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We report the discovery of TOI-1444b, a 1.4-$R_\oplus$ super-Earth on a0.47-day orbit around a Sun-like star discovered by {\it TESS}. Precise radialvelocities from Keck/HIRES confirmed the planet and constrained the mass to be$3.87 \pm 0.71 M_\oplus$. The RV dataset also indicates a possiblenon-transiting, 16-day planet ($11.8\pm2.9M_\oplus$). We report a tentativedetection of phase curve variation and secondary eclipse of TOI-1444b in the{\it TESS} bandpass. TOI-1444b joins the growing sample of 17ultra-short-period planets with well-measured masses and sizes, most of whichare compatible with an Earth-like composition. We take this opportunity toexamine the expanding sample of ultra-short-period planets ($<2R_\oplus$) andcontrast them with the newly discovered sub-day ultra-hot Neptunes($>3R_\oplus$, $>2000F_\oplus$ TOI-849 b, LTT9779 b and K2-100). We find that1) USPs have predominately Earth-like compositions with inferred iron core massfractions of 0.32$\pm$0.04; and have masses below the threshold of runawayaccretion ($\sim 10M_\oplus$), while ultra-hot Neptunes are above the thresholdand have H/He or other volatile envelope. 2) USPs are almost always found inmulti-planet system consistent with a secular interaction formation scenario;ultra-hot Neptunes ($P_{\rm orb} \lesssim$1 day) tend to be ``lonely' similarto longer-period hot Neptunes($P_{\rm orb}$1-10 days) and hot Jupiters. 3) USPsoccur around solar-metallicity stars while hot Neptunes prefer highermetallicity hosts. 4) In all these respects, the ultra-hot Neptunes show moreresemblance to hot Jupiters than the smaller USP planets, although ultra-hotNeptunes are rarer than both USP and hot Jupiters by 1-2 orders of magnitude.

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