---
title: 'HAT-P-26 b: Warm Neptune Benchmark'
url: https://www.emergentmind.com/topics/hat-p-26-b
type: topic
---

# HAT-P-26 b: Warm Neptune Benchmark

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HAT-P-26 b is a transiting warm Neptune orbiting the K1 dwarf HAT-P-26 (GSC 0320-01027) with an orbital period near 4.2345 days. Since its discovery as a low-density Neptune-mass planet, it has become a benchmark target for transmission spectroscopy, atmospheric metallicity studies, transit-timing analyses, and, more recently, JWST sulfur-chemistry measurements. The system is notable both for the planet’s extended H/He-rich atmosphere and for persistent evidence that the HAT-P-26 system may be dynamically more complex than a single-planet configuration [1010.1008].

## 1. Discovery, host star, and observational identification

HAT-P-26 b was discovered by the HATNet survey, which monitored the host star in 2009 and identified a significant transit signal with \(P = 4.234516 \pm 0.000015\) d, \(T_c = 2455304.65122 \pm 0.00035\) (BJD), and transit duration \(0.1023 \pm 0.0010\) d. The host star was characterized in the discovery analysis as a moderately bright \(V=11.744\) K1 dwarf with \(M_\star = 0.82 \pm 0.03\,M_\odot\), \(R_\star = 0.79^{+0.10}_{-0.04}\,R_\odot\), \(T_{\rm eff} = 5079 \pm 88\) K, and \([\mathrm{Fe/H}] = -0.04 \pm 0.08\). Reconnaissance spectroscopy, Keck/HIRES radial velocities, bisector analysis, archival imaging, and explicit blend modeling were used to exclude eclipsing-binary and hierarchical-triple interpretations, establishing the object as a bona fide transiting planet [1010.1008].

Subsequent stellar analyses refined the stellar picture without changing its qualitative classification. A HARPS-N and Gaia-based study reported \(T_{\rm eff} = 5100 \pm 20\) K, \([{\rm Fe/H}] = +0.05 \pm 0.10\), \(v\sin i_\star = 1.9 \pm 0.4\ {\rm km\,s^{-1}}\), \(M_\star = 0.796 \pm 0.015\ (\text{stat}) \pm 0.026\ (\text{sys})\ M_\odot\), and \(R_\star = 0.916 \pm 0.062\ (\text{stat}) \pm 0.010\ (\text{sys})\ R_\odot\), while also noting no close Gaia companions within \(10''\) and no detectable rotational modulation in HATNet photometry [2205.10549].

The star’s low activity has been a recurring feature of the system’s interpretation. The discovery paper measured \(S_{\rm HK} \approx 0.182\) and \(\log R'_{\rm HK} \approx -4.99\), identifying HAT-P-26 as chromospherically quiet. Later timing studies likewise treated stellar activity as an unlikely explanation for the system’s coherent transit-timing behavior, which is significant because transit-timing variation analyses are often limited by starspot-induced timing offsets rather than by purely dynamical effects [1010.1008].

## 2. Orbital architecture and bulk planetary properties

In the discovery solution, HAT-P-26 b had \(M_p = 0.059 \pm 0.007\,M_{\rm Jup}\), \(R_p = 0.565^{+0.072}_{-0.032}\,R_{\rm Jup}\), and mean density \(0.40 \pm 0.10\ {\rm g\,cm^{-3}}\). Expressed in Earth units, later atmospheric work summarized the planet as \(M_p \approx 18.6\,M_\oplus\) and \(R_p \approx 6.33\,R_\oplus\), with surface gravity \(g_p \approx 4.47\ {\rm m\,s^{-2}}\) or \(4.17\ {\rm m\,s^{-2}}\) depending on the adopted solution, and an equilibrium temperature near \(990\) K. These values place HAT-P-26 b in the low-density Neptune-mass regime, with a radius substantially larger than Neptune’s and an atmospheric scale-height signal correspondingly favorable for transmission spectroscopy [1705.04354].

Later homogeneous photometric and spectroscopic reanalyses preserved the same physical classification while modestly revising some parameters. The GAPS study reported \(M_p = 0.0577 \pm 0.0069\ (\text{stat}) \pm 0.0013\ (\text{sys})\ M_{\rm Jup}\), \(R_p = 0.652 \pm 0.055\ (\text{stat}) \pm 0.007\ (\text{sys})\ R_{\rm Jup}\), \(\rho_p = 0.195 \pm 0.055\ (\text{stat}) \pm 0.002\ (\text{sys})\ \rho_{\rm Jup}\), \(g_p = 3.37 \pm 0.69\ {\rm m\,s^{-2}}\), and \(T_{\rm eq} = 1080 \pm 39\) K, with \(a = 0.04748 \pm 0.00030\ (\text{stat}) \pm 0.00052\ (\text{sys})\ {\rm au}\), \(i = 87.20 \pm 0.86^\circ\), \(R_p/R_\star = 0.0732 \pm 0.0011\), and \(b = 0.55^{+0.17}_{-0.13}\) [2205.10549].

The orbital eccentricity has remained an interpretive uncertainty rather than a settled parameter. The discovery paper found \(e = 0.124\) and \(\omega \approx 54^\circ\), but also emphasized that the eccentricity was only marginally significant. The GAPS work retained \(e = 0.124 \pm 0.060\) from the discovery analysis, whereas the 2023 TransitFit study fixed \(e=0\) and reported \(i = 87.82 \pm 0.05^\circ\) and \(a/R_* = 12.51 \pm 0.07\). This suggests that HAT-P-26 b is securely established as a close-in warm Neptune, while the precise degree of orbital non-circularity remains model-dependent [1010.1008; 2205.10549; 2303.03610].

Interior interpretations have consistently required a substantial H/He envelope. The discovery paper concluded that HAT-P-26b is consistent with theoretical models of an irradiated Neptune-mass planet with a \(10\,M_\oplus\) heavy element core that comprises \(\gtrsim 50\%\) of its mass with the remainder contained in a significant hydrogen-helium envelope. Later atmospheric studies, using the planet’s low density and large spectral feature amplitudes, reinforced the same basic inference: HAT-P-26 b is not a compact ice-giant analogue, but a puffier Neptune-mass planet with a highly extended atmosphere [1010.1008; 1705.04354].

## 3. Atmospheric characterization before JWST

The first dedicated atmospheric study combined Magellan/LDSS-3C transmission spectroscopy and Spitzer/IRAC photometry. That analysis found that the measured spectrum is best explained by the presence of water vapor, a lack of potassium, and either a high-metallicity, cloud-free atmosphere or a solar-metallicity atmosphere with a cloud deck at \(\sim 10\) mbar. The authors also noted the emergence of multi-scale-height spectral features and explicitly framed the principal degeneracy as metallicity versus clouds. In that study, the transit ephemeris was updated to \(t_0 = 2455304.65218(25)\ {\rm BJD_{TDB}}\) and \(p = 4.2345023(7)\) days [1511.08226].

A much broader HST+Spitzer analysis then extended the transmission spectrum to \(0.5\)–\(5\ \mu{\rm m}\), using STIS G750L, WFC3 G102, WFC3 G141, and IRAC 3.6 and \(4.5\ \mu{\rm m}\). That work detected a prominent H\(_2\)O absorption band with a base-to-peak amplitude of \(525 \pm 43\) ppm and an \(8.8\sigma\) detection significance. Using ATMO retrievals combined through Bayesian Model Averaging, it inferred an atmospheric heavy-element abundance \(Z/Z_\odot = 4.8^{+21.5}_{-4.0}\), with H\(_2\)O as the dominant metallicity proxy. The optical transmission spectrum was interpreted as consistent with a cloud deck, and Na and K yielded only upper limits; sulfur-based condensates, especially Na\(_2\)S, were proposed as representative cloud species compatible with the pressure range probed in transmission [1705.04354].

That metallicity result immediately made HAT-P-26 b unusual in comparative planetology. The 2017 analysis argued that its atmosphere is likely less enriched than those of Neptune and Uranus and has a median metallicity closer to Jupiter and Saturn, placing the planet about \(1\sigma\) below the combined mass–metallicity trend. The paper interpreted this as evidence that the gaseous envelope of HAT-P-26b is primordial and was acquired late in the disk lifetime, with little contamination from metal-rich planetesimals. This suggests that formation location and the timing of gas accretion are at least as important as bulk planetary mass in setting atmospheric metallicity [1705.04354].

A later reanalysis combined 13 new ground-based transits with HST/WFC3, TESS, and earlier data, and retrieved a limb temperature of \(590^{+60}_{-50}\) K together with an H\(_2\)O abundance of \(2.4^{+2.9}_{-1.6}\%\). In that framework, the cloud-top pressure was \(\log_{10} P_{\rm clouds} (\mathrm{Pa}) = 4.2^{+0.5}_{-0.6}\), implying a deep cloud deck and an effectively cloud-free upper atmosphere over the optical-to-near-IR wavelength range used in the retrieval. The same study did not find compelling evidence for metal hydrides in its adopted dataset and attributed the discrepancy with previous claims to the omission of STIS and Magellan optical spectra where those signatures are strongest [2303.03610].

## 4. Transit timing variations and the possibility of additional planets

Transit-timing variability has become one of the defining dynamical features of HAT-P-26 b. The GAPS analysis compiled discovery, HST, and ground-based timings and found that a sinusoidal timing model is statistically preferred over linear, quadratic, and cubic ephemerides. Its preferred solution gave \(T_0 = 2455304.65234 \pm 0.00035\ \text{BJD(TDB)}\), \(P_{\rm orb} = 4.23450213 \pm 0.00000076\ \text{days}\), a TTV sine period \(P_{\rm sine} = 275.5 \pm 9.1\) epochs, and amplitude \(A = 93 \pm 15\) s. The authors concluded that the HAT-P-26 system shows transit timing variations that may tentatively be attributed to the presence of a third body, but they also showed that a simple light-time interpretation would imply a minimum companion mass \(M_{\rm TB} \sim 0.07\,M_\odot\) and an RV acceleration much larger than the measured drift, making that specific scenario problematic [2205.10549].

A more extensive 2023 timing analysis combined 33 re-fitted transits obtained over seven years and reported a highly significant periodic signal in the O–C diagram. Its generalized Lomb–Scargle periodogram identified a peak at \(f = 0.0045 \pm 0.0001\) cycles per orbital period with \(\mathrm{FAP} \sim 10^{-7}\,\%\), and the preferred sinusoidal model had amplitude \(A_{\rm TTV} = 1.98 \pm 0.05\) minutes and phase \(\phi = -0.22 \pm 0.04\). Using the analytic formalism of Lithwick et al. (2012), the authors argued that the signal could result from an additional \(0.02\,M_{\rm Jup}\) planet near the 1:2 mean-motion resonance at \(P_2 \approx 8.47\) days [2303.03610].

These two timing analyses agree on the existence of coherent non-linear transit timing but not on a unique physical origin. The GAPS paper favored caution because simple third-body light-time explanations conflict with the observed RV drift, while the 2023 work treated a resonant planetary perturber as a plausible explanation. Taken together, the published timing literature supports the statement that HAT-P-26 is very likely dynamically non-trivial, even though the architecture responsible for the TTVs is not yet established [2205.10549; 2303.03610].

Independent support for additional complexity came from a TESS-based SHERLOCK analysis. After recovering HAT-P-26 b in Sector 50 with \( \mathrm{S/N} = 42.2 \) and \( \mathrm{SDE} = 15.6 \), the pipeline masked the known transits and found a second periodic transit-like signal at \(P = 6.594^{+0.009}_{-0.007}\,\mathrm{days}\), \(R_p = 1.97 \pm 0.20\,R_\oplus\), depth \(0.59 \pm 0.11\,\mathrm{ppt}\), and \(T_{\mathrm{eq}} = 785 \pm 15\) K. The signal was released as CTOI-420779000.02, with TRICERATOPS values \(\mathrm{FPP} = 0.24\) and \(\mathrm{NFPP} = 0.0075\), placing it in the paper’s “likely planet” regime rather than among fully validated planets. A two-planet transit model was modestly favored over a one-planet model with \(\Delta \ln Z \approx 4.7\), and MEGNO calculations identified stable solutions for a low-eccentricity outer candidate, typically \(e_c \lesssim 0.05\) for most explored masses [2407.14602].

If CTOI-420779000.02 is planetary, the period ratio \(P_c/P_b \approx 1.556\) places the putative companion near, but not exactly at, the \(3\!:\!2\) commensurability with HAT-P-26 b. The SHERLOCK paper explicitly connected this candidate to earlier TTV work by noting the prior suggestion of a non-transiting planet at \(P \approx 8.47\) d and by emphasizing that a few-Earth-mass planet at \(\sim 6.6\) d could plausibly contribute to the observed TTVs, although the paper did not attempt a full TTV model. This suggests that the presently available evidence allows multiple distinct outer-perturber scenarios, none of which has yet been confirmed [2407.14602].

## 5. Spin–orbit geometry, ephemerides, and follow-up constraints

The first Rossiter–McLaughlin attempt for HAT-P-26 b was carried out with HARPS-N on 2015-03-26 using 27 spectra of 600 s each. Because the in-transit RV sequence was of limited quality, the analysis fixed \(v\sin i_\star = 1.9 \pm 0.4\ {\rm km\,s^{-1}}\) and fit only for the projected obliquity, obtaining \(\lambda = 18^\circ \pm 49^\circ\). This left the spin–orbit angle poorly constrained, but the data favored a prograde orbit and did not support a strongly retrograde or polar geometry [2205.10549].

The same study found no measurable stellar rotation period in HATNet photometry and therefore could not infer the stellar spin-axis inclination \(i_\star\) or the true 3D obliquity \(\psi\). As a result, HAT-P-26 b remains one of the cases where the RM effect suggests prograde motion but does not yet locate the planet securely within broader obliquity population trends. For a Neptune-mass planet around a cool star, that limitation is primarily observational rather than conceptual: the RM signal is intrinsically modest and the host star is relatively faint [2205.10549].

Ephemeris maintenance has been unusually important for this planet. Ground-based, HST, Spitzer, TESS, and SHERLOCK analyses have successively refined the linear orbital period from the discovery value \(P = 4.234516 \pm 2\times10^{-5}\,\mathrm{days}\) to values near \(4.234502\)–\(4.234503\) d, while at the same time showing that a purely linear ephemeris is not sufficient once TTVs are included. The 2023 study summarized this with \(T_m^c(E) = 2455304.65211^{+0.00036}_{-0.00035} + 4.234503^{+0.000001}_{-0.000001}\,E\) for 33 re-fitted epochs, whereas the GAPS work advocated an explicitly sinusoidal timing term [2303.03610; 2205.10549].

The SHERLOCK analysis also emphasized a more general ephemeris problem for the newly proposed outer candidate: with only one TESS sector available, the uncertainty in future transit times had already grown to \(\Delta T \sim 22.7\) h. The same work argued that “ground-based instrumentation is inadequate” for such shallow candidates in the HAT-P-26 and WASP-16 systems, and recommended space-based facilities, notably CHEOPS, for repeated “filler campaign” monitoring. Although this specific limitation concerns CTOI-420779000.02 rather than HAT-P-26 b itself, it illustrates why the HAT-P-26 system has become a case study in the interaction between transit-timing uncertainty, cadence-limited survey data, and follow-up strategy [2407.14602].

## 6. JWST spectroscopy and the current atmospheric picture

JWST NIRSpec G395H transmission spectroscopy transformed the atmospheric interpretation of HAT-P-26 b by resolving the \(2.8\)–\(5.1\ \mu{\rm m}\) spectral region containing strong CO\(_2\) and SO\(_2\) features. A single transit observed on 2023 June 15 yielded robust detections of H\(_2\)O, CO\(_2\), and SO\(_2\), with \(\ln \mathcal{B} = 4.06\), \(85.64\), and \(13.46\), respectively, while H\(_2\)S and CO remained marginal with \(\ln \mathcal{B} < 0.5\). The preferred free-chemistry retrieval reported \(\log_{10} X_{\rm H_2O} = -2.07^{+0.39}_{-0.59}\), \(\log_{10} X_{\rm CO_2} = -2.91^{+0.56}_{-0.71}\), \(\log_{10} X_{\rm SO_2} = -4.40^{+0.35}_{-0.40}\), and a cloud top pressure \(\log_{10} P_{\rm clouds} ({\rm Pa}) = 2.36^{+0.60}_{-0.42}\) [2509.16082].

The JWST retrievals also refined the basic thermodynamic picture of the terminator. They inferred \(T \simeq 667^{+64}_{-58}\ \text{K}\), substantially cooler than the commonly quoted global equilibrium temperature \(T_{\rm eq} \sim 1000\ \text{K}\), and a retrieved mean molecular weight \({\rm MMW} = 2.55^{+0.32}_{-0.20}\ \text{amu}\), consistent with a hydrogen-helium dominated envelope. This “cold terminator” result is consistent with earlier transmission studies that had already suggested a limb temperature lower than the global equilibrium value [2509.16082; 2303.03610].

In compositional terms, the JWST analysis converged on a metal-rich, oxygen-rich atmosphere. Using retrieved elemental abundances, it found \(\frac{M/H}{(M/H)_\odot} = 11.4^{+13.3}_{-8.1}\), together with \({\rm C/O} = 0.14^{+0.21}_{-0.08}\), and noted that a self-consistent PICASO grid fit preferred roughly \(10\times\) solar metallicity with a similarly sub-solar C/O ratio. Those values are consistent with the earlier HST-based conclusion that HAT-P-26 b has an atmospheric metallicity of only a few to a few tens of times solar, while adding a much richer molecular inventory than HST and Spitzer alone could supply [2509.16082; 1705.04354].

The detection of sulfur dioxide is especially important because it places HAT-P-26 b in the emerging comparative context of JWST sulfur photochemistry. Photochemical modeling reproduced the observed SO\(_2\) abundance through the net reaction
\[
\mathrm{H_2S} + 2\:\mathrm{H_2O} \rightarrow \mathrm{SO_2} + 3\:\mathrm{H_2},
\]
driven by photogenerated radicals from H\(_2\)O photolysis and UV irradiation. The study further argued that HAT-P-26 b bridges the gap between previous SO\(_2\) detections in hot Jupiters and sub-Neptunes, and that its SO\(_2\) abundance is consistent with the predicted trend of increasing SO\(_2\) with atmospheric metallicity [2509.16082].

Taken together, the pre-JWST and JWST literature now portray HAT-P-26 b as a warm Neptune with a low-density bulk structure, an H\(_2\)/He-dominated atmosphere, strong H\(_2\)O absorption, super-solar but not extreme metallicity, sub-solar C/O, high-altitude clouds or cloud decks whose inferred depth depends on wavelength coverage and model assumptions, and clear evidence for disequilibrium sulfur chemistry. On the dynamical side, the planet’s coherent TTVs, the unresolved origin of those TTVs, and the appearance of a second transiting candidate in TESS data all indicate that HAT-P-26 b is central to a system whose architecture is not yet fully mapped.

Source: https://www.emergentmind.com/topics/hat-p-26-b