HR 2877 Geminorum

(The Planet Project)

Gem Like HR 2447, the system is unusual in that the planet belongs not to an ordinary dwarf, as most do, but to a giant star, one that is in the act of evolving and dying.


The lower circle shows the location of the class K giant star HR 2877, found in the constellation Gemini. The planet, with a mass estimated to be at least 6.5 times that of Jupiter, orbits its star with a period of 303 days at an average distance of roughly 0.8 Astronomical Units (120 million kilometers, 75 million miles, 10 percent greater than the size of Venus's orbit). The orbital eccentricity is not known. The parameters of giant-star planets are not well constrained, as they first require an estimate of stellar mass. Ordinary dwarfs have luminosities and temperatures that are very sensitive to mass, and it is therefore easy to obtain it and thus to find the needed values from the velocity data. The luminosities and masses of giants, however, are nowhere near as sensitive to mass, which yields larger errors in the results.


HR 2877, also called HD 59686, is a just-barely fifth magnitude (5.45) class K2 orange giant in Gemini. 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 (needed to evaluate the amount of infrared radiation) has never been measured, but from the star's class should be about 4400 Kelvin. Given that figure, from a distance of 300 light years, it shines with a luminosity of 84 times that of the Sun and has a radius 16 times solar, appropriate for an evolved, helium-fusing giant. From a distance of 0.8 AU, the central giant star would appear 20 times as big as the Sun, roughly 10 degrees across, twice the angular separation between the front bowl stars of the Big Dipper. Unlike most stars with planets, the metal abundace of the star appears to be normal, that is, close to the solar value.

Pollux and HD 50554 Geminorum, near the center of the picture, also have orbiting planets.
Written by Jim Kaler. Return to The Planet Project or go to STARS.