HD 110067 d is the largest of six sub-Neptunes around HD 110067, a star a little smaller than the Sun about 100 light-years away, at about 2.85 times Earth's width. It orbits every 20.52 days, and its detection by ESA's CHEOPS satellite is what revealed the system's structure.
NASA's TESS satellite had recorded the star's dips in 2020 and 2022, but they could not be uniquely matched to orbits. When a team led by Rafael Luque used CHEOPS to pin down a 20.5-day planet, they saw that the inner three periods, 9.1, 13.7 and 20.5 days, each step up by a factor of about one and a half. That chain of 3 to 2 resonances allowed them to predict the remaining three planets' periods from the other TESS dips. ESA described CHEOPS's confirmation of this third planet as the key to the whole system.
Its mass, about 8.5 Earth masses, was measured from the star's wobble, one of three in the system. That mass is low for its size, and the discovery team concluded that the planets with measured masses likely have large atmospheres dominated by hydrogen. In 2023 Kevin Apps and colleagues pointed out that HD 110067 is not a single star, as first described, but has a distant companion pair about 13,400 times Earth's distance from the Sun away. HD 110067 d receives over 20 times the light Earth gets from the Sun.