One of the most intriguing and rare types of exoplanets are ultra-short-period (USP) planets. As the name indicates, these planets are defined by having orbital periods shorter than one day. Out of the more than 5500 confirmed exoplanets known to date, only 130 are USP planets, and approximately 40 have mass and radius measurements. Since the only parameter used to define this class of planets is their orbital period, USP planets cover a wide range of sizes and masses.
Small, rocky (Rp < 3 Rearth) USP planets are mysterious sub-category of USP planets. Owing to their short planet-to-star distance, they experience levels of irradiation of the order of hundreds or thousands of times stronger than Earth's, and are thus subject to extreme temperatures. These planets typically have small sizes and appear to have mostly Earth-like compositions (i.e., rocky worlds with little to no atmosphere).
Small USP planets appear to exhibit unique characteristics, suggesting they may constitute a distinct population. For instance, the occurrence rates of USP planets seem to vary with spectral type. Another interesting feature of small USP planets is that, unlike hot Jupiters, they tend to be found in multi-planetary systems.
The formation process of small USP planets is still unknown. Since they are close to their star and subject to temperatures that cause the sublimation of most of the dust present in the planetary disk, it is expected that they formed farther from their stars and migrated inward. Hence, most proposed scenarios involve some migration mechanism to explain their current orbit.
More interestingly, rocky USP planets may provide a unique opportunity to study the surface and internal composition of exoplanets with current instrumentation. These planets likely have molten surfaces if their equilibrium temperatures exceed 1100 K, which is the melting point of an Earth-like crust. Additionally, some volatile elements associated with the lava oceans on the planet's dayside may be detectable through secondary transits (eclipses) spectroscopic observations. In this project we propose to exploit TESS data for the validation of USP planet candidates and the discovery of new ones. Our objectives include measuring their masses and identifying additional planets around stars already known to host a USP planet. Our primary focus will be on small-sized (Rp < 3 Rearth) USP planets, which are likely to have Earth-like compositions and are potential candidates for being lava-covered worlds.
We will specifically concentrate on both new and previously known TESS USP planet candidates lacking mass determination. We will use our access to ground-based facilities to observe them and confirm that the transit events are not caused by other phenomena that can mimic planetary transit signals. Once the candidates are validated using photometry, we will include them in our radial velocity (RV) followup program. By combining RV measurements with the transit events observed by TESS and ground-based facilities, we will be able to determine the masses and radii of these newly discovered exoplanets.
Our team possesses extensive experience in the discovery of exoplanets and is part of a renowned research group with a proven track record of delivering high-quality and impactful results. Through this project, we aim to further advance the forefront of exoplanet exploration, focusing particularly on unraveling the mysteries surrounding the formation of USP planets.
