HR 8799 Pegasi

(The Planet Project)

Peg Four "super-Jupiter" planets are found to orbit the class A5 dwarf, HR 8799. Along with Fomalhaut's planet, this quartet of planets contains the first to be seen by direct imaging and the first to be discovered orbiting a class A star.


The lower circle shows the location of the class A5 dwarf star HR 8799, found in the constellation Pegasus. Four giant planets are seen to orbit, HR 8799: "e", "d", "c", and "b", at average distances from the star of 14.5, 24, 38, and 68 Astronomical Units. The first three were found in 2008, the fourth ("e") in 2010. If placed here, around our Sun, the inner one would be between the orbits of Saturn and Uranus, the next one a bit inside the orbit of Neptune, while the third ("d") would be near the average distance of Pluto. With respective estimated masses of 9, 10, 10, and 7 times that of Jupiter, they take roughly 50, 100, 190, and 465 years to make full orbits.


HR 8799, also known as HD 218396, is a sixth magnitude (5.99) white, class A5 hydrogen fusing dwarf that lies just with the Great Square of Pegasus. Too faint to have a proper or Greek letter name, or even a Flamsteed number, it is known best by its numbers in the Bright Star(HR) and the Henry Draper (HD) Catalogues. The temperature is measured at 7250 Kelvin. From a distance of 129 light years, the star shines with a luminosity of just 4.9 times that of the Sun, which gives a radius 1.4 times solar. Comparisons with theory then give a mass of 1.5 times solar, and show the star to be very young, its age only about 60 million years, as compared to its 2.8 billion year hydrogen-fusing lifetime. Unlike most stars with planets, the metal abundance of the star is low, only about a third that of the Sun. Consistent with having a planet, the star is also surrounded by an infra-red- radiating debris disk. Slightly unstable, HR 8799 has distinction of being a Gamma Doradus type variable, one that varies subtly with multiple periods, this one with periods of 0.505 and 0.579 days. It also appears to be low in iron and other elements characteristic of a Lambda Bootis star.

By odd coincidence, HR 8799 is a near-neighbor of 51 Pegasi, the first star known to have an orbiting planet.