HD 188753 Cygni

(The Planet Project)

At nearly eighth magnitude, HD 188753 (in the constellation Cygnus is just barely visible at the center of the labelled circle and requires a telescope or large binoculars to see. HD 188753 is unique in that the planet orbits within a triple star system. The planet, a "hot Jupiter," goes around the primary of the system as does a remarkably-nearby close double star. How such a system could have developed is a mystery since the secondary double star should have destroyed the raw material from which the planet should have formed.


The planet, with a mass at least 1.14 times that of Jupiter, orbits very close to the primary (brighter) star of the triple system HD 188753 with a period of only 3.3481 days at a distance of but 0.0446 Astronomical Units (6.7 million miles, 10.8 million kilometers), 11.5 percent the distance of Mercury from the Sun.


HD 188753 is a near-eighth-magnitude class G or K (K0 or G9) star 145 light years away that falls more or less in between Delta and Gamma Cygni. It is a triple in which the primary is orbited by a close double at a mean separation of 12.3 Astronomical Units (29 percent farther than Saturn is from the Sun) with a looping orbit that takes the double as close as 6.2 AU and as far as 18.5 AU over a period of 25.7 years. The primary, with a temperature of 5750 Kelvin, carries a mass of 1.06 times that of the Sun. The double- star companion orbits with a period of 155 days, an average separation of 0.66 AU, and has a total combined mass of 1.63 solar. With an age estimated at 5.6 billion years, the system is just a bit older than the Sun. (Principal data from M. Konacki in Nature for 14 July, 2005 and references therein).

Cygnus has another star with a planet, the double star 16 Cygni.
Written by Jim Kaler. Return to The Planet Project or go to STARS.