---
title: 'TOI-270 b: Hot Super-Earth in Compact System'
url: https://www.emergentmind.com/topics/toi-270-b
type: topic
---

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

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 [1903.06107, 2308.10763, 2403.03244, 2509.14224].

## 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 [1903.06107].

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 \(10.416\), together with low activity indicators, makes the system favorable for precise transit and radial-velocity work [2308.10763].

The three known planets form a compact near-resonant chain. TOI-270 b orbits every \(\sim 3.36\) d, TOI-270 c every \(\sim 5.66\) d, and TOI-270 d every \(\sim 11.38\) d. The period ratios \(P_c/P_b \approx 1.684\) and \(P_d/P_c \approx 2.011\) 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 [1903.06107, 2308.10763].

## 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 [1903.06107, 2308.10763, 2403.03244, 2509.14224].

The orbital period has been measured at progressively higher precision. The discovery paper reported
\[
P_b = 3.360080^{+0.000065}_{-0.000070}\ \mathrm{days},
\]
while a later TTV+RV analysis gave
\[
P_b = 3.35992 \pm 0.00005\ \mathrm{days},
\]
noting consistency with a purely photometric value of \(3.36016 \pm 0.000004\) d. JWST white-light analyses adopted \(P_b = 3.3601538\) d from earlier literature. Geometrically, the planet is nearly edge-on: the discovery fit found \(i_b = 88.65^{+0.85}_{-1.40}\) deg, the TTV-era photometric fit implied \(i \approx 88.5^\circ\) via \(\cos i = 0.0267^{+0.0012}_{-0.0184}\), and the JWST white-light fit yielded \(i_b = 89.39^{\circ}{}_{-0.29}^{+0.32}\) [1903.06107, 2308.10763, 2403.03244].

Radius estimates likewise evolved as the transit modeling improved. The discovery value was
\[
R_b = 1.247^{+0.089}_{-0.083}\,R_\oplus,
\]
with \(R_b/R_* = 0.0300^{+0.0015}_{-0.0011}\). The later TESS+ground analysis obtained \(R_b = 1.28 \pm 0.05\,R_\oplus\) from \(R_b/R_* = 0.0307^{+0.0012}_{-0.0025}\). JWST white-light fits then reported \(R_b = 1.302 \pm 0.028\,R_\oplus\) from \(R_p/R_* = 0.03142_{-0.00015}^{+0.00014}\), and subsequently \(R_p = 1.306 \pm 0.028\,R_\oplus\) from \(R_p/R_\star = 0.03144 \pm 0.00010\). This systematic upward revision in radius is central to later reinterpretations of the planet’s bulk composition [2308.10763, 2403.03244, 2509.14224].

The mass was initially only predicted from mass-radius relations, with values near \(1.5\,M_\oplus\) or \(1.9^{+1.5}_{-0.7}\,M_\oplus\). Incorporating radial-velocity constraints later produced an adopted mass
\[
M_b = 1.48^{+0.19}_{-0.18}\,M_\oplus,
\]
while JWST atmospheric work also used the RV-based value \(M_b = 1.58 \pm 0.26\,M_\oplus\) from Van Eylen (2021). Corresponding mean densities shifted from an implied rocky value of order \(5\ \mathrm{g\,cm^{-3}}\) in early work to explicitly quoted values of \(3.89 \pm 0.66\ \mathrm{g\,cm^{-3}}\) and \(3.7 \pm 0.5\ \mathrm{g\,cm^{-3}}\) in later studies [1903.06107, 2308.10763, 2403.03244, 2509.14224].

## 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 [2308.10763].

The discovery paper anticipated large TTVs from the near-resonant geometry and modeled them with *ttvfast* and direct \(N\)-body integrations. It found expected TTV amplitudes of \(\gtrsim 10\) min for planet c and \(\gtrsim 30\) min for planet d, with a super-period of \(1000\)–\(1100\) 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 [1903.06107].

A subsequent multi-observatory campaign detected clear TTVs for planets c and d, with amplitudes of \(\sim 10\) minutes and a super-period of \(\sim 3\) 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,
\[
e_b = 0.0167^{+0.0084}_{-0.0089},
\]
along with the mass measurement quoted above [2308.10763].

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 \(2\), and stability probabilities \(\gtrsim 70\)–\(90\%\) in FeatureClassifier and \(\gtrsim 97\%\) in DeepRegressor across explored mass-eccentricity maps. Small values of \(e_b\), including the measured \(e_b \sim 0.017\), 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 [2308.10763].

## 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 \(R_b \approx 1.25\,R_\oplus\), it lies below the planetary radius gap around \(1.7\)–\(2.0\,R_\oplus\), 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 [1903.06107].

That interpretation was strengthened by the 2023 TTV+RV analysis. Combining \(M_b = 1.48^{+0.19}_{-0.18}\,M_\oplus\) with \(R_b = 1.28 \pm 0.05\,R_\oplus\), the authors derived
\[
\rho_b = 3.89 \pm 0.66\ \mathrm{g\,cm^{-3}}
\]
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\(_2\)O atmosphere [2308.10763].

Later JWST-based analyses substantially complicated that picture. Using the updated white-light radius \(R_p = 1.306 \pm 0.028\,R_\oplus\) together with \(M_p = 1.48 \pm 0.18\,M_\oplus\), Coulombe et al. reported
\[
\rho_p = 3.7 \pm 0.5\ \mathrm{g\,cm^{-3}},
\]
arguing that TOI-270 b is inconsistent with an Earth-like composition at \(4.4\sigma\) and lies \(2.4\sigma\) 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 [2509.14224].

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 \(\mathrm{Fe/Mg} = 0.81 \pm 0.22\) as a proxy for planetary refractory composition, corresponding to \(f'_\mathrm{core} = 0.407 \pm 0.074\). Under these priors, the inferred water mass fraction is \(f_{\mathrm{H_2O}} = 5.1 \pm 1.1\%\) or \(5.1 \pm 1.0\%\), depending on whether stellar or solar Fe/Mg is used; a broader prior gives \(5.6^{+2.5}_{-2.2}\%\) [2509.14224].

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 [2308.10763, 2509.14224].

## 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 [2004.12475].

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 \(2.73\)–\(5.17\ \mu\mathrm{m}\), 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 \(3.3\)–\(3.5\sigma\), and, when informative priors on stellar heterogeneity are imported from TOI-270 d, an H\(_2\)-rich atmospheric model is preferred over the stellar-heterogeneity model at \(2.7\sigma\). The authors therefore concluded that there is marginal evidence for an H\(_2\)-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 [2403.03244].

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\(_2\)/He, and water-bearing atmospheres. They found that low- and moderate-metallicity H\(_2\)/He atmospheres produce features much larger than observed and are therefore strongly disfavored, while a pure H\(_2\)O atmosphere fits better than a flat line. More generally, retrievals favored a steam-rich H\(_2\)O atmosphere over a flat spectrum with \(\ln\mathcal{B} = 0.3\)–\(3.2\), the exact value depending on whether a free offset between the two NIRSpec detectors is allowed [2509.14224].

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 \(1.7\) 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 \(3\,\mu\mathrm{m}\) 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 [2509.14224].

Taken together, the atmospheric literature now contains two tentative but distinct scenarios for TOI-270 b: a low-\(\mu\), H\(_2\)-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 [2403.03244, 2509.14224].

## 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 \(\approx 530\) K assuming \(A = 0.3\) and inferred that the planet receives roughly \(18\times\) Earth’s insolation at \(a_b = 0.0306^{+0.0033}_{-0.0057}\) AU, far inside the habitable-zone interval \(0.10\)–\(0.28\) AU for the host star. Subsequent studies reported \(T_{\mathrm{eq},b}(A=0.3) = 548 \pm 15\) K and \(T_{\mathrm{eq},b}(A=0) = 600 \pm 16\) K, or \(T_{\rm eq} = 581 \pm 14\) K for zero albedo, and later \(T_{\mathrm{eq}, A_B=0} = 569^{+14}_{-13}\) K and \(T_{\mathrm{eq}, A_B=0.3} = 521^{+13}_{-12}\) 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 [1903.06107, 2308.10763, 2403.03244, 2509.14224].

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\(_2\)+H\(_2\)O atmosphere to a steam-dominated atmosphere to an H\(_2\)-dominated atmosphere with \(\sim 15\%\) H\(_2\)O by volume [1903.06107, 2509.14224].

TOI-270 b is also significant observationally. A Transmission Spectra Metric of \(102.1\) was reported for the planet in the terrestrial class, well above the suggested threshold of \(12\) 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 [2308.10763, 2509.14224].

In that sense, TOI-270 b has become more consequential as the literature has become less settled. It remains securely defined as a \( \sim 1.3\,R_\oplus \), \( \sim 1.5\,M_\oplus \), 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\(_2\)-rich envelope, or a warm steam-rich world with a few percent water by mass [2308.10763, 2403.03244, 2509.14224].

Source: https://www.emergentmind.com/topics/toi-270-b