Psi-1 = 91 Aquarii

(The Planet Project)

Aquarius Psi-1 Aquarii, an evolved class K (K0) giant, and its Jupiter-like planet are both orbited by a low-mass double star.


The circle toward lower left shows the location of the evolved class K giant star Psi-1 Aquarii (in Aquarius, the Water Bearer), which is probably better known as 91 Aquarii. The planet, which has at least 2.9 times the mass of Jupiter, orbits Psi-1 every 182 days at a mean separation of 0.3 Astronomical Units, or about 80 percent Mercury's distance from the Sun. Nothing else is known about it.

The upper-left circle shows HR 8734 in Pisces.


Psi-1 Aquarii is the western-most of a tight and obvious but unrelated trio, the others Psi-2 and Psi-3, which is why "Flamsteed 91" is perhaps more commonly used. Planetary systems continue to amuse and confound. Several planets are known to orbit giants and members of widely spaced double stars. Psi-1 does both, but with a twist in that the distant "companion," Psi-1 BC (the main bright class K0 giant thus being Psi-1 A, the planet then Psi-1 Ab), is itself an orbiting double! At a distance of 150 light years (second Hipparcos reduction), the star shines at the rate of 50 Suns, which leads to a radius of 10 solar and an approximate mass of 2.5 Suns (and thus born as a B8 dwarf). The BC pair, class K3 dwarfs, orbit every 84 years at a mean separation of 21.5 AU but with a high eccentricity that takes them between 31 and 12 AU apart. Kepler's laws give a mass-sum of 1.4 solar, very close to that expected. The pair orbits Psi-1 A and its planet at a distance greater than 2300 AU, taking at least 56,000 years to go all the way around. From the planet, which is obviously uninhabited, the BC pair would be spectacular, appearing as twin ultra-bright yellow-orange Venus's up to half or so a degree apart.
Written 3/10/11 by Jim Kaler. Return to The Planet Project or go to STARS.