54 Piscium

(The Planet Project)

And 54 Piscium hosts a low mass body, measured at only two-thirds the mass of Saturn, in an elliptical orbit that takes it close to its cool, low-mass star. Remarkably, the system also contains a distant brown dwarf, a substar whose mass is too low to run hydrogen fusion, showing that a large range of binary systems can posses planets.


The circle at lower right shows the location of the class K dwarf star 54 Piscium relative to stars of Andromeda, which is depicted here. Though technically in Pisces, the star is just barely over the formal border between the two constellations. 54 Piscium is the star to the right of the pair within the circle, the one at the exact center. The one to the left is 55 Psc, a class K giant or supergiant with an F dwarf companion that is irrelevant here. 54 Psc's planet has a mass measured to at 0.2 times that of Jupiter, which is only 2/3 that of Saturn and one of the lower known, though since the orbital tilt is not determined, the mass is a lower limit and could be significantly higher. The planet orbits its star with a period of 62.23 days at an average distance of 0.284 Astronomical Units (42.6 million kilometers, 26.5 million miles), or 73 percent of the distance of Mercury from the Sun. A high orbital eccentricity takes the planet between 0.46 and 0.11 AU from its star, making it a "hot Saturn" -- if in fact the mass is really that low.


54 Piscium is a sixth magnitude (5.87) class K0 dwarf star in Pisces that lies just over the constellation's border with Andromeda, to which it appears better related. Known professionally as HD 3651, from a distance of 36 light years, it shines with a luminosity only 0.54 that of the Sun from a surface at 5173 Kelvin (610 or so degrees cooler than the Sun), which tells of a star with a radius 0.92 solar and a mass only 0.76 solar. Direct measure of angular diameter gives a radius of 0.94 solar, the two values in excellent agreement. Like most stars with planets, 54 Psc is relatively rich in metals, its iron abundance (relative to hydrogen) 1.12 times that of the Sun. The star has a reputation for variability. While the Bright Star Catalogue (where it is called HR 166) gives the magnitude at 5.87, measures from other sources suggest 5.80, which is odd, since a K0 dwarf should be stable. And while 54 Psc is supposedly fainter than 55 Psc, it looks brighter within the circle above, though given the faintness of the stars, that is probably a photographic effect (or caused by possible variability of 55 Psc).


In addition to its planet, 54 Piscium also has a low-mass substar companion, a brown dwarf called 54 Psc B (or HD 3651 B) located 43 seconds of arc away, which translates to a physical distance of at least 475 Astronomical Units. Given the mass of the primary K0 dwarf, the orbital period must be at least 12,000 years. But don't try to look for it, as this class T (T7.5) is so cool (810 Kelvin, 537 C, 1000 F) that it is invisible to the eye, and can be seen only in the infrared part of the spectrum, where it was discovered. Even in the IR, the little one is dim, radiating at a rate of just two millionths that of the Sun. The low temperature is primarily the result of low mass, which from different authors ranges from 35 to 65 times that of Jupiter (0.03 to 0.06 times that of the Sun), all of which are below the crtical value of 0.073 times solar, below which a star cannot run hydrogen fusion to helium: hence the star must be a brown dwarf. Part of its dimness is the result of its age (unlike real stars, brown dwarfs fading with time), which is constrained to between roughly 3 and 12 billion years. The system thus shows more of the great variety of other planetary systems. A similar kind of dim star was once speculated to orbit our Sun. Called "Nemesis," it could disturb our surrounding cloud of comets, sending them pounding down toward Earth. Deep surveys (as well as the orbits of our planets), however, show that it does not exist.

The circle at left shows Upsilon Andromedae, which has a planetary system with three orbiting planets.
Written by Jim Kaler. Return to The Planet Project or go to STARS.