TOI-561 b
TOI-561 b is an USP Super-Earth with a radius of roughly 1.4 Earths. It has an extremely short orbital period of under 11 hours, less than half of an Earth day, resulting in an equilibrium temperature of. The planet was shown to have an atmosphere in a 2025 study.
Composition and atmosphere
The planet is believed to be far too small and irradiated to hold onto its primordial Hydrogen and Helium envelope. However, the composition of the planet varies greatly between the two studies. Weiss 2020 found a mass of around 3.2 Earths and a density of 5.5 g/cm3, around the same as Earth and implying a rocky but iron-poor composition. Lacedelli 2020, on the other hand, found a mass of only 1.59 Earths and a density of 3.0 g/cm3, abnormally low for a planet of its size and suggesting a composition made of 50% or more of water. Even their higher mass estimate of 1.83 Earths is consistent with a water-world. With an insolation 5,100 times that of Earth, TOI-561 b should have lost its gaseous layer and have little volatiles, so the authors believe if the planet has a significant amount of water, it has been evaporated into a puffy steam atmosphere that makes the planet seem larger, less dense, and more water-rich. If it is an extremely water-rich world, TOI-561 b would prove formation scenarios about Super-Earths forming beyond the "Snow Line" and migrating inwards.Emission spectrum measurement with James Webb [Space Telescope] in 2025 has revealed that the planet effectively transfers the heat from its dayside to its nightside, which proves the existence of a thick atmosphere surrounding the planet.