WASP-156 b: Aligned Exo-Neptune

• WASP-156 b is a short-period exo-Neptune characterized by a mass of 42 M⊕, a radius of 6.15 R⊕, and a density of about 1 g/cm³, placing it between the hot-Neptune desert and the 'savanna'.
• The planet’s orbital geometry was refined through high-resolution transit spectroscopy using ESPRESSO and MAROON-X, revealing a nearly aligned configuration with a projected obliquity of approximately -8°.
• Integrated photometric and radial-velocity analyses ruled out nearby massive companions, supporting formation scenarios such as in situ origin or early disc-driven migration.

WASP-156 b is a short-period exo-Neptune whose orbital geometry has been reassessed through high-resolution transit spectroscopy and joint photometric–radial-velocity analysis. The principal result is that the planet does not follow a polar orbit; instead, it is consistent with an aligned configuration, with a projected obliquity measured from ESPRESSO data of $λ=-8\pm16^\circ$ (Lafarga et al., 26 May 2026). In the same study, the system parameters were updated, the stellar projected rotation was found to be substantially lower than previously reported, and long-term radial-velocity monitoring was used to exclude Jupiter-mass companions within 5 au. Collectively, the measured aligned and circular orbit, together with the absence of nearby massive companions, are consistent with in situ formation or early disc-driven migration (Lafarga et al., 26 May 2026).

1. System classification and physical parameters

WASP-156 b is described as a warm “ridge” Neptune with $P\approx3.84$ d, $R\approx6.15\,R_\oplus$, $M\approx42\,M_\oplus$, and $\rho\approx1$ g/cm$^3$ (Lafarga et al., 26 May 2026). In the terminology used for the short-period Neptune population, it resides between the hot-Neptune “desert” ($P\lesssim3$ d) and the longer-period “savanna” (Lafarga et al., 26 May 2026). This placement is relevant because the population of short-period exo-Neptunes is thought to be shaped by an interplay between orbital migration, tidal effects, and photoevaporation.

The stellar parameters derived from ESPRESSO spectra with the PAWS spectral-synthesis pipeline are $T_{\rm eff}=5036\pm104$ K, $\log g=4.46\pm0.08$, and ${\rm [Fe/H]}=+0.32\pm0.15$ (Lafarga et al., 26 May 2026). The same analysis gives a projected stellar rotation of $P\approx3.84$0 km/s as an upper limit, superseding the literature SOPHIE-based value $P\approx3.84$1 km/s (Lafarga et al., 26 May 2026).

From a joint fit to all TESS and NGTS photometry together with archival out-of-transit radial velocities from CORALIE, SOPHIE, HARPS, and HIRES, the orbital period was updated to $P\approx3.84$2 d and the transit epoch to $P\approx3.84$3 BJD (Lafarga et al., 26 May 2026). The same fit yielded $P\approx3.84$4, impact parameter $P\approx3.84$5, stellar density $P\approx3.84$6 g cm$P\approx3.84$7, and radial-velocity semi-amplitude $P\approx3.84$8 m/s (Lafarga et al., 26 May 2026). Eccentricity was fixed to $P\approx3.84$9, with $R\approx6.15\,R_\oplus$0 at $R\approx6.15\,R_\oplus$1 (Lafarga et al., 26 May 2026).

The derived planetary properties are $R\approx6.15\,R_\oplus$2, $R\approx6.15\,R_\oplus$3, mean density $R\approx6.15\,R_\oplus$4 g cm$R\approx6.15\,R_\oplus$5, and semi-major axis $R\approx6.15\,R_\oplus$6 au (Lafarga et al., 26 May 2026).

2. Observational basis

The reassessment of the system relied on a multi-instrument campaign centered on transit spectroscopy. Two transits were observed with ESPRESSO on ESO’s VLT in high-resolution mode at $R\approx6.15\,R_\oplus$7, on 2022-09-02 and 2022-09-29 (Lafarga et al., 26 May 2026). Each night produced 21 spectra, with signal-to-noise ratios of approximately 85 per pixel at 703 nm for the first night and approximately 70 for the second; fibre A tracked the star and fibre B monitored sky, with exposure times of 600–700 s (Lafarga et al., 26 May 2026).

A third transit was observed with MAROON-X on Gemini North on 2021-11-04 at $R\approx6.15\,R_\oplus$8, spanning 5000–9200 Å (Lafarga et al., 26 May 2026). Thirty-eight spectra were obtained, 17 of them in transit, using 400 s exposures; the signal-to-noise ratio increased from approximately 60 to 100 toward egress (Lafarga et al., 26 May 2026).

Simultaneous ground-based photometry was obtained with NGTS during the two ESPRESSO transits (Lafarga et al., 26 May 2026). NGTS used 4–5 20 cm telescopes, a custom 520–890 nm filter, and 10 s exposures, achieving approximately 1 ppt precision after airmass detrending (Lafarga et al., 26 May 2026).

The ESPRESSO and MAROON-X spectra were reduced to 2D blaze-corrected, sky-subtracted spectra, and disc-integrated radial velocities were extracted both via cross-correlation functions and via template matching with SERVAL to maximize precision (Lafarga et al., 26 May 2026). These high-cadence, high-S/N in-transit radial-velocity time series contain the Rossiter–McLaughlin anomaly, produced because the transiting planet blocks light from different line-of-sight velocity regions across the rotating stellar disk (Lafarga et al., 26 May 2026).

3. Rossiter–McLaughlin measurement and projected obliquity

Two independent approaches were used to measure the sky-projected obliquity $R\approx6.15\,R_\oplus$9 from the Rossiter–McLaughlin signal (Lafarga et al., 26 May 2026). The first was a classical RM analysis based on disc-integrated radial velocities, implemented with ironman/rmfit and using the ESPRESSO SERVAL radial velocities only, because they were more precise than the CCF radial velocities (Lafarga et al., 26 May 2026).

In that framework, the in-transit velocity anomaly was modeled as

$M\approx42\,M_\oplus$0

with

$M\approx42\,M_\oplus$1

The fit adopted Gaussian priors on orbital parameters from Table 1, uninformative priors on $M\approx42\,M_\oplus$2 and $M\approx42\,M_\oplus$3, specifically $M\approx42\,M_\oplus$4 and $M\approx42\,M_\oplus$5 km/s, and a prior on the total intrinsic broadening $M\approx42\,M_\oplus$6 km/s (Lafarga et al., 26 May 2026). The joint fit to both ESPRESSO nights yielded $M\approx42\,M_\oplus$7 and $M\approx42\,M_\oplus$8 km/s (Lafarga et al., 26 May 2026).

The second approach was the reloaded RM technique, using spatially resolved local CCFs following Cegla et al. 2016 (Lafarga et al., 26 May 2026). For each exposure, the out-of-transit master-out CCF was subtracted from the in-transit CCF, after normalization by the instantaneous photometric flux, to isolate the local CCF from the blocked stellar patch (Lafarga et al., 26 May 2026). Gaussian fits to these local CCFs provided local radial velocities $M\approx42\,M_\oplus$9, which were then modeled under a rigid-body rotation law,

$\rho\approx1$0

An MCMC fit with emcee, using uniform priors $\rho\approx1$1 and $\rho\approx1$2 km/s, gave $\rho\approx1$3 and $\rho\approx1$4 km/s from the ESPRESSO data taken over both nights (Lafarga et al., 26 May 2026).

These two analyses are mutually consistent and both indicate an aligned orbit rather than a polar one. The lower measured $\rho\approx1$5 is central to that inference, because the study explicitly identifies a reduction of the $\rho\approx1$6–$\rho\approx1$7 degeneracy when the stellar projected rotation is constrained near 0.4 km/s rather than near 4 km/s (Lafarga et al., 26 May 2026).

4. Orbital architecture and companion constraints

The updated global fit supports a circular short-period orbit, with eccentricity fixed to zero and a $\rho\approx1$8 upper bound of $\rho\approx1$9 (Lafarga et al., 26 May 2026). Long-term radial-velocity residuals from the juliet analysis, spanning approximately 7.8 yr and combining CORALIE, SOPHIE, HARPS, and HIRES, show no additional trends or periodicities (Lafarga et al., 26 May 2026).

A Monte-Carlo injection–recovery analysis rules out, at 5σ confidence, any companion $^3$0 interior to approximately 5 au (Lafarga et al., 26 May 2026). Independent high-resolution imaging constraints are also reported: Gemini/Zorro contrast curves at 562/832 nm exclude stellar companions with $^3$1–7 from 0.2″–1.2″, corresponding to approximately 25–150 au, while Shane adaptive optics excludes companions of approximately 3–7 mag from approximately 60–500 au (Lafarga et al., 26 May 2026). Gaia DR3 RUWE of approximately 1.1 and the El-Badry et al. wide-binary catalog similarly show no stellar-mass companion out to 1 pc (Lafarga et al., 26 May 2026).

These results define the current observational picture of the system’s architecture: a close-in Neptune on an aligned, nearly circular orbit, without evidence for massive nearby perturbers on either planetary or stellar scales. A plausible implication is that mechanisms requiring a present external driver are not supported by the available data.

5. Reassessment of the earlier polar-orbit claim

A previous study by Bourrier et al. (2023), based on CARMENES data, reported $^3$2 and therefore described the system as having a “polar orbit” (Lafarga et al., 26 May 2026). That analysis adopted $^3$3 km/s from SOPHIE line-profile calibrations (Lafarga et al., 26 May 2026).

The later reassessment identifies several reasons for the discrepancy. The CARMENES RM fit is described as suffering from low signal-to-noise, since the observations were obtained with the 3.5 m Calar Alto telescope rather than 8 m class telescopes; from strong telluric contamination and airmass-correlated systematics; and from a restrictive but inaccurate prior on $^3$4 based on an empirical calibration outside its valid $^3$5 range (Lafarga et al., 26 May 2026). By contrast, the ESPRESSO and MAROON-X spectra, combined with PAWS spectral modelling, indicate $^3$6 km/s, while both the classical and reloaded RM analyses yield values near 0.4 km/s (Lafarga et al., 26 May 2026).

The same paper also notes an independent classical RM analysis by Jiang et al. (2026), using the first ESPRESSO night and CCF radial velocities with old system parameters, which found $^3$7 (Lafarga et al., 26 May 2026). That result is also consistent with alignment, although less precise.

The central methodological point is therefore not merely that a different best-fit obliquity was obtained, but that the inferred obliquity changed once the stellar projected rotation was re-estimated using higher-quality spectroscopy and once the RM signal was analyzed with updated system parameters. This suggests that the earlier polar interpretation was sensitive to prior assumptions and data quality limitations.

6. Dynamical interpretation and evolutionary implications

The aligned projected obliquity, the nearly circular orbit, and the lack of nearby massive companions are interpreted as being consistent with either in situ formation or early disc-driven migration within a primordial protoplanetary disc aligned with the stellar spin (Lafarga et al., 26 May 2026). In contrast, high-eccentricity migration scenarios such as planet–planet scattering or Kozai–Lidov forcing would require an external companion capable of exciting $^3$8 and would generically produce misaligned orbits (Lafarga et al., 26 May 2026). The observational constraints reported for the system do not support such a configuration.

The study further evaluates the true three-dimensional obliquity using the Fabrycky & Winn (2009) relation,

$^3$9

When the distribution is simulated from $P\lesssim3$0 and random $P\lesssim3$1, the resulting median is $P\lesssim3$2, which makes a true polar configuration “extremely unlikely” (Lafarga et al., 26 May 2026). The paper also reports that the stellar tidal-realignment timescale is $P\lesssim3$3 Gyr, much longer than the system age of approximately 5–6 Gyr, so tides cannot erase a primordial misalignment (Lafarga et al., 26 May 2026). This strengthens the interpretation that the currently observed alignment is likely primordial rather than the result of later tidal damping.

Photoevaporative evolution is also discussed. The study states that any low-density Neptune migrating into the desert or ridge would likely be eroded, and that WASP-156 b, with $P\lesssim3$4 g/cm$P\lesssim3$5, lies near the “density brink,” suggesting that it survived moderate atmospheric loss (Lafarga et al., 26 May 2026). He I 1083 nm and Ly$P\lesssim3$6 observations show no detectable escape to date (Lafarga et al., 26 May 2026). This suggests a scenario in which the planet’s present-day density reflects some atmospheric sculpting, but not the level of mass loss required to remove the envelope entirely.

Within the broader short-period Neptune population, the principal significance of the revised result is taxonomic as well as dynamical. WASP-156 b is moved from a tentative cluster of close-in Neptunes in polar orbits to the group of aligned Neptunes (Lafarga et al., 26 May 2026). That reclassification matters because stellar obliquity is used as a diagnostic of formation and migration history; for WASP-156 b, the currently available evidence is most naturally read as favoring formation in, and migration through, a well-aligned disc, with moderate photoevaporation shaping the current bulk density (Lafarga et al., 26 May 2026).