TOI-1759A b: Temperate Sub-Neptune

• TOI-1759A b is a temperate sub-Neptune identified via coordinated TESS photometry and ground-based follow-up that resolved a period ambiguity.
• Its measured properties include a radius of ~3.1 R⊕ and an 18.85-day orbital period, with low density suggesting a significant H2-rich envelope.
• The system is promising for transmission spectroscopy and atmospheric escape studies, offering insights into M-dwarf planet evolution.

Searching arXiv for the specified exoplanet to ground the article in the cited literature. {"query":"TOI-1759 b arXiv", "max_results": 5} TOI-1759A b, usually abbreviated TOI-1759 b, is a transiting temperate sub-Neptune orbiting the early-M dwarf TOI-1759 (TIC 408636441). It was independently characterized in two contemporaneous studies that combined TESS photometry with precision radial velocities, one using CARMENES in the optical and the other using SPIRou in the near infrared. Both analyses resolved an initial factor-of-two period ambiguity in the TESS discovery data and converged on an orbital period of approximately 18.85 days, a planetary radius near $3.1\,R_\oplus$, and a low-density, gas-rich bulk structure in a regime that is favorable for transmission spectroscopy and potentially for atmospheric escape studies (Espinoza et al., 2022, Martioli et al., 2022).

1. Discovery and period disambiguation

TOI-1759 was observed by TESS at 2-minute cadence in Sectors 16, 17, and 24. In each sector, only one transit was recorded, so the transit sequence admitted more than one ephemeris. The data validation initially favored a period near 37.7 days, but the light curve was also consistent with a half-period solution near 18.85 days, with a transit depth around $\sim 2700$ ppm. Ground-based photometric follow-up then captured a full on-target transit whose duration and depth matched the TESS events, decisively confirming the shorter-period solution (Espinoza et al., 2022).

The independent SPIRou-based analysis describes the same sequence in slightly different terms: three clear transits in the TESS Presearch Data Conditioning flux, an initial SPOC Data Validation solution of $37.696\pm0.002$ d, and ground-based $I_c$ photometry confirming $P \approx 18.85$ d (Martioli et al., 2022). Taken together, the two studies establish that the defining observational problem for the system was not transit detection itself, but alias resolution caused by sparse transit sampling.

This discovery pathway is methodologically notable because it illustrates a common limitation of TESS for longer-period planets around compact stars: when only one event is observed per sector, an apparently well-defined transit sequence can still remain degenerate until supplemented by either targeted photometric recovery or phase-coherent radial-velocity measurements. TOI-1759A b is therefore a representative case of coordinated validation in which sector-limited survey photometry, ground-based transit recovery, and precision spectroscopy each supplied a nonredundant constraint.

2. Host star and validation architecture

The host system consists of a single, high-proper-motion star at $40.12\pm0.02$ pc, identified as TIC 408636441 and TYC 4266-736-1. The designation “A” does not denote a known luminous binary component; rather, one study explicitly states that TOI-1759A is the sole luminous component and that the label marks the primary in an otherwise single system. Both studies classify the star as an M0 dwarf or M0V dwarf, with stellar properties centered near $R_* \approx 0.60\,R_\odot$, $M_* \approx 0.61\,M_\odot$, and $T_{\mathrm{eff}} \approx 4000$ K, although the second study reports several internally consistent spectroscopic and SED-based estimates spanning $3972\pm40$ K to $\sim 2700$0 K (Martioli et al., 2022).

Validation relied on both image-domain and statistical vetting. High-resolution imaging with Gemini/NIRI, ‘Alopeke speckle, and Keck/NIRC2 AO detected no stellar companions, with reported contrast limits including no companions down to $\sim 2700$1 at $\sim 2700$2–5 mag, $\sim 2700$3 mag at $\sim 2700$4 in the 832 nm ‘Alopeke data, and $\sim 2700$5 mag at $\sim 2700$6 in Keck/NIRC2 Br$\sim 2700$7. The TESS DV centroid test localized the transit to within $\sim 2700$8 arcsec of the target, and nearby catalog sources within that radius were more than 7 mag fainter. TRICERATOPS yielded a false positive probability of $\sim 2700$9 from TESS alone, which dropped below $37.696\pm0.002$0 when the high-resolution imaging constraints were incorporated, with negligible Nearby FPP of $37.696\pm0.002$1 (Martioli et al., 2022).

One characterization describes the host as a quiet, nearby M0 dwarf, whereas the other describes it as a moderately active M0V star. This is not a contradiction in the planetary validation itself; rather, it reflects different emphases in the follow-up programs. The optical CARMENES study foregrounded the planetary solution and activity filtering in the RV time series, while the SPIRou study explicitly incorporated spectropolarimetry and magnetic-field reconstruction. A plausible implication is that the host’s activity level is moderate enough to be astrophysically consequential for RV extraction and atmospheric escape, but not so high as to prevent validation or mass measurement.

3. Orbital solution and measured planetary properties

The two studies agree closely on the orbital architecture and radius, while differing more substantially in the inferred planetary mass and hence density. Both ultimately adopt a circular orbit. In the CARMENES analysis, eccentricity was fixed to $37.696\pm0.002$2 as consistent with the data and the argument of periastron was fixed to $37.696\pm0.002$3 degrees. In the SPIRou analysis, fits that allowed $37.696\pm0.002$4 and $37.696\pm0.002$5 to float produced formally nonzero eccentricities, but these were disfavored by BIC, and the adopted solution also fixed $37.696\pm0.002$6 (Espinoza et al., 2022, Martioli et al., 2022).

Parameter	CARMENES + TESS	SPIRou + TESS
Orbital period	$37.696\pm0.002$7 d	$37.696\pm0.002$8 d
Mid-transit epoch	$37.696\pm0.002$9 BJD	$I_c$0 BJD
Semi-major axis	$I_c$1 AU	$I_c$2 au
Radius	$I_c$3	$I_c$4
Mass	$I_c$5	$I_c$6
RV semi-amplitude	$I_c$7 m s$I_c$8	$I_c$9 m s$P \approx 18.85$0
Inclination	$P \approx 18.85$1 deg	$P \approx 18.85$2 deg
Impact parameter	$P \approx 18.85$3	$P \approx 18.85$4

The transit geometry is consistently close to edge-on. The CARMENES-based joint fit gives $P \approx 18.85$5 h, $P \approx 18.85$6, and $P \approx 18.85$7, corresponding to $P \approx 18.85$8 ppm. The SPIRou-based fit gives $P \approx 18.85$9, $40.12\pm0.02$0 h, and $40.12\pm0.02$1 (Espinoza et al., 2022, Martioli et al., 2022).

The equilibrium irradiation is similarly stable across the two analyses. The CARMENES study reports $40.12\pm0.02$2 and quotes two equilibrium temperatures under full day-night heat redistribution: $40.12\pm0.02$3 K for $40.12\pm0.02$4 and $40.12\pm0.02$5 K for $40.12\pm0.02$6. The SPIRou study reports $40.12\pm0.02$7 and $40.12\pm0.02$8 K for $40.12\pm0.02$9. The numerical differences therefore arise from explicit differences in the assumed Bond albedo, not from disagreement over the orbit (Espinoza et al., 2022, Martioli et al., 2022).

The mass discrepancy is the main parameter-level divergence between the optical and NIR characterizations. One solution yields $R_* \approx 0.60\,R_\odot$0 g cm$R_* \approx 0.60\,R_\odot$1 and $R_* \approx 0.60\,R_\odot$2 m s$R_* \approx 0.60\,R_\odot$3, whereas the other gives $R_* \approx 0.60\,R_\odot$4 g cm$R_* \approx 0.60\,R_\odot$5, $R_* \approx 0.60\,R_\odot$6, and $R_* \approx 0.60\,R_\odot$7 km s$R_* \approx 0.60\,R_\odot$8 (Espinoza et al., 2022, Martioli et al., 2022). This suggests that the planet is securely low density in either case, but that its envelope mass fraction and detailed internal structure remain sensitive to the treatment of activity and to the RV extraction methodology.

4. Radial velocities, stellar activity, and magnetic topology

The optical and NIR RV programs differed markedly in cadence and auxiliary diagnostics. The CARMENES campaign used the VIS channel on the 3.5 m Calar Alto telescope and obtained 57 RV measurements over approximately 175 days, from 2020-07-24 to 2021-01-17, with typical RV errors of $R_* \approx 0.60\,R_\odot$9 m s$M_* \approx 0.61\,M_\odot$0. The RVs showed a clear $M_* \approx 0.61\,M_\odot$1-day signal consistent with the transit period, and the activity component was modeled with a quasi-periodic Gaussian process. The joint photometry-plus-RV analysis fixed the orbital period and ephemeris and broke the TESS single-transit ambiguity (Espinoza et al., 2022).

The SPIRou campaign was longer and more magnetically diagnostic: 218 spectra across 54 nights over 447 days, from 2020-06-05 to 2021-08-26, with near-IR S/N per pixel at $M_* \approx 0.61\,M_\odot$2m ranging from 43 to 210 and a median of 184. RVs were extracted using both a cross-correlation function pipeline and a line-by-line method adapted from Dumusque (2018), with simultaneous Fabry-Perot monitoring in fiber C to correct instrumental drifts. Drift correction reduced the CCF RMS from 10.6 to 8.4 m s$M_* \approx 0.61\,M_\odot$3 and the LBL RMS from 9.5 to 5.6 m s$M_* \approx 0.61\,M_\odot$4; typical internal uncertainties were $M_* \approx 0.61\,M_\odot$5 m s$M_* \approx 0.61\,M_\odot$6 for LBL and $M_* \approx 0.61\,M_\odot$7–8 m s$M_* \approx 0.61\,M_\odot$8 for CCF. Stacked Bayesian GLS periodograms showed coherent power growth near 18.85 d, especially after subtracting activity GP models (Martioli et al., 2022).

The SPIRou study also measured the stellar magnetic field directly. A LSD analysis of the Stokes $M_* \approx 0.61\,M_\odot$9 polarized spectra detected Zeeman signatures, and ZDI over two seasons reconstructed a predominantly poloidal large-scale field with poloidal fraction 97–99%, mean surface field strengths of $T_{\mathrm{eff}} \approx 4000$0 G and $T_{\mathrm{eff}} \approx 4000$1 G, dipole components of $T_{\mathrm{eff}} \approx 4000$2 G and $T_{\mathrm{eff}} \approx 4000$3 G, and year-to-year changes in dipole tilt and axisymmetry. The longitudinal field time series yielded a rotation period of $T_{\mathrm{eff}} \approx 4000$4 d, with an inferred $T_{\mathrm{eff}} \approx 4000$5–0.85 km s$T_{\mathrm{eff}} \approx 4000$6 (Martioli et al., 2022).

The GP formalism used in the SPIRou analysis was explicitly

$T_{\mathrm{eff}} \approx 4000$7

with $T_{\mathrm{eff}} \approx 4000$8. On $T_{\mathrm{eff}} \approx 4000$9, the fit gave $3972\pm40$0 G, $3972\pm40$1 G, $3972\pm40$2 G, $3972\pm40$3 d, $3972\pm40$4, and $3972\pm40$5 d. On RVs, the activity GP parameters were less constrained, with representative values $3972\pm40$6 m s$3972\pm40$7, $3972\pm40$8 d, and $3972\pm40$9, while $\sim 2700$00 remained consistent with the $\sim 2700$01 prior (Martioli et al., 2022).

The CARMENES study identified two additional RV signals: one near 80 d and one at periods longer than 200 d. Activity indicators including BIS, Ca II IRT, H$\sim 2700$02, and long-term photometry showed power near $\sim 2700$03 d and its harmonic, consistent with stellar rotation and starspot modulation; the $\sim 2700$04-day signal could not be robustly pinned down within the available baseline, although some models localized it near $\sim 2700$05 d. The authors therefore interpreted the $\sim 2700$06-day signal as likely stellar activity and recommended longer-baseline RV monitoring to determine whether the $\sim 2700$07-day excess is also activity-related or traces an additional companion (Espinoza et al., 2022).

5. Interior structure, atmospheric observables, and escape

The measured radius and mass place TOI-1759A b securely in the sub-Neptune class and well below the density expected for a purely rocky planet. In the CARMENES interpretation, the mass-radius position is consistent with an Earth-like rocky interior plus a modest $\sim 2700$08–5% H$\sim 2700$09-rich envelope, or with a scaled-down Neptune-like composition. In the SPIRou interpretation, the planet is a likely gas-dominated sub-Neptune with an expected high rate of photoevaporation (Espinoza et al., 2022, Martioli et al., 2022).

Transmission spectroscopy is a central scientific motivation for the system. The CARMENES study computes a Transmission Spectroscopy Metric of over 80, specifically $\sim 2700$10, and places TOI-1759 b among the top five temperate, small exoplanets with $\sim 2700$11 K and $\sim 2700$12 having the highest TSM discovered to date. It is discussed alongside L 98-59 d, TOI-178 g, TOI-1231 b, and LHS 1140 b. The same study reports that for plausible compositions at $\sim 2700$13 K, water and methane features at the $\sim 2700$14 ppm level could be differentiable in a single transit across combined JWST modes, unless high metallicity and haze substantially mute the spectral features (Espinoza et al., 2022).

The SPIRou analysis emphasizes complementary escape diagnostics. For a hydrogen-rich atmosphere with mean molecular weight $\sim 2700$15 amu, the scale height is estimated as $\sim 2700$16 m at $\sim 2700$17. Using empirical scalings from surface magnetic field strength to X-ray flux and from X-ray to EUV flux, the stellar surface XUV flux is estimated at $\sim 2700$18 to $\sim 2700$19 erg cm$\sim 2700$20 s$\sim 2700$21, implying an XUV flux at the planet of $\sim 2700$22 erg cm$\sim 2700$23 s$\sim 2700$24. Hydrodynamic modeling then yields a hydrogen mass-loss rate of $\sim 2700$25 g s$\sim 2700$26, comparable to GJ 3470 b even though TOI-1759A b orbits farther out at $\sim 2700$27 au (Martioli et al., 2022).

These two lines of argument are complementary rather than competing. One foregrounds the detectability of deep-atmosphere transmission features in a temperate sub-Neptune, while the other foregrounds the detectability of upper-atmosphere escape. A plausible implication is that TOI-1759A b can simultaneously constrain atmospheric chemistry, haze prevalence, and mass-loss physics in the low-irradiation M-dwarf regime.

6. Demographic context, open questions, and follow-up priorities

TOI-1759A b occupies a parameter space of particular interest for small-planet population studies. Its equilibrium temperature of roughly 400–443 K and insolation of about $\sim 2700$28 place it in a temperate regime rather than in the highly irradiated hot sub-Neptune desert. The SPIRou study notes that the planet lies well above the M-dwarf radius gap and far from the hot sub-Neptune desert, while the CARMENES study stresses that its temperature falls within the 300–600 K regime in which haze formation may be favored. Both discussions connect the system to broader questions about the radius valley, envelope retention, and the relative roles of photoevaporation and formation history around low-mass stars (Espinoza et al., 2022, Martioli et al., 2022).

Several uncertainties remain explicit in the literature. The RV semi-amplitude is robustly phase-coherent with the transit ephemeris, but the NIR mass measurement is only a $\sim 2700$29 detection in the favored LBL+GP solution, and the optical and NIR analyses do not yield identical masses. The SPIRou study also notes that eccentricity is unconstrained and limb darkening is weakly constrained in the TESS bandpass, while the RV GP can overfit if not regularized by external activity information. The CARMENES study adds that the long-period $\sim 2700$30-day RV excess cannot be securely interpreted within the available baseline (Espinoza et al., 2022, Martioli et al., 2022).

The observational priorities are correspondingly clear. Longer-baseline RV monitoring is needed to refine $\sim 2700$31 and test the nature of the long-period RV signal. Multiwavelength transit photometry and transmission spectroscopy can probe atmospheric composition and haze, and the SPIRou study specifically highlights both J-band H$\sim 2700$32O sensitivity and searches for neutral hydrogen in Lyman-$\sim 2700$33. The CARMENES study emphasizes JWST and capable ground-based facilities for transmission work, whereas the SPIRou study notes that continued NIR RV and spectropolarimetric follow-up can improve both the planetary mass and the stellar activity model (Espinoza et al., 2022, Martioli et al., 2022).

In synthesis, TOI-1759A b is a temperate, low-density sub-Neptune around a nearby early-M dwarf whose discovery required resolving a TESS alias and whose subsequent characterization has linked transit photometry, optical and NIR RVs, GP-based activity modeling, and stellar magnetic mapping. The system’s combination of modest irradiation, favorable transit geometry, high transmission-spectroscopy potential, and plausible ongoing atmospheric escape makes it a benchmark target for comparative studies of temperate sub-Neptune atmospheres and M-dwarf planet evolution.