HAT-P-17: A Cool K-Type Star and Its Two Known Worlds

HAT-P-17 is a K-type star located 301 light-years away. It is smaller than the Sun in both mass and radius, at 0.857 solar masses and 0.838 solar radii, and its temperature of 5250 K is cooler than the Sun's. The star is also older than the Sun, with an age of 7.8 billion years. Two planets are known to orbit it.

HAT-P-17 b was discovered in 2012 by the transit method, which detects the small dip in a star's brightness when a planet passes in front of it. This Jupiter-like planet completes an orbit in 10.3 days, making it the shortest-period planet among the two listed worlds. Its radius is 11.8 Earth radii, larger than Earth's, and its mass of 184 Earth masses is likewise larger than Earth's. Its mean density is 0.623 g/cm³, which is 0.113 times Earth's.

HAT-P-17 c was also announced in 2012, but it was found by the radial velocity method, which tracks the wobble a planet's gravity induces in its star. Its minimum mass is 1080 Earth masses, roughly 3.4 times Jupiter's mass. Because this is a minimum mass, it represents a lower bound on the planet's true mass. The planet's orbital period is 5580 days, the longest among the listed planets.