NGTS-11 c: a transiting Neptune-mass planet interior to the warm Saturn NGTS-11 b

Abstract

We report the discovery of NGTS-11 c, a transiting warm Neptune ($P \approx 12.8$ d; $M_{p} = 1.2^{+0.3}_{-0.2} M_{\mathrm{Nep}}$; $R_{p} = 1.24 \pm 0.03 R_{\mathrm{Nep}}$), in an orbit interior to the previously reported transiting warm Saturn NGTS-11 b ($P \approx 35.5$ d). We also find evidence of a third outer companion orbiting the K-dwarf NGTS-11. We first detected transits of NGTS-11 c in TESS light curves and confirmed them with follow-up transits from NGTS and many other ground-based facilities. Radial-velocity monitoring with the HARPS and FEROS spectrographs revealed the mass of NGTS-11 c and provides evidence for a long-period companion ($P > 2300$ d; $M_{p} \sin i > 3.6 M_{\mathrm{Jup}}$). Taking into account the two additional bodies in our expanded datasets, we find that the mass of NGTS-11 b ($M_{p} = 0.63 \pm 0.09 M_{\mathrm{Sat}}$; $R_{p} = 0.97 \pm 0.02 R_{\mathrm{Sat}}$) is lower than previously reported ($M_{p} = 1.2 \pm 0.3 M_{\mathrm{Sat}}$). Given their near-circular and compact orbits, NGTS-11 c and b are unlikely to have reached their present locations via high-eccentricity migration. Instead, they probably either formed in situ or formed farther out and then underwent disk migration. A comparison of NGTS-11 with the eight other known systems hosting multiple well-characterized warm giants shows that it is most similar to Kepler-56. Finally, we find that the commonly used 10-day boundary between hot and warm Jupiters is empirically well supported.

Figures (6)

• Figure 1: Transit light curves of NGTS-11 c (left panel) and NGTS-11 b (right panel), shown offset in relative flux for clarity and plotted to the same scale for comparison. The data were phase-binned to 10 min, except for TESS Sector 3 (30 min) and TRAPPIST and PEST (20 min for both). The error bars represent formal uncertainties and do not include the photometric jitter terms (\ref{['tab:props']}). The transit model (\ref{['sec:anal']}), including limb darkening, is shown for each case.
• Figure 2: Top panel: RV data from FEROS (red) and HARPS (blue) and the adopted two-planet circular Keplerian model with a quadratic trend (black). The Keplerian signal alone is shown in gray. For the FEROS data, the thicker error bars indicate the formal uncertainties, whereas the thinner error bars show the quadrature sums of the formal uncertainties and the jitter term, $\sigma_{\rm FEROS}$ (\ref{['sec:rv']}). Middle panel: Residual RVs after subtracting the best-fitting model. Lower-left panel: Phase-folded RVs for the orbit of NGTS-11 c, after subtraction of the quadratic trend and the Keplerian signal due to NGTS-11 b. Lower-right panel: Phase-folded RVs for the orbit of NGTS-11 b, after subtraction of the quadratic trend and the Keplerian signal due to NGTS-11 c.
• Figure 3: Speckle imaging of NGTS-11 by 'Alopeke at the Gemini North telescope (left) and HRCam at the Southern Astrophysical Research telescope (right). The contrast curve plots show the linear fits to the 5-$\sigma$ contrast curves on either side of 01 ('Alopeke) and 02 (HRCam). The autocorrelation functions are shown in the insets.
• Figure 4: Times of mid-transit (\ref{['tab:phot']}) relative to the constant orbital ephemeris of \ref{['tab:props']}, the $T_{\rm c}$ values of which define the zeroth epochs shown here. A few mid-transit times with large uncertainties fall outside of the plotted area.
• Figure 5: Mass--radius distribution of known exoplanets (gray dots have $P<10$ d and orange triangles have 10 $\le P \le 200$ d), solar system giants (blue circles), and NGTS-11 c and b (brown diamonds, with error bars). At ${M_{\rm pl}}$$\sim 0.2$${M_{\rm Jup}}$, NGTS-11 b approximately marks the mass boundary above which hot Jupiters are often inflated, whereas warm Jupiters generally are not. Data were retrieved on 2025 Oct 1 from the composite data table of the NASA Exoplanet Archive. Planets with masses inferred from a mass--radius relationship were excluded, as were those with imprecise parameters ($\Delta$${M_{\rm pl}}$/${M_{\rm pl}}$$>$ 0.5 or $\Delta$$R_{\rm pl}$/$R_{\rm pl}$$>$ 0.2).