HD 189733 Vulpeculae

(The Planet Project)

Vul A Jupiter-like planet in short orbit about the eighth magnitude K dwarf HD 189733 (63 light years away) is also seen to transit across the star, allowing the orbital tilt, exact planetary mass, density, and something of the chemistry to be found.


The class K1.5 dwarf star HD 189733, found in the constellation Vulpecula, lies at the center of the circle just a bit southeast of 14 Vul. (The planetary nebula M 27 is at the circle's upper-right edge.) The planet orbits the star in a circular path at a very small distance of 0.031 Astronomical Units, just 10 percent Mercury's distance from the Sun, giving it an orbital period of 2.219 days. At every round, the planet crosses in front of the star, slightly dimming it (by about 0.03 magnitude), which tells of an orbital tilt of 4 degrees to the line of sight. That combined with velocity shifts in the star's spectrum give a precise planetary mass of 1.13 times that of Jupiter. Direct measure of the star's angular diameter (hence physical diameter) plus the small "eclipse" then give a planetary radius of 1.19 Jupiters and a first-determined density of 0.91 grams per cubic centimeter, 68 percent Jupiter's, probably as a result of the planet's proximity to its parent star. Spectroscopy of the light from the star- plus-planet reveals the presence of water, carbon dioxide, and carbon monoxide in the planetary atmosphere.


HD 218396 is an eighth magnitude (7.68) class K1.5 hydrogen-fusing dwarf that lies in the modern constellationVulpecula only a quarter of a degree to the east of the famed planetary nebula Messier 27 (the Dumbbell Nebula). It requires at least large binoculars to see. Far too faint to have a proper or Greek letter name, or even a Flamsteed number, it is known best by its listing in the Henry Draper (HD) Catalogue . The temperature is measured at 4980 Kelvin, 800 Kelvin cooler than our own Sun. From a distance of 63 light years, the star shines with a luminosity of just 0.33 times that of the Sun, which gives a radius 0.78 solar. Direct measure of angular diameter leads to the same radius, showing that all the parameters are accurate. Comparisons with theory then give a mass of 80 percent solar. The metal content is similar to that of the Sun. Tiny variations in the stellar brightness caused by starspots and bright magnetic patches moving in and out of view as the star rotates give a rotation period of 12.0 days, about half that of the Sun.
Written by Jim Kaler. Return to The Planet Project or go to STARS.