HD 20781 and Its Four Known Planets

HD 20781 is a star 117 light-years from Earth with four known planets. The star's spectral type is G0, and its reported measurements include a mass of 0.9 solar masses, a radius of 0.865 solar radii, and a temperature of 5260 K. Its radius is smaller than the Sun's, and its temperature is cooler than the Sun's.

The planets were all discovered in 2019 using the radial velocity method, which measures the wobble a planet's gravity induces in its star's motion. Repeated wobbles reveal the planet and its orbital period.

HD 20781 b has a minimum mass of 1.93 Earth masses and an orbital period of 5.31 days. Among the listed planets, it has the shortest year and the smallest minimum mass. HD 20781 c has a minimum mass of 5.33 Earth masses and an orbital period of 13.9 days. Among the listed planets, it has the second-shortest year and the second-smallest minimum mass. HD 20781 d has a minimum mass of 10.6 Earth masses and an orbital period of 29.2 days. Among the listed planets, it has the second-longest year and the second-largest minimum mass. HD 20781 e has a minimum mass of 14 Earth masses and an orbital period of 85.5 days. Among the listed planets, it has the longest year and the largest minimum mass.

Each of the four planets has a minimum mass larger than Earth's, meaning each value is a lower bound on the planet's true mass rather than an exact figure.