LTT 3780 b: Ultra-Short-Period Rocky Planet

• LTT 3780 b is an ultra-short-period super-Earth with an Earth-like radius and high density, serving as a benchmark for rocky composition studies.
• It has been robustly characterized using TESS photometry, high-precision radial velocities, and JWST thermal emission measurements that confirm its rocky nature.
• Its location below the radius valley in a dual-planet system offers a controlled case to study atmospheric loss, interior structure, and exoplanet evolutionary processes.

LTT 3780 b is an ultra-short-period transiting super-Earth orbiting the nearby mid-M dwarf LTT 3780 (TOI-732, TIC 36724087) at a distance of about 22 pc. It is notable for combining an Earth-sized radius, a precisely measured mass, and extreme irradiation with membership in a two-planet system that straddles the small-planet radius valley: LTT 3780 b lies below the valley in the rocky regime, while the outer planet LTT 3780 c lies above it in the volatile-rich regime. The planet was first confirmed from TESS photometry and high-precision radial velocities, subsequently reanalyzed with larger multi-instrument RV data sets and host-star abundance measurements, and later observed in thermal emission with JWST/MIRI; across these studies, LTT 3780 b has emerged as a reference case for rocky-planet composition, atmospheric loss, and comparative exoplanetology around M dwarfs (Cloutier et al., 2020, Nowak et al., 2020, Weisserman et al., 8 Apr 2026, Allen et al., 19 Aug 2025).

1. Discovery and stellar environment

LTT 3780 b was discovered in TESS Sector 9 by the SPOC pipeline. The target, a Cool Dwarf list star observed at 2-minute cadence, showed two periodic transit signals in the PDCSAP light curve, with the inner signal corresponding to TOI-732.01, now LTT 3780 b. The initial TESS detection identified 28 transits at a period of approximately 0.768 d, establishing the planet immediately as an ultra-short-period (USP) candidate (Cloutier et al., 2020).

The host star has been consistently characterized as a nearby mid-M dwarf, though different analyses report somewhat different stellar parameter sets because they use different spectroscopic pipelines and global fits. One analysis reported an M4V star with $d = 21.981 \pm 0.040$ pc, $T_{\rm eff} = 3331 \pm 157$ K, $M_s = 0.401 \pm 0.012\,M_\odot$, $R_s = 0.374 \pm 0.011\,R_\odot$, and [Fe/H] $= +0.28^{+0.11}_{-0.13}$ dex (Cloutier et al., 2020). An independent CARMENES-based characterization gave an M3.5 V dwarf with $T_{\rm eff} = 3360 \pm 51$ K, $\log g = 4.81 \pm 0.04$, [Fe/H] $= +0.09 \pm 0.16$, $M_\star = 0.379 \pm 0.016\,M_\odot$, and $R_\star = 0.382 \pm 0.012\,R_\odot$ (Nowak et al., 2020). A later NIRPS abundance study re-derived $T_{\rm eff} = 3331 \pm 157$0, $T_{\rm eff} = 3331 \pm 157$1, $T_{\rm eff} = 3331 \pm 157$2, and $T_{\rm eff} = 3331 \pm 157$3 K (Weisserman et al., 8 Apr 2026).

The star appears magnetically quiet. Low chromospheric activity was reported with $T_{\rm eff} = 3331 \pm 157$4, no photometric rotation signal was detected in one analysis, and a rotation period of $T_{\rm eff} = 3331 \pm 157$5 d was inferred from activity–rotation relations (Cloutier et al., 2020). Another study found no significant long-baseline ASAS-SN variability and obtained a quasi-periodic GP hint of $T_{\rm eff} = 3331 \pm 157$6 d with amplitude below 300 ppm, again consistent with modest activity (Nowak et al., 2020). The system is also the primary of a wide binary with LP 729-55 at $T_{\rm eff} = 3331 \pm 157$7, corresponding to a projected separation of about 354 AU; this companion was found not to affect the measured RVs over the observational baseline (Cloutier et al., 2020).

2. Validation and measurement methodology

The confirmation of LTT 3780 b relied on a broad validation program combining reconnaissance spectroscopy, multi-site transit photometry, high-resolution imaging, and precise radial velocities. Reconnaissance TRES spectroscopy showed a single-lined spectrum, H$T_{\rm eff} = 3331 \pm 157$8 in absorption, and no resolved rotational broadening, with $T_{\rm eff} = 3331 \pm 157$9 km s$M_s = 0.401 \pm 0.012\,M_\odot$0. Ground-based transit observations were obtained with LCOGT 1 m telescopes at CTIO and SAAO in $M_s = 0.401 \pm 0.012\,M_\odot$1 and $M_s = 0.401 \pm 0.012\,M_\odot$2, LCOGT-SSO in $M_s = 0.401 \pm 0.012\,M_\odot$3, the OSN 1.5 m in $M_s = 0.401 \pm 0.012\,M_\odot$4 and $M_s = 0.401 \pm 0.012\,M_\odot$5, TRAPPIST-North in $M_s = 0.401 \pm 0.012\,M_\odot$6, and MEarth-North in its red band. High-resolution imaging with SOAR speckle in $M_s = 0.401 \pm 0.012\,M_\odot$7 band and Gemini/NIRI AO in Br$M_s = 0.401 \pm 0.012\,M_\odot$8 found no nearby contaminating sources at $M_s = 0.401 \pm 0.012\,M_\odot$9 within $R_s = 0.374 \pm 0.011\,R_\odot$0 and $R_s = 0.374 \pm 0.011\,R_\odot$1, respectively, and all follow-up transits were on-target without chromatic depth variations (Cloutier et al., 2020).

An independent discovery-and-confirmation campaign employed a partly overlapping but distinct follow-up network. It included MuSCAT2 multi-band transit photometry, additional LCOGT observations, TRAPPIST-North, SNO-T150, and Observatori Astronòmic Albanyà, together with FastCam/TCS lucky imaging, SOAR/HRCam speckle, Gemini North ‘Alopeke speckle, and Gemini North NIRI+ALTAIR AO. That campaign also measured 52 CARMENES spectra and found no activity-indicator power at the planetary frequencies in CRX, dLW, H$R_s = 0.374 \pm 0.011\,R_\odot$2, or CCF diagnostics, supporting the planetary interpretation and a low-activity RV environment (Nowak et al., 2020).

Radial-velocity characterization evolved substantially across the literature. The initial HARPS/HARPS-N study combined 33 HARPS spectra and 30 HARPS-N spectra, extracted with the TERRA template-matching pipeline, and analyzed them with two independent approaches: a fiducial model in which a GP-detrended TESS transit fit supplied strong ephemeris priors to a quasi-periodic GP RV model with two Keplerians, and a global EXOFASTv2 fit to all transit and RV data; both approaches returned consistent results (Cloutier et al., 2020). The later NIRPS program added 37 NIRPS spectra yielding 17 RVs over 17 nights, combined them with MAROON-X, CARMENES, HARPS, and HARPS-N, and modeled stellar variability with a per-instrument Gaussian process using celerite2’s SHO kernel with shared hyperparameters $R_s = 0.374 \pm 0.011\,R_\odot$3 and $R_s = 0.374 \pm 0.011\,R_\odot$4 and instrument-specific amplitudes $R_s = 0.374 \pm 0.011\,R_\odot$5 (Weisserman et al., 8 Apr 2026). This progression is central to understanding why the reported mass of LTT 3780 b has gradually tightened while remaining mutually consistent.

3. Orbital architecture and measured bulk properties

All major analyses agree that LTT 3780 b is a USP planet on a nearly circular orbit with a semimajor axis close to $R_s = 0.374 \pm 0.011\,R_\odot$6 AU. The orbit is generally modeled as circular because the tidal circularization time is very short at $R_s = 0.374 \pm 0.011\,R_\odot$7 d, and the planet is expected to be tidally locked (Cloutier et al., 2020).

Study	Orbit and size	Mass and bulk properties
HARPS/HARPS-N global fit (Cloutier et al., 2020)	$R_s = 0.374 \pm 0.011\,R_\odot$8 d; $R_s = 0.374 \pm 0.011\,R_\odot$9; $= +0.28^{+0.11}_{-0.13}$0 AU	$= +0.28^{+0.11}_{-0.13}$1; $= +0.28^{+0.11}_{-0.13}$2 g cm$= +0.28^{+0.11}_{-0.13}$3; $= +0.28^{+0.11}_{-0.13}$4 m s$= +0.28^{+0.11}_{-0.13}$5
CARMENES global fit (Nowak et al., 2020)	$= +0.28^{+0.11}_{-0.13}$6 d; $= +0.28^{+0.11}_{-0.13}$7; $= +0.28^{+0.11}_{-0.13}$8 au	$= +0.28^{+0.11}_{-0.13}$9; $T_{\rm eff} = 3360 \pm 51$0 g cm$T_{\rm eff} = 3360 \pm 51$1; $T_{\rm eff} = 3360 \pm 51$2 m s$T_{\rm eff} = 3360 \pm 51$3
NIRPS joint transit+RV analysis (Weisserman et al., 8 Apr 2026)	$T_{\rm eff} = 3360 \pm 51$4 d; $T_{\rm eff} = 3360 \pm 51$5; $T_{\rm eff} = 3360 \pm 51$6 AU	$T_{\rm eff} = 3360 \pm 51$7; $T_{\rm eff} = 3360 \pm 51$8 g cm$T_{\rm eff} = 3360 \pm 51$9; $\log g = 4.81 \pm 0.04$0 m s$\log g = 4.81 \pm 0.04$1

The transit geometry is correspondingly well constrained. In the EXOFASTv2 solution, the transit duration is $\log g = 4.81 \pm 0.04$2 h, the impact parameter is $\log g = 4.81 \pm 0.04$3, the inclination is $\log g = 4.81 \pm 0.04$4 deg, and the transit depth is $\log g = 4.81 \pm 0.04$5 ppt (Cloutier et al., 2020). The CARMENES fit reported $\log g = 4.81 \pm 0.04$6 h, $\log g = 4.81 \pm 0.04$7, $\log g = 4.81 \pm 0.04$8 deg, and $\log g = 4.81 \pm 0.04$9 (Nowak et al., 2020). The NIRPS analysis, adopting a re-derived stellar radius, found $= +0.09 \pm 0.16$0 (Weisserman et al., 8 Apr 2026).

Derived bulk quantities are straightforwardly computed from the reported mass and radius through

$= +0.09 \pm 0.16$1

and

$= +0.09 \pm 0.16$2

The literature correspondingly reports mean densities between about $= +0.09 \pm 0.16$3 and $= +0.09 \pm 0.16$4 g cm$= +0.09 \pm 0.16$5 and surface gravities between about $= +0.09 \pm 0.16$6 and $= +0.09 \pm 0.16$7 m s$= +0.09 \pm 0.16$8 (Cloutier et al., 2020, Nowak et al., 2020). Reported equilibrium temperatures also depend on the adopted stellar solution and thermal assumptions: $= +0.09 \pm 0.16$9 K for zero albedo and full redistribution in one analysis, with $M_\star = 0.379 \pm 0.016\,M_\odot$0 K for $M_\star = 0.379 \pm 0.016\,M_\odot$1 (Cloutier et al., 2020); $M_\star = 0.379 \pm 0.016\,M_\odot$2 K in the CARMENES study (Nowak et al., 2020); and $M_\star = 0.379 \pm 0.016\,M_\odot$3 K for zero Bond albedo in the NIRPS work (Weisserman et al., 8 Apr 2026). All of these values place the planet in a strongly irradiated regime.

4. Interior structure and composition

LTT 3780 b has consistently been classified as a rocky planet. In the HARPS/HARPS-N analysis, its mass–radius combination was found to be consistent with an Earth-like bulk composition and specifically with a fully differentiated interior of approximately 33% Fe core plus 67% MgSiO$M_\star = 0.379 \pm 0.016\,M_\odot$4 mantle. Its density and surface gravity were interpreted as those of a rocky, H/He-poor planet (Cloutier et al., 2020). The CARMENES study similarly placed the planet on rocky, terrestrial composition curves, describing it as consistent with an Earth-like interior ranging from silicate–iron mixtures of about 50/50 to silicate-dominated compositions up to 100% silicate (Nowak et al., 2020).

A later compositional reanalysis framed the problem in terms of core mass fraction,

$M_\star = 0.379 \pm 0.016\,M_\odot$5

Using the exopie interior model with an iron core and silicate mantle, and allowing silicon in the core and iron in the mantle to vary over $M_\star = 0.379 \pm 0.016\,M_\odot$6, the NIRPS study inferred

$M_\star = 0.379 \pm 0.016\,M_\odot$7

From host-star refractory abundances measured in the same work, exopie’s chemical module gave

$M_\star = 0.379 \pm 0.016\,M_\odot$8

For LTT 3780 b, the difference $M_\star = 0.379 \pm 0.016\,M_\odot$9 was explicitly described as not significant for the single system given the uncertainties (Weisserman et al., 8 Apr 2026).

That same study nevertheless used LTT 3780 b as part of a broader sample-wide argument that hot M-dwarf super-Earths may have core mass fractions slightly smaller than predicted from host-star chemistry. In that interpretation, the missing density is plausibly supplied by small water reservoirs rather than by a retained H/He atmosphere. For LTT 3780 b, the reported water-mass-fraction constraints were a 95% upper limit of $R_\star = 0.382 \pm 0.012\,R_\odot$0 for sequestered water in core+mantle partitioning, and $R_\star = 0.382 \pm 0.012\,R_\odot$1 for a differentiated surface-layer case (Weisserman et al., 8 Apr 2026). The paper favored interior sequestration for this planet because atmospheres are disfavored at $R_\star = 0.382 \pm 0.012\,R_\odot$2 K and because recent thermal-emission observations are consistent with a bare rock. A plausible implication is that LTT 3780 b is compositionally close to terrestrial rocky models while still allowing a low-level volatile inventory in high-pressure interior phases.

5. Radius-valley placement, dynamical regime, and evolutionary interpretation

The principal system-level significance of LTT 3780 b is that it shares a host star with LTT 3780 c while lying on the opposite side of the radius valley. LTT 3780 b, with a radius near $R_\star = 0.382 \pm 0.012\,R_\odot$3, lies below the valley in the rocky regime, whereas LTT 3780 c, with a radius near $R_\star = 0.382 \pm 0.012\,R_\odot$4–$R_\star = 0.382 \pm 0.012\,R_\odot$5, lies above it in the non-rocky or volatile-rich regime (Cloutier et al., 2020, Nowak et al., 2020). The pair therefore provides a controlled comparison under nearly identical stellar environment, and the original discovery papers explicitly emphasized the system as a laboratory for testing the origin of the radius valley around low-mass stars.

The dynamical coupling between the two planets is weak. The period ratio is $R_\star = 0.382 \pm 0.012\,R_\odot$6, far from first-order resonances, and predicted transit-timing-variation amplitudes are very small: about $R_\star = 0.382 \pm 0.012\,R_\odot$7 s for b and about $R_\star = 0.382 \pm 0.012\,R_\odot$8 s for c over two years (Cloutier et al., 2020). No evidence for additional planets was found in the RV residuals: BGLS periodograms showed no significant periodicity, three-planet tests did not converge on a third signal, and the quoted upper limit was $R_\star = 0.382 \pm 0.012\,R_\odot$9 m s$T_{\rm eff} = 3331 \pm 157$00 at 95% confidence (Cloutier et al., 2020). The CARMENES periodogram analysis likewise reported that after fitting the two planets, no other signals remained (Nowak et al., 2020).

Both principal atmospheric-loss frameworks discussed in the discovery literature are compatible with the measured masses. In the photoevaporation framework, the authors used comparative timescale arguments based on energy-limited escape and found that the relevant inequalities reduce, for this system, to $T_{\rm eff} = 3331 \pm 157$01 and $T_{\rm eff} = 3331 \pm 157$02, both trivially or comfortably satisfied by the measured RV masses (Cloutier et al., 2020). In the core-powered mass-loss scenario, the corresponding constraint gave $T_{\rm eff} = 3331 \pm 157$03 and $T_{\rm eff} = 3331 \pm 157$04, again consistent with the data (Cloutier et al., 2020). The system therefore does not uniquely distinguish between photoevaporation and core-powered mass loss. The authors further noted that the radius-valley slope around low-mass stars is closer to gas-poor terrestrial formation in occurrence-rate space, but the LTT 3780 mass measurements themselves do not rule out thermally driven loss mechanisms. This is an important point of interpretation: the system is an empirical anchor for radius-valley models, not a decisive discriminator among them.

6. Atmospheric constraints and thermal-emission measurements

Even before direct atmospheric observations, LTT 3780 b was identified as unusually favorable for emission studies. The initial HARPS/HARPS-N paper estimated an emission spectroscopy metric of about 13.4 and stated that a single JWST/MIRI eclipse should discriminate a bare rock from a $T_{\rm eff} = 3331 \pm 157$05 bar atmosphere (Cloutier et al., 2020). That forecast was later tested directly with JWST.

As part of the Hot Rocks Survey, two JWST/MIRI F1500W secondary eclipses of LTT 3780 b were observed back-to-back on 2024-05-04 and 2024-05-05. The combined analyses yielded a joint eclipse depth of $T_{\rm eff} = 3331 \pm 157$06 ppm in the transitspectroscopy reduction and $T_{\rm eff} = 3331 \pm 157$07 ppm in an independent alternate reduction. These measurements imply a dayside brightness temperature of

$T_{\rm eff} = 3331 \pm 157$08

which is $T_{\rm eff} = 3331 \pm 157$09 of the maximum possible dayside temperature for a zero-albedo, zero-heat-redistribution blackbody, with $T_{\rm eff} = 3331 \pm 157$10 K (Allen et al., 19 Aug 2025). The result is therefore consistent with negligible heat redistribution and a low-albedo bare rock.

The atmospheric constraints are strongest for CO$T_{\rm eff} = 3331 \pm 157$11 in the 15 $T_{\rm eff} = 3331 \pm 157$12m band. Using self-consistent 1D plane-parallel emission spectra computed with HELIOS, the JWST study found that pure CO$T_{\rm eff} = 3331 \pm 157$13 atmospheres produce too little 15 $T_{\rm eff} = 3331 \pm 157$14m emission and can be ruled out down to 0.01 bar surface pressure at greater than $T_{\rm eff} = 3331 \pm 157$15. O$T_{\rm eff} = 3331 \pm 157$16–CO$T_{\rm eff} = 3331 \pm 157$17 mixtures down to 10% CO$T_{\rm eff} = 3331 \pm 157$18 are ruled out to the same total pressures because CO$T_{\rm eff} = 3331 \pm 157$19 dominates opacity in the band. By contrast, a pure O$T_{\rm eff} = 3331 \pm 157$20 atmosphere cannot be ruled out in F1500W alone, and a 1 bar pure H$T_{\rm eff} = 3331 \pm 157$21O atmosphere is only weakly constrained at $T_{\rm eff} = 3331 \pm 157$22 (Allen et al., 19 Aug 2025). The paper argued that such an H$T_{\rm eff} = 3331 \pm 157$23O-rich atmosphere is intrinsically unlikely for such a highly irradiated planet.

The same JWST work placed LTT 3780 b in an atmospheric-escape context using order-of-magnitude energy-limited calculations. Adopting a conservative saturated-XUV timescale of 100 Myr and integrated saturated instellation $T_{\rm eff} = 3331 \pm 157$24 erg cm$T_{\rm eff} = 3331 \pm 157$25, it estimated a total atmospheric mass loss of about $T_{\rm eff} = 3331 \pm 157$26 kg over the saturated phase. The mass of 1 bar of atmosphere on the planet was estimated as $T_{\rm eff} = 3331 \pm 157$27 kg per bar, implying that more than 20,000 bars could be stripped during saturation; present-day loss was estimated at about $T_{\rm eff} = 3331 \pm 157$28 kg yr$T_{\rm eff} = 3331 \pm 157$29, or roughly 1 bar per 15 Myr (Allen et al., 19 Aug 2025). These estimates were explicitly identified as order-of-magnitude and uncertain in their treatment of M-dwarf XUV histories, magnetic effects, and outgassing, but they reinforce the inference that long-lived volatile-rich atmospheres are difficult to retain.

Surface composition remains less certain than atmospheric state. The JWST study tested Fe-oxidized, basaltic, ultramafic, metal-rich, granitoid, and feldspathic bare-rock surface models and found that all are consistent with the single F1500W data point (Allen et al., 19 Aug 2025). It therefore recommended multi-band secondary eclipses or phase curves with NIRSpec/G395H or MIRI/LRS to distinguish among bare-rock surface compositions and to test for any thin atmospheres that are spectrally inactive at 15 $T_{\rm eff} = 3331 \pm 157$30m.

7. Position in current exoplanet research

LTT 3780 b occupies a distinctive intersection of several active research programs. It is simultaneously a precisely weighed USP rocky planet, a component of a radius-valley pair around a bright M dwarf, a target for comparative atmospheric evolution, and a case study in linking planetary interiors to host-star refractory chemistry (Cloutier et al., 2020, Weisserman et al., 8 Apr 2026). The planet’s observational accessibility follows from the brightness and small size of the host star, while its interpretive value derives from the fact that the inner rocky planet and outer mini-Neptune share the same stellar environment.

Several misconceptions are usefully excluded by the present literature. First, the system does not yet decide between photoevaporation and core-powered mass loss; the measured masses are consistent with both frameworks (Cloutier et al., 2020). Second, the bare-rock interpretation from JWST does not mean that every atmosphere is ruled out: CO$T_{\rm eff} = 3331 \pm 157$31-dominated atmospheres are strongly excluded in the observed band, but pure O$T_{\rm eff} = 3331 \pm 157$32 and some H$T_{\rm eff} = 3331 \pm 157$33O-rich cases are not ruled out by F1500W alone (Allen et al., 19 Aug 2025). Third, the NIRPS compositional argument for low-level water reservoirs is not a high-significance result for LTT 3780 b individually; the $T_{\rm eff} = 3331 \pm 157$34 versus $T_{\rm eff} = 3331 \pm 157$35 offset is only about $T_{\rm eff} = 3331 \pm 157$36–$T_{\rm eff} = 3331 \pm 157$37 for this system, and the statistical weight comes from the ensemble trend across nine hot M-dwarf super-Earths (Weisserman et al., 8 Apr 2026).

Within those limits, the current synthesis is stable. LTT 3780 b is a hot, Earth-sized, high-density planet on a $T_{\rm eff} = 3331 \pm 157$38 d orbit, with RV semi-amplitude near 3.3–3.5 m s$T_{\rm eff} = 3331 \pm 157$39, radius near $T_{\rm eff} = 3331 \pm 157$40–$T_{\rm eff} = 3331 \pm 157$41, and mass near $T_{\rm eff} = 3331 \pm 157$42–$T_{\rm eff} = 3331 \pm 157$43 depending on the adopted global analysis (Cloutier et al., 2020, Nowak et al., 2020, Weisserman et al., 8 Apr 2026). Its bulk properties are consistent with a rocky composition, its present dayside emission is consistent with a low-albedo bare rock, and its location below the radius valley continues to make it one of the more informative M-dwarf planets for testing how small close-in planets lose or avoid retaining gaseous envelopes (Allen et al., 19 Aug 2025).