HD 15337

Characteristics

The two stars orbit each other once every 760 years or so, separated by an average of 88 AU on a very stretched-out path. The larger star is a K1V orange dwarf, 15% smaller than the Sun in both mass and radius, and shines with less than half the Sun’s light. Its surface temperature is 5131 K. Billions of years old, it has a metal content and activity level similar to the Sun’s. The smaller companion weighs in at just 26% of the Sun’s mass.

Planetary system

In May 2019, two exoplanets were discovered transiting the primary star: HD 15337 b and c. Both orbit much closer to their star than Mercury is to the Sun, with equilibrium temperatures of 1001 K and 642 K, respectively—hot enough to melt lead. The inner planet, HD 15337 b, has a radius of 1.770 Earth radii and a mass of 6.519 Earth masses, giving it a density higher than Earth's, indicating a rocky super-Earth.

The outer planet, c, has a mass of 6.792 Earth masses but a radius over 40% larger, resulting in a much lower density suggestive of a mini-Neptune with a thick hydrogen-helium envelope. This difference places the two planets on opposite sides of the small planet radius gap. In 2024, CHEOPS and HARPS observations refined the parameters: HD 15337 b's mass and radius uncertainties were reduced to under 2% and 7%, and HD 15337 c's radius was constrained to within 3%.