Following over two decades of discoveries, we now have a substantial sample of exoplanets that can be characterized in detail with current facilities, enabling the emerging field of comparative exoplanetology. Studies of hot Jupiters at low spectral resolution have revealed a rich diversity of planetary atmospheres that range from cloudy to cloud-free, and exhibit scattering slopes from aerosols and absorption features from species such as Na, K, and TiO. High-spectral-resolution observations are emerging as a particularly powerful and robust probe of exoplanetary atmospheres. The use of high-resolution spectroscopy, where we can uniquely identify atoms and molecules, distinguishes it from other comparative studies. These type of observations can place strong constraints on the abundances, altitudes, and local temperatures where each species forms. High-resolution observations have also been used to measure atmospheric winds, structure, and planetary rotation. In this talk, we will discuss the results of our large survey exploring the diversity of exoplanet atmospheres at high-resolution utilizing multiple telescopes. Currently, we have observed 28 exoplanets. The observed planets span a wide range of sizes, from 1.4 Earth radii to 2 Jupiter radii; masses, from 2.8 Earth mass to 2.8 Jupiter mass; and effective temperatures, from 700 to 4500 K. Some highlights include the first detection of chromium hydride, a robust indicator of temperature, at high-resoution in the atmosphere of the hot Jupiter WASP-31b, the first detection of ionizied calcium in the atmosphere of the hottest (~4500 K) known planet KELT-9b, and a detection of many species (neutral sodium, ionized calcium, iron) in the atmosphere of ultra-hot Jupiter WASP-76b. The results from KELT-9b and WASP-76b indicate that the atmospheric layers probed from our observations are much hotter than expected from hydrodynamic and radiative equilibrium. Finally, we will also discuss the upper-limits on the atmosphere found for the terrestrial exoplanet GJ 486b, which are very informative for the JWST observations of the same planet. In summary, our survey hopes to advance our understanding of the diversity of exoplanet atmospheres in significant ways, and shed new light on how this diversity depends on the physical properties of the planets and their host stars.
