0.12) and measured the mass of the planet (0.273 $\pm$ 0.006 M$_J$). Based on these measurements, TOI-4860 b joins the small set of massive planets ($>$80 M$_E$) found around mid to late M dwarfs ($<$0.4 R$\odot$), providing both an interesting challenge to planet formation theory and a favourable target for further atmospheric studies with transmission spectroscopy. We identified an additional signal in the radial velocity data that we attribute to an eccentric planet candidate ($e=0.66\pm0.09$) with an orbital period of $427\pm7$~days and a minimum mass of $1.66\pm 0.26$ M$_J$, but additional data would be needed to confirm this.

• The paper identifies TOI-4860 b as a giant exoplanet with a 1.52-day orbit transiting an M3.5 dwarf, defying core-accretion model expectations.
• It employs transit photometry from TESS and precise radial velocity measurements from SPIRou and ESPRESSO to determine its mass (≈0.273 Jupiter masses) and radius (≈0.77 Jupiter radii).
• The findings suggest that alternative formation mechanisms, such as pebble accretion or disk instability, may better explain giant planet formation around low-mass stars.

Analyzing TOI-4860 b: A Giant Planet Orbiting an M3.5 Dwarf

The paper under analysis presents the identification and characterization of TOI-4860 b, a giant exoplanet with a notably short orbital period transiting an M3.5-type dwarf star. This discovery presents significant interest due to the rarity of massive planets orbiting low-mass stars, challenging prevailing planet formation theories predominantly governed by core accretion models.

Key Findings

The star TOI-4860 is located approximately 80.4 parsecs away and has a mass of about 0.340 solar masses. The investigation conducted using data primarily from TESS, as well as other photometric and spectrometric observations, particularly through the ExTrA facility and high-resolution spectroscopy via the CFHT/SPIRou and ESO/ESPRESSO instruments, led to the determination of precise characteristics of the planet TOI-4860 b.

• Planetary Characteristics: TOI-4860 b features a radius of 0.77 Jupiter radii and a mass of approximately 0.273 Jupiter masses. Notably, it has a very short orbital period of 1.52 days.
• Density and Structure: With a calculated average density of ~0.75 g/cm³, this planet falls into the category occupied by planets within or near the 'hot Neptune desert,' characterized by a scarcity of similar-radius planets at short orbital periods.
• Metallicity and Stellar Environment: The host star exhibits a metallicity significantly higher than the solar value ([Fe/H] = 0.27), which supports the metal-rich context often associated with the presence of massive planets.

The paper emphasizes the remarkable aspect of TOI-4860 b in that its formation presents a contradiction to the core-accretion model’s prediction, which predominantly foresees giant planet formation around stars with greater than 0.4 solar masses. The traditional models suggest that formation at such stellar mass should not sufficiently produce the required core mass for giant planet formation within the limited lifespan of protoplanetary disks.

Observational Techniques

The discovery is attributed to a combination of transit photometry and precise RV measurements. The TESS mission's encompassing sky survey capabilities allowed the detection of the transit, while ExTrA photometry validated its planetary nature by ruling out potential false positive conditions like blended binaries. RV data from SPIRou and ESPRESSO furnished an accurate measurement of the planet's mass and provided further stellar parameter refinement.

Theoretical Implications

The presence of TOI-4860 b extends the catalog of giant planets around low-mass stars and compels a re-evaluation of planet formation scenarios. For instance, alternative theories, like pebble accretion or disk instability, merit consideration as potential explanations for such atypical formations.

Moreover, the eccentric candidate planet detected in RV data introduces additional dynamics warranting investigation. The available data suggests an eccentric orbit for this second planetary body, potentially implicating mechanisms such as planet-planet scattering in the current architecture of the system.

Future Directions

The planetoid's close proximity to its host star incites interest for atmospheric studies via transmission spectroscopy, which can provide insight into the composition and evolution of this class of celestial bodies. The paper proposes TOI-4860 b as an exceptional opportunity for atmospheric characterization due to its favorable measurement conditions and relative size, propelling further research into the interplay between stellar radiation and planetary atmospheres in tightly bound systems.

Additionally, continued observation campaigns could clarify the dynamics implied by RV variations and confirm the existence and nature of the candidate planet. These findings could shed light on the influence of secondary planets on the development of giant planets in similar low-mass systems.

In conclusion, the examination of TOI-4860 b contributes substantially to our understanding of planetary formation in environments previously considered unfavorable for such giants. It stresses the broader implications for planet formation theory and underscores the need for continuous advancement of observational techniques to remain receptive to such unanticipated discoveries.