This star system, in the southern constellation Ara, the altar, is about 50 light-years away. Its star is a little larger and heavier than the Sun, and bright enough to see with the unaided eye.
The system appears to include at least four planets.
One of the planets, known as Mu Arae b, is at least 1.7 times the mass of Jupiter and orbits the star once every 1.75 years. The other, Mu Arae c, is 3.1 times Jupiter’s mass and orbits once every 8.2 years. Planets “d” and “e” are closer to the star and less massive than “b” and “c.” None of the four planets appear to reside inside the “habitable zone,” which is the distance from the star at which temperatures are right for liquid water.
A problem with the Mu Arae system, however, is that some simulations show that if the planets all orbit in the same plane, they should interact in such a way that the entire system would be disrupted in less than 100 million years. That suggests that the orbits of these planets are tilted at different angles, so the system would not look like marbles rolling on a tabletop, as ours does.
Mu Arae is a top target for future efforts to take pictures and spectra of extrasolar planets.
This yellow-orange star, which takes its name from its listing in the Henry Draper star catalog, is about 54 light-years away and is about 79 percent as massive as the Sun. It is a highly active star, which means that lots of dark “starspots” break out on its surface. This activity complicates efforts to find and study planets in the system.
HD 128311 appears to have two planets. Planet “b” is 2.58 times as massive as Jupiter, and orbits the star once every 1.15 years. Planet “c” is 3.24 times Jupiter’s mass and orbits every 2.52 years.
The most distant of the study’s four stars at 151 light-years, HD 202206, in Capricornus, is slightly smaller and less massive than the Sun, and a few hundred degrees cooler.
Observations to date confirm that the system consists of either two stars with a planet orbiting both of them, or a single star with two companions — a planet and a brown dwarf (a “failed” star that is between the mass of a star and a planet).
One of the objects appears to be at least 17.4 times the mass of Jupiter, which would make it a brown dwarf. The other object is about 2.5 times the mass of Jupiter and orbits the star once every 3.8 years.