An M-class dwarf star such as Gliese 581 has a much lower mass than the Sun, causing the core region of the star to fuse hydrogen at a significantly lower rate. From the apparent magnitude and distance, astronomers have estimated an effective temperature of 3200 K and a visual luminosity of 0. 2% of that of the Sun. However, a red dwarf such as Gliese 581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly 830 nm (estimated using Wien’s displacement law, which assumes the star radiates as a black body), so such an estimate will underestimate the star’s total luminosity. (For comparison, the peak emission of the Sun is roughly 530 nm, in the middle of the visible part of the spectrum. ) When radiation over the entire spectrum is taken into account (not just the part that humans are able to see), something known as the bolometric correction, this star has a bolometric luminosity 1. 3% of the Sun’s total luminosity. A planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth. The region of space around a star where a planet would receive roughly the same energy as the Earth is sometimes termed the “Goldilocks Zone”, or, more prosaically, the habitable zone. The extent of such a zone is not fixed and is highly specific for each planetary system. Gliese 581 is a very old star. Its slow rotation makes it very inactive, making it better suited than most red dwarfs for having habitable planets.
A moon is defined as a natural satellite in orbit around another body that, in turn, is in orbit around its Star. The moon is kept in its position by both its own gravity, as well as its host's gravitational grip. Some planets have many moons, some have only a small number, and still others have none at all. Several asteroids inhabiting our Solar System are circled by very small moons, and some dwarf planets--such as Pluto--also host moons. The very productive Cassini mission might attain some indirect information by analyzing the ring arc material--however, it is unlikely to come close to the little moon again before the mission ends in 2017. Several possibilities could provide an answer as to why the moon would have charcoal-black surface patches, even though it is circling a dwarf planet that is as bright as freshly fallen snow. One theory that has been suggested proposes that, unlike larger objects such as Makemake, its own little companion moon is so small that it cannot gravitationally keep a grip onto a bright and icy crust, which then sublimates, undergoing a sea-change from solid to gas under the melting influence of warming sunlight. This would make the little moon akin to comets and other KBOs, many of which are well-coated with very dark material.