K2-280 b – a low density warm sub-Saturn around a mildly evolved star.

We present an independent discovery and detailed characterization of K2-280 b, a transiting low density warm sub-Saturn in a 19.9-d moderately eccentric orbit (e  = |$0.35_{-0.04}^{+0.05}$|  ) from K2 campaign 7. A joint analysis of high precision HARPS, HARPS-N, and FIES radial velocity measurements and K2 photometric data indicates that K2-280 b has a radius of R b = 7.50 ± 0.44 R⊕ and a mass of M b = 37.1 ± 5.6 M⊕, yielding a mean density of ρb = |$0.48 _{ - 0.10 } ^ { + 0.13 }$|   |${\rm g\, cm^{-3}}$|⁠. The host star is a mildly evolved G7 star with an effective temperature of T eff = 5500 ± 100 K, a surface gravity of |$\log \, g_{\star }$|  = 4.21 ± 0.05 (cgs), and an iron abundance of [Fe/H] = |${0.33}\, {\pm }\, {0.08}$|  dex, and with an inferred mass of M ⋆ = 1.03 ± 0.03 M⊙ and a radius of R ⋆ = 1.28 ± 0.07 R⊙. We discuss the importance of K2-280 b for testing formation scenarios of sub-Saturn planets and the current sample of this intriguing group of planets that are absent in the Solar system. [ABSTRACT FROM AUTHOR]