HR 7272 B Lyrae

(The Planet Project)

Lyr A massive planet orbits a fainter companion to HR 7272 A (HR 7272 B) in Lyra, which is itself a close double, making the planet part of a triple star system.


The purple circle shows the location of the class G dwarf star HR 7272 B, found in the constellation Lyra. HR 7272 A, the brighter of a wide double, is the brighter of the two close to the center of the circle. The other is not the companion and is of no consequence here. HR 7272 B is only 16 seconds of arc from HR 7272 A and over a magnitude fainter, and is not visible in the picture (but can be seen easily through a small telescope). With a hefty mass of at least 6.92 times that of Jupiter, HR 7272 B's planet orbits with a period of 71.49 days at an average distance of 0.32 Astronomical Units (48 million kilometers, 30 million miles), 83 percent of Mercury's distance from the Sun. A modest eccentricity takes the planet between 0.36 and 0.28 AU from its star.


HR 7272, also called HD 178911, a sixth magnitude (6.4) class G star in Lyra, is near the limit of naked eye vision, but can be seen easily in binoculars. Too faint to have a proper or Greek letter name, or even Flamsteed number, it is known best by its numbers in the Bright Star(HR) and the Henry Draper (HD) Catalogues. Through the telescope it appears as a classic wide double star that consists of a brighter class G1 dwarf (HR 7272 A) that shines at magnitude 6.74 and an eighth magnitude (7.97) G5 (perhaps G8) dwarf 16 seconds of arc away. The magnitudes are ambiguous. Catalogues give magnitude 6.7 for both the combined light and for HR 7272 A alone (the latter course adopted here). No orbital motion between A and B has been detected, but both move together at high speed relative to the Sun (roughly 4 times normal). At a distance of 130 light years, the two are at least 640 Astronomical Units apart and orbit with a period greater than 9800 years. HR 7272 A is itself a close double that consists of class G1 and K1 dwarfs with masses of 1.1 and 0.8 solar that orbit in 3.54 years at an average separation of 3.82 AU. (The distance of 130 light years is found from the binary star orbit, and is less than the parallax, from which 160 light years is found.) HR 7272 B, the class G5 planetary host, shines with a luminosity 92 percent that of the Sun from a 5595 Kelvin surface (190 degrees cooler than the Sun), from which we find a radius about the same as that of the Sun and a mass of around 90 percent solar (though some suggest higher, up to 0.98 solar). From the planet, the distant pair that makes HR 7272 A would appear as two bright stars a third of a degree apart that together would shine with the light of 10 full Moons.
Written by Jim Kaler. Return to The Planet Project or go to STARS.