TOI-2431 b: Ultra-Short-Period Lava World

• TOI-2431 b is an ultra-short-period exoplanet with a ~5.38-hour orbit, offering key insights into extreme tidal evolution and planetary structure.
• Its high bulk density (9.4 g/cm³) and Earth-like composition, with a mix of iron and silicate rock, classify it as a 'lava-world' under intense stellar irradiation.
• A multi-instrument approach using TESS, NEID, and HPF validated its parameters and orbital decay, making it a benchmark for studying ultra-short-period exoplanets.

TOI-2431 b is an ultra-short-period (USP), Earth-sized exoplanet orbiting a nearby late K dwarf star. Exhibiting one of the shortest orbital periods known for any exoplanet, TOI-2431 b provides a critical benchmark for studies of planetary structure, extreme irradiation, tidal evolution, and atmospheric escape under extreme conditions. Its high bulk density, extreme stellar insolation, and close approach to the Roche limit situate the planet in a parameter regime of particular importance for understanding the interior and atmospheric properties of rocky exoplanets exposed to intense tidal and radiative environments (Taş et al., 11 Jul 2025).

1. Stellar Host Characterization

TOI-2431 serves as the host for TOI-2431 b and is classified as a mildly metal-poor late K-type main-sequence star. The following fundamental parameters were determined through an SED (Spectral Energy Distribution) fit using ARIADNE, anchored by HPF‐SpecMatch spectroscopic measurements:

Parameter	Value
Spectral Type	K7 V
Effective Temperature	$T_{\star} = 4109^{+28}_{-27}$ K
Mass	$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$
Radius	$R_{\star} = 0.651 \pm 0.012 R_\odot$
Distance	$d = 36.01^{+0.06}_{-0.02}$ pc
Apparent Magnitude ($V$)	$10.89 \pm 0.01$
Apparent Magnitude ($K_S$)	$7.55 \pm 0.02$

The relatively bright apparent magnitudes facilitate high signal-to-noise follow-up opportunities, including emission spectroscopy and photometric phase-curve studies.

2. Orbital and Physical Properties of TOI-2431 b

TOI-2431 b was identified and confirmed via joint analysis of TESS photometry and high-precision radial velocities (RVs) obtained from NEID and HPF spectrographs. The combined floating-chunk-offset model yields the planet's parameters:

Parameter	Value
Orbital Period	$P = 0.22419577 \pm 5\times10^{-8}$ d ($\approx$5.38 h)
Semi-major Axis	$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$0 AU
Radius	$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$1
Mass	$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$2
Bulk Density	$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$3 g\,cm$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$4
Equilibrium Temperature	$M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$5 K (with $M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$6)

The planet's period of 5.38 hours establishes it as one of the shortest-period transiting exoplanets discovered to date, placing it at the extreme end of the USP population and allowing for unique physical and dynamical constraints.

3. Interior Composition and Physical Interpretation

In the planetary mass–radius plane (cf. Fig. 9 in (Taş et al., 11 Jul 2025)), TOI-2431 b aligns with the "Earth-like" density curve as defined by Zeng et al. (2019), corresponding to an interior composition of approximately $M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$7 iron and $M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$8 silicate rock. Its measured bulk density ($M_{\star} = 0.661^{+0.021}_{-0.024} M_\odot$9 g\,cm$R_{\star} = 0.651 \pm 0.012 R_\odot$0) notably exceeds that of Earth ($R_{\star} = 0.651 \pm 0.012 R_\odot$1 g\,cm$R_{\star} = 0.651 \pm 0.012 R_\odot$2), yet remains within the uncertainty bounds for a differentiated terrestrial planet. This composition is consistent with limited or no volatile envelope retention, as expected due to extreme irradiation and atmospheric escape.

Given its high dayside temperature ($R_{\star} = 0.651 \pm 0.012 R_\odot$3 K), which exceeds the melting point of common silicates, TOI-2431 b is categorized as a "lava-world." In such planets, surface rock may be partially or wholly molten, and the planet may possess only a tenuous or entirely absent refractory atmosphere.

4. Roche Limit Proximity and Orbital Evolution

TOI-2431 b's orbital period is only $R_{\star} = 0.651 \pm 0.012 R_\odot$4 greater than its calculated Roche period, the theoretical lower limit for a planet's survival against tidal disruption:

$R_{\star} = 0.651 \pm 0.012 R_\odot$5

For TOI-2431 b,

$R_{\star} = 0.651 \pm 0.012 R_\odot$6

Thus, TOI-2431 b resides at the edge of long-term tidal stability for a USP.

Assuming a constant-lag-angle tidal model, the characteristic orbital period decay timescale ($R_{\star} = 0.651 \pm 0.012 R_\odot$7) is:

$R_{\star} = 0.651 \pm 0.012 R_\odot$8

For $R_{\star} = 0.651 \pm 0.012 R_\odot$9, this gives:

$d = 36.01^{+0.06}_{-0.02}$0

By integrating inward to the Roche limit, the remaining lifetime of TOI-2431 b is $d = 36.01^{+0.06}_{-0.02}$1. This proximity to Roche instability implies the planet is in an advanced stage of orbital decay, and the current occurrence rate of similar systems constrains the timescale for tidal inspiral and possible destruction.

5. Detection and Validation Methodologies

The identification and characterization of TOI-2431 b employed a comprehensive suite of observational strategies:

• TESS Photometry: Five sectors (31, 42, 43, 70, 71), each providing 2-minute cadence PDCSAP light curves, were detrended using a Matérn-3/2 Gaussian Process to reveal the transit signal at a 5.38-hour period.
• Radial Velocity Measurements: NEID (12 exposures, median precision 2.9 m\,s$d = 36.01^{+0.06}_{-0.02}$2; analyzed with NEID‐SERVAL) and HPF (3 exposures, median precision 7.3 m\,s$d = 36.01^{+0.06}_{-0.02}$3; analyzed with HPF‐SERVAL) provided robust determination of the planetary mass.
• High-Contrast Imaging: NESSI speckle imaging at 562 nm and 832 nm excluded stellar companions between 0.2″ and 1.2″ with $d = 36.01^{+0.06}_{-0.02}$4–$d = 36.01^{+0.06}_{-0.02}$5 mag at 5$d = 36.01^{+0.06}_{-0.02}$6, and archival POSS/ZTF images ruled out background contaminants along the star's proper motion track.

This multi-instrument approach eliminates false positives from eclipsing binaries or background objects and secures the planetary interpretation.

6. Atmospheric Signatures and Phase-Curve Opportunities

Due to the high stellar flux and significant planet-to-star size ratio, TOI-2431 b presents a favorable target for atmospheric characterization by emission and phase-curve spectroscopy. The Emission Spectroscopy Metric (ESM) of Kempton et al. (2018):

$d = 36.01^{+0.06}_{-0.02}$7

For TOI-2431 b, this metric yields $d = 36.01^{+0.06}_{-0.02}$8, positioning it among the most promising USP planets for JWST phase-curve or secondary-eclipse observations.

Given the dayside temperature ($d = 36.01^{+0.06}_{-0.02}$9 K), silicate minerals on the surface are expected to be at or above melting temperatures, providing an opportunity to probe refractory surface compositions and potential residual atmospheres. JWST phase-curve data could measure surface mineralogy, detect trace refractory gases, and provide stringent constraints on atmospheric escape processes. This suggests the system may serve as a testbed for modeling the radiative and geochemical consequences of extreme irradiation on rocky exoplanets.

7. Scientific Context and Implications

TOI-2431 b exemplifies the most extreme class of rocky exoplanets: large, dense, and highly irradiated USP "lava-worlds." Its measurable orbital decay, as inferred from proximity to the Roche limit and from standard tidal theory, provides a critical datum for theoretical models of planetary tides and dynamical stability. The system is also a prime target for detailed JWST characterization, enabling constraints on the surface and atmospheric composition of lava worlds. A plausible implication is that such planets, found at the threshold of tidal disruption, may represent the end-stage for rocky planet evolution in close-in orbits (USP regime).

The discovery and detailed measurement of TOI-2431 b underscore the importance of joint photometric, spectroscopic, and imaging campaigns for identifying and characterizing extreme planetary systems (Taş et al., 11 Jul 2025). The star's brightness and proximity, combined with the planet's extreme properties, make TOI-2431 b a reference point for future studies on USP planet populations, tidal evolution, planetary structure, and atmospheric dynamics in high-irradiation regimes.