HD 50554 d is the outer of two small planets found in 2026 crossing in front of HD 50554, an F-type star about 100 light-years away that also hosts a giant planet on a wide orbit. HD 50554 d orbits every 28.1 days and is about 1.4 times Earth's width, a little larger than its inner sibling, HD 50554 c.
A team led by Quanyi Liu found both while looking for small planets around stars already known to have cold giants. Using NASA's TESS satellite, which catches the dip in starlight as a planet passes in front of its star, they searched 132 such stars. They then confirmed the two planets around HD 50554 by combining the TESS data with archived measurements of the star's wobble and with the star's movement measured by the Gaia satellite, which together rule out other explanations for the dips.
The system is a small-scale example of the study's main result: inner super-Earths are about six and a half times more common around stars that have a cold giant planet, which the team took as strong evidence that the two kinds of planet tend to form together. HD 50554 d is cooler than c but still far too hot for liquid water, receiving about 40 times the light Earth gets from the Sun, according to the NASA Exoplanet Archive. Its mass has not been measured, so its makeup is unknown.