Kavli Affiliate: Avi Shporer
| First 5 Authors: David R. Anderson, David R. Anderson, , ,

| Summary:
We report the discovery of NGTS-11 c, a transiting warm Neptune ($P approx 12.8$ d; $M_p = 1.2^+0.3_-0.2 M_mathrmNep$; $R_p = 1.24 pm 0.03 R_mathrmNep$), 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; $M_p sin i > 3.6 M_mathrmJup$). Taking into account the two additional bodies in our expanded datasets, we find that the mass of NGTS-11 b ($M_p = 0.63 pm 0.09 M_mathrmSat$; $R_p = 0.97 pm 0.02 R_mathrmSat$) is lower than previously reported ($M_p = 1.2 pm 0.3 M_mathrmSat$). 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.
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