LHS 3844 b: Ultra-Short-Period Rocky Exoplanet

• LHS 3844 b is an ultra-short-period terrestrial exoplanet characterized by its ~11-hour orbit, rocky composition, and absence of a substantial atmosphere.
• Observations from TESS, ESPRESSO, and JWST have precisely constrained its mass, radius, and surface mineralogy, reinforcing its archetype status among airless rocky worlds.
• Its well-defined tidal locking, surface weathering patterns, and evidence of atmospheric loss provide a benchmark for studying exogeology and planetary evolution.

LHS 3844 b is a canonical ultra-short-period (USP), terrestrial exoplanet orbiting the nearby mid-M dwarf LHS 3844, distinguished by its proximity, large planet-to-star flux contrast, and the absence of a substantial atmosphere. Located 15 parsecs from Earth, the system has enabled precision constraints on planetary structure, surface composition, and atmospheric loss processes through an array of observational and modeling campaigns spanning the TESS discovery, ground- and space-based spectroscopy, and the James Webb Space Telescope (JWST). This exoplanet has emerged as an archetype for the empirical study of airless, tidally locked rocky worlds.

1. System Parameters and Discovery

LHS 3844 b was identified as a transiting planet in TESS photometry with a 0.46293-day (11.1-hour) period (Vanderspek et al., 2018). Transit depth analysis yields a robust planetary radius $R_p = 1.286^{+0.043}_{-0.044}\, R_\oplus$, with independent measurements converging at $R_p = 1.303 \pm 0.022 R_\oplus$ (Hacker et al., 30 Mar 2026). Radial velocity campaigns employing ESPRESSO and CRIRES+ have converged on $M_p = 2.27 \pm 0.23\, M_\oplus$ (Hacker et al., 30 Mar 2026), with consistent results from multiple datasets and RV models (Nagel et al., 30 Mar 2026). This infers a bulk density $\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$, consistent with a predominantly rocky interior.

The host M5.0 V star has $M_*=0.151\pm0.014\,M_\odot$, $R_*=0.189\pm0.006\,R_\odot$, $T_\mathrm{eff}=3080\pm50$ K, and an age of $7.8\pm1.6$ Gyr, and is photometrically and chromospherically inactive with a rotation period of $\sim$130 days (Nagel et al., 30 Mar 2026). The planet orbits at $a=0.00622\pm0.00017$ AU ($R_p = 1.303 \pm 0.022 R_\oplus$0), yielding an insolation $R_p = 1.303 \pm 0.022 R_\oplus$1 and (for Bond albedo $R_p = 1.303 \pm 0.022 R_\oplus$2) an equilibrium temperature $R_p = 1.303 \pm 0.022 R_\oplus$3 K.

2. Orbital Dynamics, Tidal Locking, and Eccentricity

Extremely short-period, nearly circular orbits ($R_p = 1.303 \pm 0.022 R_\oplus$4 d, $R_p = 1.303 \pm 0.022 R_\oplus$5 (Lyu et al., 2023, Zieba et al., 30 Apr 2026)) imply rapid tidal synchronization. Analysis of phase-curve data and the absence of observable tidal heating rule out non-synchronous rotation states (e.g., Mercury-like 3:2 spin-orbit resonance) (Lyu et al., 2023). The combination of mass, semimajor axis, and M-dwarf properties ensures tidal-locking timescales $R_p = 1.303 \pm 0.022 R_\oplus$6 system age, yielding a permanent dayside and nightside and strong spatial gradients in irradiation, surface temperatures, and potential weathering processes.

The planet's eccentricity is tightly limited ($R_p = 1.303 \pm 0.022 R_\oplus$7 from phase-curve modeling; $R_p = 1.303 \pm 0.022 R_\oplus$8 at 2σ from orbital fits (Zieba et al., 30 Apr 2026)). The low eccentricity severely constrains ongoing tidal heating, with plausible explanations for the observed thermal flux focusing on space weathering as the dominant process altering surface properties.

3. Bulk Composition, Interior Structure, and Formation

Density and mass-radius constraints are consistent with an Earth-like rocky composition (Hacker et al., 30 Mar 2026, Nagel et al., 30 Mar 2026). Bayesian interior-structure inversions yield core mass fractions of $R_p = 1.303 \pm 0.022 R_\oplus$9–$M_p = 2.27 \pm 0.23\, M_\oplus$0, highly consistent with Earth's value of $M_p = 2.27 \pm 0.23\, M_\oplus$1 and requiring minimal water/volatile content ($M_p = 2.27 \pm 0.23\, M_\oplus$2 wt%). All interior models disfavor a substantial H/He envelope and are inconsistent with high-density “super-Mercury” cores, thus excluding a giant impact scenario with extreme mantle stripping.

Thermal evolution and geodynamical models demonstrate that LHS 3844 b's lack of outgassed volatiles is consistent with a volatile-poor mantle, likely tracing to formation inside the system snow-line. Simulations indicate that an initial mantle C inventory $M_p = 2.27 \pm 0.23\, M_\oplus$3 wt% is required to remain compatible with current atmosphere limits. Catastrophic impact stripping or late heavy bombardment scenarios require implausibly large or numerous impactors to account for the observed lack of outgassed material without simultaneous core-enrichment (Kane et al., 2020).

4. Surface Composition and Exogeology

Mid-infrared JWST/MIRI/LRS emission spectroscopy provides direct constraints on LHS 3844 b's surface mineralogy (Zieba et al., 30 Apr 2026). The observed disk-averaged spectrum (5.06–12.37 μm, eclipse depth $M_p = 2.27 \pm 0.23\, M_\oplus$4 ppm) is fit by a blackbody of $M_p = 2.27 \pm 0.23\, M_\oplus$5 K. The surface is best described by basaltic to ultramafic (olivine-clinopyroxenite) rock types (SiO$M_p = 2.27 \pm 0.23\, M_\oplus$6 47–51 wt%), with strong exclusion ($M_p = 2.27 \pm 0.23\, M_\oplus$7) of felsic (high-silica) crusts, fresh rock powders, or high-albedo mineralogies.

Space weathering signatures dominate: the lack of spectral features and low Bond albedo $M_p = 2.27 \pm 0.23\, M_\oplus$8 are consistent with nanophase iron (npFe; $M_p = 2.27 \pm 0.23\, M_\oplus$9 by volume) and carbon darkening, paralleling surface maturation processes of airless Solar System bodies. The absence of Si–O stretch features and strong constraints against SO$\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$0 or CO$\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$1 emission lines ($\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$2 mbar CO$\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$3 excluded at $\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$4, $\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$5b SO$\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$6 at $\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$7) indicate a long-term quiescence and evolved surface (Zieba et al., 30 Apr 2026).

There is no evidence for high-SiO$\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$8 crust analogous to Earth's continents, nor for recent volcanic resurfacing. The dark, low-albedo surface closely matches Mercury and the lunar maria, supporting parallels in igneous composition and regolith maturation (Zieba et al., 30 Apr 2026).

5. Atmospheric Loss, Escape, and Current Constraints

Comprehensive constraints indicate an absence of any significant atmosphere. Transmission spectroscopy (620–1020 nm, 13 transits) rules out clear, low–$\rho = 5.67^{+0.65}_{-0.61}\,\mathrm{g\,cm}^{-3}$9 (solar composition, $M_*=0.151\pm0.014\,M_\odot$0) atmospheres at $M_*=0.151\pm0.014\,M_\odot$1 bar to $M_*=0.151\pm0.014\,M_\odot$2, water vapor atmospheres ($M_*=0.151\pm0.014\,M_\odot$3) at $M_*=0.151\pm0.014\,M_\odot$4, and cloud-deck atmospheres with $M_*=0.151\pm0.014\,M_\odot$5 bar at $M_*=0.151\pm0.014\,M_\odot$6 (Diamond-Lowe et al., 2020). Spitzer phase-curve brightness temperatures ($M_*=0.151\pm0.014\,M_\odot$7 K, $M_*=0.151\pm0.014\,M_\odot$8 K) require $M_*=0.151\pm0.014\,M_\odot$9 bar for thick atmospheres. JWST/MIRI observations further exclude CO$R_*=0.189\pm0.006\,R_\odot$0-rich secondary atmospheres (surface pressures $R_*=0.189\pm0.006\,R_\odot$1 mbar ruled out at $R_*=0.189\pm0.006\,R_\odot$2; O$R_*=0.189\pm0.006\,R_\odot$3 bar at $R_*=0.189\pm0.006\,R_\odot$4) (Zieba et al., 30 Apr 2026).

The host's high accumulated XUV/UV output—even in its current inactive phase ($R_*=0.189\pm0.006\,R_\odot$5 erg cm$R_*=0.189\pm0.006\,R_\odot$6 s$R_*=0.189\pm0.006\,R_\odot$7)—drives hydrodynamic escape sufficient to fully remove a primordial H/He envelope in $R_*=0.189\pm0.006\,R_\odot$850 Myr (Diamond-Lowe et al., 2021). Geodynamical models reinforce this picture: degassing in the earliest epochs may have reached $R_*=0.189\pm0.006\,R_\odot$9 bar, but the present volatile budget indicates efficient loss or original depletion (Kane et al., 2020). Impact-driven escape is secondary given the low current volatile content and is strongly disfavored as the dominant mechanism.

The possibility of trace, high-μ secondary atmospheres (O$T_\mathrm{eff}=3080\pm50$0, CO$T_\mathrm{eff}=3080\pm50$1 at $T_\mathrm{eff}=3080\pm50$2 bar) cannot be excluded by current constraints, but no observational signatures have been detected (Diamond-Lowe et al., 2020, Zieba et al., 30 Apr 2026).

6. Atmospheric Retrieval, Spectroscopy, and JWST Prospects

Atmospheric retrieval frameworks (e.g., HyDRo) reveal that with as few as 8 JWST/MIRI secondary eclipses, CO$T_\mathrm{eff}=3080\pm50$3-rich atmospheres ($T_\mathrm{eff}=3080\pm50$4Venus-like) could be detected or excluded at $T_\mathrm{eff}=3080\pm50$5 (Piette et al., 2021). Ruling out a bare-rock spectrum under highly agnostic assumptions requires $T_\mathrm{eff}=3080\pm50$620 eclipses at $T_\mathrm{eff}=3080\pm50$7 when the composition is unknown or potentially water-dominated. Temperature profiles across the MIRI photosphere are constrained to $T_\mathrm{eff}=3080\pm50$8200 K, and even with high-albedo or cloudy atmospheres, inferred temperatures would deviate appreciably from surface blackbody predictions.

JWST’s mid-infrared sensitivity enables distinguishing mineralogies through Si–O vibrational bands and surface emissivity contrasts. The confirmed low albedo and featureless spectrum substantiate the methodology by demonstrating that JWST/MIRI can directly detect the mineralogy and weathering history of rocky exoplanet surfaces. LHS 3844 b serves as the first sub-$T_\mathrm{eff}=3080\pm50$9 exoplanet with unambiguous mid-IR surface characterization and is a prime benchmark for future surface and phase-curve studies (Zieba et al., 30 Apr 2026, Piette et al., 2021).

7. Astrophysical and Comparative Context

LHS 3844 b is the nearest, best-characterized ultra-short-period super-Earth lacking a significant atmosphere. Its properties exemplify the effects of intense irradiation, M-dwarf driven atmospheric loss, and subsequent surface weathering on rocky exoplanets. The surface parallels Mercury and the Moon in albedo, weathering processes, and basal igneous mineralogy. There is no evidence for plate-tectonic–style surface evolution, nor for significant interior water. Its exclusion of volatile-rich compositions and evidence for ancient surface darkening provide a critical empirical anchor for understanding atmospheric retention as a function of planetary mass, orbit, and host-star spectral type.

Potential additional planetary companions (e.g., a tentative $7.8\pm1.6$0 d RV signal) remain unconfirmed (Nagel et al., 30 Mar 2026, Hacker et al., 30 Mar 2026). The system provides a crucial platform for exploring the boundary conditions for atmospheric erosion, super-Earth bulk structure, and the onset of surface spectroscopy for terrestrial exoplanets. LHS 3844 b’s unique combination of high planet-to-star flux contrast, proximity, and absence of confounding atmospheric effects positions it as a reference point for the exoplanet “exogeology” discipline.