  Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like
starthat hosts a transiting planet candidate in a 10.6 d orbit. From
spectroscopy, we estimate the stellar temperature to be 5687 +/- 50 K, its
metallicity to be [m/H] = 0.32 +/- 0.08, and the projected rotational velocity
to be v sin i <2.4 km s-1. We combine these values with a study of the
asteroseismic frequencies from short cadence Kepler data to estimate the
stellar mass to be 1.028+0:04-0:03 M_Sun, the radius to be 1.066 +/- 0.012
R_Sun and the age to be 5.25+1:41-1:39 Gyr. We estimate the radius of the 10.6
d planet as 2.37 +/- 0.13 R_Earth. Using 63 radial velocity observations
obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and
36 observations made with the HIRES spectrograph at Keck Observatory, we
measure the mass of this planet to be 6.8 +/- 1.4M_Earth. We also detect two
additional non-transiting companions, a planet with a minimum mass of 4.46 +/-
0.12 M_J in a nearly circular 524 d orbit and a massive companion with a period
>10 years and mass >12.1M_J . The twelve exoplanets with radii <2.7 R_Earth and
precise mass measurements appear to fall into two populations, with those <1.6
R_Earth following an Earth-like composition curve and larger planets requiring
a significant fraction of volatiles. With a density of 2.76 +/- 0.73 g cm-3,
Kepler-454b lies near the mass transition between these two populations and
requires the presence of volatiles and/or H/He gas.
