The circle shows the location of the class G subdwarf-dwarf Mu Arae
(in the constellation Ara). Like 55 Cancri, it has four planets,
described below. The innermost has the one of the lowest masses
(actually a lower limit to the true mass, as the tilt of the orbit
is not known) of any planet discovered so far, while the outermost
has one of the longest periods yet known. Photo courtesy of Chris
Grohusko.
THE PLANETS
*Planet 1: From the star outward, planet 1 orbits in 9.55 days, has a mass at least 0.03 times that of Jupiter (only 10 times that of Earth, one of the lowest mass-limits known), and is 0.09 Astronomical Units (AU) from the star (13.5 million kilometers, 8 million miles, or 23 percent Mercury's distance from the Sun). As expected for such a close planet, the orbit is circular.*Planet 2 orbits in 311 days (0.85 years), has a mass of at least 0.52 times that of Jupiter, and averages 0.92 Astronomical Units from the star (138 million kilometers, 86 million miles). The orbit has only a seven percent eccentricity, which carries the planet between 0.98 and 0.86 AU from the star.
*Planet 3 orbits in 1.8 years, has a mass at least 1.7 times that of Jupiter, and averages 1.5 Astronomical Units from the star (225 million kilometers, 140 million miles, or 1.5 Earth's distance from the Sun). The orbit is fairly eccentric, the planet moving between 2.0 and 1.0 AU from the star.
*Planet 4 has a long orbit of 11.3 years, a mass at least 1.8 times that of Jupiter, and averages 5.2 Astronomical Units from the star (780 million kilometers, 485 million miles, close to Jupiter's distance from the Sun). The orbit is particularly eccentric, the ellipse taking the planet from as far as 8 AU to as close as 2 AU.