TKS X: Confirmation of TOI-1444b and a Comparative Analysis of the Ultra-short-period Planets with Hot Neptunes

We report the discovery of TOI-1444b, a 1.4-$R_\oplus$ super-Earth on 0.47-day orbit around Sun-like star discovered by {\it TESS}. Precise radial velocities from Keck/HIRES confirmed planet and constrained mass to be $3.87 \pm 0.71 M_\oplus$. The RV dataset also indicates possible non-transiting, 16-day ($11.8\pm2.9M_\oplus$). tentative detection phase curve variation secondary eclipse TOI-1444b in TESS} bandpass. joins growing sample 17 ultra-short-period planets with well-measured masses sizes, most which are compatible an Earth-like composition. take this opportunity examine expanding ($<2R_\oplus$) contrast them newly sub-day ultra-hot Neptunes ($>3R_\oplus$, $>2000F_\oplus$ TOI-849 b, LTT9779 b K2-100). find that 1) USPs have predominately compositions inferred iron core fractions 0.32$\pm$0.04; below threshold runaway accretion ($\sim 10M_\oplus$), while above H/He or other volatile envelope. 2) almost always found multi-planet system consistent secular interaction formation scenario; ($P_{\rm orb} \lesssim$1 day) tend ``lonely' similar longer-period hot Neptunes($P_{\rm orb}$1-10 days) Jupiters. 3) occur solar-metallicity stars prefer higher metallicity hosts. 4) In all these respects, show more resemblance Jupiters than smaller USP planets, although rarer both 1-2 orders magnitude.