TOI-270 b: Hot Super-Earth in Compact System

• TOI-270 b is a short-period super-Earth with a radius around 1.3 R⊕ and mass near 1.5 M⊕, sparking debate over its rocky versus volatile-rich composition.
• It was discovered via TESS and validated with extensive follow-up, including ground-based photometry, spectroscopy, and JWST transit observations to refine its properties.
• Its placement below the radius valley, near-resonant orbital dynamics, and tentative atmospheric signals make it a key target for comparative exoplanetology.

TOI-270 b is the innermost known planet in the TOI-270 system, a compact three-planet architecture around the nearby M3V dwarf TOI-270 (L 231-32, TIC 259377017). It is a short-period transiting super-Earth with an orbital period of about $3.36$ days, a radius measured in the range $1.247^{+0.089}_{-0.083}$ to $1.306 \pm 0.028\,R_\oplus$, and a measured or adopted mass near $1.5\,M_\oplus$. Since its TESS discovery, TOI-270 b has been central to discussions of the super-Earth/sub-Neptune divide because it lies below the radius valley while its sibling planets TOI-270 c and d lie above it; correspondingly, the literature has moved from an early interpretation of TOI-270 b as a likely Earth-like rocky planet to later JWST-based arguments for a significant volatile inventory and possible atmosphere (Günther et al., 2019, Kaye et al., 2023, Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).

1. Discovery, host star, and system architecture

TOI-270 was identified in TESS short-cadence observations in Sectors 3–5, where the SPOC pipeline detected three periodic transit signals corresponding to planets b, c, and d. Follow-up validation used the TESS Follow-up Observing Program, including ground-based photometry, reconnaissance spectroscopy, high-resolution imaging, archival imaging, and statistical validation with vespa; TOI-270 b was thereby established as a genuine planet transiting the target star rather than a blended false positive (Günther et al., 2019).

The host star is consistently described as a nearby, bright, and relatively quiet mid-M dwarf. Refined stellar parameters reported for TOI-270 include distance $22.453 \pm 0.021$ pc, mass $0.386 \pm 0.008\,M_\odot$, radius $0.380 \pm 0.008\,R_\odot$, effective temperature $3506$ K, and mean stellar density $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$. Its brightness, with $K = 8.25$ and TESS magnitude $1.247^{+0.089}_{-0.083}$0, together with low activity indicators, makes the system favorable for precise transit and radial-velocity work (Kaye et al., 2023).

The three known planets form a compact near-resonant chain. TOI-270 b orbits every $1.247^{+0.089}_{-0.083}$1 d, TOI-270 c every $1.247^{+0.089}_{-0.083}$2 d, and TOI-270 d every $1.247^{+0.089}_{-0.083}$3 d. The period ratios $1.247^{+0.089}_{-0.083}$4 and $1.247^{+0.089}_{-0.083}$5 place the pairs near the 5:3 and 2:1 mean-motion resonances, respectively. This architecture is dynamically important because it drives strong transit timing variations for the outer pair and places TOI-270 b at the inner end of a resonant-like chain that cleanly samples both sides of the radius valley (Günther et al., 2019, Kaye et al., 2023).

2. Observational basis and measured properties

The physical characterization of TOI-270 b rests on several successive datasets and modeling frameworks. The discovery analysis used a global fit to TESS and follow-up photometry with allesfitter, ellc transit models, and Gaussian Process noise with a Matern 3/2 kernel, assuming circular orbits because eccentric fits were not favored. Later work re-analyzed TESS Sectors 3–5, 30, and 32 together with extensive ground-based photometry using BATMAN, LDTk-constrained quadratic limb darkening, Gaussian Processes for correlated noise, and DEMCMC TTV fitting with ttvfast. JWST studies added NIRSpec/G395H transit spectroscopy, using batman transit models, MultiNest or emcee sampling, and independent reduction pipelines such as JExoRES, exoTEDRF, Eureka!, and Tiberius (Günther et al., 2019, Kaye et al., 2023, Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).

The orbital period has been measured at progressively higher precision. The discovery paper reported

$1.247^{+0.089}_{-0.083}$6

while a later TTV+RV analysis gave

$1.247^{+0.089}_{-0.083}$7

noting consistency with a purely photometric value of $1.247^{+0.089}_{-0.083}$8 d. JWST white-light analyses adopted $1.247^{+0.089}_{-0.083}$9 d from earlier literature. Geometrically, the planet is nearly edge-on: the discovery fit found $1.306 \pm 0.028\,R_\oplus$0 deg, the TTV-era photometric fit implied $1.306 \pm 0.028\,R_\oplus$1 via $1.306 \pm 0.028\,R_\oplus$2, and the JWST white-light fit yielded $1.306 \pm 0.028\,R_\oplus$3 (Günther et al., 2019, Kaye et al., 2023, Holmberg et al., 2024).

Radius estimates likewise evolved as the transit modeling improved. The discovery value was

$1.306 \pm 0.028\,R_\oplus$4

with $1.306 \pm 0.028\,R_\oplus$5. The later TESS+ground analysis obtained $1.306 \pm 0.028\,R_\oplus$6 from $1.306 \pm 0.028\,R_\oplus$7. JWST white-light fits then reported $1.306 \pm 0.028\,R_\oplus$8 from $1.306 \pm 0.028\,R_\oplus$9, and subsequently $1.5\,M_\oplus$0 from $1.5\,M_\oplus$1. This systematic upward revision in radius is central to later reinterpretations of the planet’s bulk composition (Kaye et al., 2023, Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).

The mass was initially only predicted from mass-radius relations, with values near $1.5\,M_\oplus$2 or $1.5\,M_\oplus$3. Incorporating radial-velocity constraints later produced an adopted mass

$1.5\,M_\oplus$4

while JWST atmospheric work also used the RV-based value $1.5\,M_\oplus$5 from Van Eylen (2021). Corresponding mean densities shifted from an implied rocky value of order $1.5\,M_\oplus$6 in early work to explicitly quoted values of $1.5\,M_\oplus$7 and $1.5\,M_\oplus$8 in later studies (Günther et al., 2019, Kaye et al., 2023, Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).

3. Orbital dynamics and transit timing variations

TOI-270 b occupies the dynamically important inner node of a near-resonant chain. The system is near low-order commensurabilities, but later dynamical analysis showed that the resonant angles circulate rather than librate, so the architecture is near-resonant rather than resonantly locked. In this configuration, the strongest observed transit timing variations arise from the c–d pair near 2:1, whereas the b–c pair near 5:3 produces weaker timing effects (Kaye et al., 2023).

The discovery paper anticipated large TTVs from the near-resonant geometry and modeled them with ttvfast and direct $1.5\,M_\oplus$9-body integrations. It found expected TTV amplitudes of $22.453 \pm 0.021$0 min for planet c and $22.453 \pm 0.021$1 min for planet d, with a super-period of $22.453 \pm 0.021$2–$22.453 \pm 0.021$3 days. For TOI-270 b, TTVs were regarded as potentially detectable but more uncertain, and the short initial timing baseline sampled only an approximately linear segment of the full waveform (Günther et al., 2019).

A subsequent multi-observatory campaign detected clear TTVs for planets c and d, with amplitudes of $22.453 \pm 0.021$4 minutes and a super-period of $22.453 \pm 0.021$5 years, but did not claim a comparably clean detection for TOI-270 b. The authors explicitly stated that, in the TTV-only fit, they used only linear ephemerides for planet b and fixed both its eccentricity and its mass to isolate the dominant c–d dynamics. In the combined TTV+RV solution, however, TOI-270 b acquired a small but statistically non-zero eccentricity,

$22.453 \pm 0.021$6

along with the mass measurement quoted above (Kaye et al., 2023).

Long-term dynamical analyses also place TOI-270 b in a highly stable configuration. Using REBOUND, SPOCK, and MEGNO, later work found regular, non-chaotic motion with MEGNO clustered around $22.453 \pm 0.021$7, and stability probabilities $22.453 \pm 0.021$8–$22.453 \pm 0.021$9 in FeatureClassifier and $0.386 \pm 0.008\,M_\odot$0 in DeepRegressor across explored mass-eccentricity maps. Small values of $0.386 \pm 0.008\,M_\odot$1, including the measured $0.386 \pm 0.008\,M_\odot$2, lie well within the stable regime. A plausible implication is that tidal damping may have reduced the eccentricity of this close-in planet, while ongoing secular forcing from c and d maintains a slight residual excitation (Kaye et al., 2023).

4. Interior structure and competing composition interpretations

From its discovery onward, TOI-270 b was recognized as occupying a different compositional regime from TOI-270 c and d. With $0.386 \pm 0.008\,M_\odot$3, it lies below the planetary radius gap around $0.386 \pm 0.008\,M_\odot$4–$0.386 \pm 0.008\,M_\odot$5, whereas c and d lie above it. The discovery study therefore interpreted the system as a compact realization of the radius-valley dichotomy: TOI-270 b likely in the Earth-like/rocky regime, and TOI-270 c and d possibly water-ice or gas-dominated sub-Neptunes (Günther et al., 2019).

That interpretation was strengthened by the 2023 TTV+RV analysis. Combining $0.386 \pm 0.008\,M_\odot$6 with $0.386 \pm 0.008\,M_\odot$7, the authors derived

$0.386 \pm 0.008\,M_\odot$8

and explicitly concluded that their findings imply “an Earth-like rocky composition for the inner planet,” contrasting TOI-270 b with the outer two planets, which they described as Earth-like cores with an additional He/H$0.386 \pm 0.008\,M_\odot$9O atmosphere (Kaye et al., 2023).

Later JWST-based analyses substantially complicated that picture. Using the updated white-light radius $0.380 \pm 0.008\,R_\odot$0 together with $0.380 \pm 0.008\,R_\odot$1, Coulombe et al. reported

$0.380 \pm 0.008\,R_\odot$2

arguing that TOI-270 b is inconsistent with an Earth-like composition at $0.380 \pm 0.008\,R_\odot$3 and lies $0.380 \pm 0.008\,R_\odot$4 above the curve for pure rock. In their interpretation, the planet requires extra low-density material and is best explained by a non-zero, percent-level water mass fraction (Coulombe et al., 17 Sep 2025).

The interior modeling in that study used the smint code and a three-layer structure: iron-rich core, silicate mantle, and hydrosphere, the latter comprising a supercritical water layer plus a steam atmosphere. A notable aspect of the analysis is its use of the stellar abundance ratio $0.380 \pm 0.008\,R_\odot$5 as a proxy for planetary refractory composition, corresponding to $0.380 \pm 0.008\,R_\odot$6. Under these priors, the inferred water mass fraction is $0.380 \pm 0.008\,R_\odot$7 or $0.380 \pm 0.008\,R_\odot$8, depending on whether stellar or solar Fe/Mg is used; a broader prior gives $0.380 \pm 0.008\,R_\odot$9 (Coulombe et al., 17 Sep 2025).

The result is a genuine interpretive tension in the literature. One line of work describes TOI-270 b as a bare or nearly bare rocky super-Earth; another, using revised radius measurements and different interior modeling, argues that it is underdense relative to an Earth-like rock and therefore a volatile-bearing super-Earth or water-world candidate below the radius valley. The disagreement is not merely semantic: it arises from small but consequential shifts in the measured radius, from updated structure models, and from different assumptions about how stellar abundances should inform planetary interiors (Kaye et al., 2023, Coulombe et al., 17 Sep 2025).

5. Atmospheric characterization and spectroscopic interpretations

Early JWST-oriented modeling of the TOI-270 system concentrated on planets c and d rather than b. Those studies treated the outer planets as prime transmission-spectroscopy targets and used petitRADTRANS and PandExo to show that clear or cloudy H-rich atmospheres on c and d should be detectable with NIRISS, NIRSpec, and MIRI in one or a few transits. TOI-270 b was not modeled as a primary target in that work, reflecting the then-prevailing view that it was likely rocky and that any atmosphere would be comparatively difficult to probe in transmission (Chouqar et al., 2020).

JWST NIRSpec/G395H observations later changed that assessment by capturing simultaneous transits of TOI-270 b and TOI-270 d. The 2024 analysis extracted a transmission spectrum for TOI-270 b over $3506$0–$3506$1, masking the overlap interval between the two transits and testing models with AURA. The main conclusion was cautious but notable: the spectrum is inconsistent with a featureless spectrum at $3506$2–$3506$3, and, when informative priors on stellar heterogeneity are imported from TOI-270 d, an H$3506$4-rich atmospheric model is preferred over the stellar-heterogeneity model at $3506$5. The authors therefore concluded that there is marginal evidence for an H$3506$6-rich atmosphere on TOI-270 b, while emphasizing that the signal-to-noise ratio is low and that no robust molecular detections or abundance constraints were obtained (Holmberg et al., 2024).

A later reanalysis of essentially the same NIRSpec/G395H event drew a different, though not incompatible, spectroscopic inference. Using exoTEDRF as the primary reduction pipeline, 113 spectroscopic channels, and the SCARLET framework, Coulombe et al. compared flat, self-consistent H$3506$7/He, and water-bearing atmospheres. They found that low- and moderate-metallicity H$3506$8/He atmospheres produce features much larger than observed and are therefore strongly disfavored, while a pure H$3506$9O atmosphere fits better than a flat line. More generally, retrievals favored a steam-rich H$10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$0O atmosphere over a flat spectrum with $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$1–$10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$2, the exact value depending on whether a free offset between the two NIRSpec detectors is allowed (Coulombe et al., 17 Sep 2025).

The 2025 study also addressed a central ambiguity in M-dwarf transmission spectroscopy: the transit-light-source effect. Because TOI-270 d transited almost simultaneously and is about $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$3 times larger in radius, it served as a built-in control on stellar contamination. Atmospheric-plus-TLS retrievals on TOI-270 d favored negligible spot coverage and spot contrast, and these constraints, propagated as priors into the TOI-270 b analysis, strongly disfavored stellar heterogeneity as the origin of the putative $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$4 water feature. Even so, the authors stressed that the evidence remains “possible” and “inconclusive to moderate,” not definitive, because the water preference weakens substantially when a detector-offset parameter is introduced (Coulombe et al., 17 Sep 2025).

Taken together, the atmospheric literature now contains two tentative but distinct scenarios for TOI-270 b: a low-$10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$5, H$10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$6-rich atmosphere weakly favored over stellar contamination, and a steam-rich, water-dominated atmosphere weakly favored over flatness once the updated radius and density are taken into account. Both interpretations are explicitly presented as provisional, and both require additional data for discrimination (Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).

6. Irradiation, habitability, and comparative significance

All published analyses place TOI-270 b well interior to the system’s habitable zone. The discovery paper estimated an equilibrium temperature of $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$7 K assuming $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$8 and inferred that the planet receives roughly $10.63 \pm 0.74\ \mathrm{g\,cm^{-3}}$9 Earth’s insolation at $K = 8.25$0 AU, far inside the habitable-zone interval $K = 8.25$1–$K = 8.25$2 AU for the host star. Subsequent studies reported $K = 8.25$3 K and $K = 8.25$4 K, or $K = 8.25$5 K for zero albedo, and later $K = 8.25$6 K and $K = 8.25$7 K. Despite small differences among adopted stellar and orbital parameters, the consistent conclusion is that TOI-270 b is a hot or warm super-Earth rather than a habitable-zone planet (Günther et al., 2019, Kaye et al., 2023, Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).

Habitability is therefore not the central issue; atmospheric retention and volatile evolution are. Early work inferred that TOI-270 b was likely a stripped rocky core, an outcome consistent with photoevaporation and the radius-valley framework. Later work argued that, despite its irradiation and its location near the empirical cosmic shoreline, the planet could retain a substantial atmosphere over Gyr timescales if it formed with a large volatile inventory and if those volatiles are buffered by a molten or partially molten interior. PACMAN-P simulations produced illustrative end states ranging from an O$K = 8.25$8+H$K = 8.25$9O atmosphere to a steam-dominated atmosphere to an H$1.247^{+0.089}_{-0.083}$00-dominated atmosphere with $1.247^{+0.089}_{-0.083}$01 H$1.247^{+0.089}_{-0.083}$02O by volume (Günther et al., 2019, Coulombe et al., 17 Sep 2025).

TOI-270 b is also significant observationally. A Transmission Spectra Metric of $1.247^{+0.089}_{-0.083}$03 was reported for the planet in the terrestrial class, well above the suggested threshold of $1.247^{+0.089}_{-0.083}$04 for follow-up, and the system’s bright, quiet M dwarf has long made it a favorable target for radial-velocity and transit work. At the same time, the contrast between TOI-270 b and the more obviously volatile-rich c and d makes the system unusually valuable for comparative planetology. If the rocky interpretation is correct, TOI-270 b is a stripped inner reference world paired with two sub-Neptunes. If the volatile-rich interpretation is correct, it becomes an important example of a super-Earth below the radius valley that nonetheless retains significant water and perhaps an atmosphere (Kaye et al., 2023, Coulombe et al., 17 Sep 2025).

In that sense, TOI-270 b has become more consequential as the literature has become less settled. It remains securely defined as a $1.247^{+0.089}_{-0.083}$05, $1.247^{+0.089}_{-0.083}$06, short-period planet orbiting a quiet nearby M dwarf, and it remains dynamically embedded in one of the best-characterized compact multiplanet systems known. What is currently under revision is not its existence or basic geometry, but its physical nature: whether it is best described as a rocky super-Earth with little atmosphere, a volatile-bearing super-Earth with a thin H$1.247^{+0.089}_{-0.083}$07-rich envelope, or a warm steam-rich world with a few percent water by mass (Kaye et al., 2023, Holmberg et al., 2024, Coulombe et al., 17 Sep 2025).