A 43 day transiting Neptune and two 25 day Saturns from TESS, NGTS and ASTEP

TL;DR

The paper demonstrates that long-period gas giants, typically missed by transit surveys, can be recovered by combining TESS single-transit candidates with dense ground-based photometry (NGTS, ASTEP) and precise RVs (HARPS, CORALIE). Through joint photometric-RV modelling with Allesfitter and stellar characterisation via SpecMatch-Emp and Ariadne, the authors confirm three warm giants: NGTS-34 b (a well-characterized Neptune-like planet at 43.13 days, Rp = 3.65 ± 0.22 R⊕, Mp = 19.1 −4.5/+4.9 M⊕), TOI-4940 b (a Saturn-mass planet at 25.87 days, Rp = 6.61 ± 0.37 R⊕, Mp < 89 M⊕), and NGTS-35 b / TOI-6669 b (a cool Saturn at 25.24 days, Rp = 10.90 ± 0.65 R⊕, Mp = 152 −19/+22 M⊕, e ≈ 0.19). These planets have equilibrium temperatures below 700 K and offer high potential for atmospheric studies; interior modelling suggests substantial heavy-element content for NGTS-34 b and NGTS-35 b. Together, they expand the sample of well-characterized long-period giants, informing theories of formation and migration and enabling future JWST/Ariel investigations of cool planetary atmospheres.

Abstract

Beyond orbital periods of 10 days, there is a dearth of known transiting gas giants. On longer orbits, planets are less affected by their host star, and become ideal probes of planet formation, migration and evolution. We report the discovery of a long period Neptune and two Saturns, each initially identified as single transits in the TESS photometry, and solved through additional transits from ground-based follow-up photometric observations by NGTS and ASTEP. High-resolution radial velocity mass measurements using CORALIE and HARPS confirm their planetary nature. From joint modelling of the photometric and spectroscopic data, we determine an orbital period of $43.12655_{-0.00017}^{+0.00012}~$days, radius of $3.65\pm0.22~\mathrm{R_{\oplus}}$, and mass of $19.1_{-4.5}^{+4.9}~\mathrm{M_{\oplus}}$ for NGTS-34b, making it one of the longest period well-characterized transiting Neptunes. Orbiting a late F-type star, bright in the K-band (Kmag$~\simeq7.9$), it is amenable for cool atmosphere studies using JWST or Ariel. TOI-4940b is a small Saturn on a $25.867811_{-0.000056}^{+0.000058}~$day orbit with a radius of $6.61\pm0.37~\mathrm{R_{\oplus}}$ and an upper mass limit $<89~\mathrm{M_{\oplus}}$. NGTS-35b(=TOI-6669b) is a larger Saturn on a $25.241192\pm0.000022~$day, moderately eccentric orbit ($e = 0.192_{-0.033}^{+0.037}$), with a radius of $10.90\pm0.65~\mathrm{R_{\oplus}}$ and a mass of $152_{-19}^{+22}~\mathrm{M_{\oplus}}$. With an assumed albedo $A=0.3$, each of these planets has an equilibrium temperature below 700K, with NGTS-35b especially cold at $450~$K. These three giants add to the small but growing population of long period planets that can further our understanding of planet formation mechanisms.

Figures (28)

• Figure 1: Speckle sensitivity curves for TOI-4940 in the I band (879 nm), taken by HRCam. The reconstructed image is shown in the top right of the figure. There is no evidence for a close stellar companion ($\Delta mag < 2.5$) within separations 0.2 to 3.0 arcseconds.
• Figure 2: Speckle sensitivity curves for NGTS-35 in both 562 nm and 832 nm filters, taken by Zorro. The reconstructed images for both filters are shown in the top right of the figure. There is no evidence for a close stellar companion ($\Delta mag < 4$) within separations 0.1 to 1.2 arcseconds.
• Figure 3: SED fits with residuals generated using Ariadne. Blue points are the broadband photometry accessed from the catalogue, and purple diamonds are the synthetic photometry fit. The black line in each plot is the most heavily weighted model included in the Bayesian Model Averaging.
• Figure 4: HR diagram of the TESS-SPOC FFI sample described in laurenHR, generated using Gaia DR3 gaiadr3 colours and parallax, where the colour bar represents the log density of stars. The three host stars NGTS-34, TOI-4940 and NGTS-35 are represented by yellow, blue and pink stars respectively.
• Figure 5: Photometry observations of NGTS-34 grouped by instrument and cadence with residuals, phase-folded on the best fitting period $P = 43.12655_{-0.00017}^{+0.00012}~$days. From left to right: NGTS binned to 2 minutes in grey and binned to 15 minutes in orange, where the red noise was modelled by a hybrid spline; TESS-SPOC 1800s (Sectors 9 and 10) in purple where the red noise was modelled by a GP (Matern-3/2); TESS-SPOC 600s (Sector 36) in pink where the red noise was modelled by a GP (Matern-3/2); and TESS-SPOC 200s (Sector 63) in grey and binned to 15 minutes in blue, where the red noise was modelled by a hybrid spline. The solid lines are the posterior median models for each instrument from the joint fit from Allesfitter.