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; $M_p=1.2_{-0.2}^{+0.3}{M}_{\mathrm{Nep}}$; $R_p=1.24\pm 0.03R_{\mathrm{Nep}}$), 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; $M_p\sin i>3.6M_{\mathrm{Jup}}$). Taking into account the two additional bodies in our expanded datasets, we find that the mass of NGTS-11 b ($M_p=0.63\pm 0.09M_{\mathrm{Sat}}$; $R_p=0.97\pm 0.02R_{\mathrm{Sat}}$) is lower than previously reported ($M_p=1.2\pm 0.3M_{\mathrm{Sat}}$). 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.

12 pages, 6 figures, submitted to ApJ