Luyten 726-8 is the sixth closest star system to our Sun, and a binary star system. Located in the constellation Cetus, it is only 8.7 light years away. Because both stars are faint, red dwarfs, with an apparent magnitude of 12.54, they are invisible to the unaided eye. The system was not discovered until 1949 by Willem Jacob Luyten. Both stars are flare stars, especially Luyten 726-8 B, commonly known by its variable star name, UV Ceti.
The two stars of the Luyten 726-8 system have a highly eccentric orbit that brings them as close as 2.1 AU and as far as 8.8 AU in their 26.5 year orbit of each other. The orbit is also highly elliptical, with an inclination of 127.3° degrees.
Luyten 726-8 A
Sometimes refereed to its variable star name, BL Ceti, Luyten 726-8 A is a main sequence red dwarf of spectral type M5.6 Ve. The star has between 10 and 11 percent of our Sun's mass, and 14 percent of its diameter. Like all class M stars, Luyten 726-8 A is extremely dim, with only 6/100,000th of our Sun's luminosity. Like its companion, Luyten 726-A is a variable flare star.
Luyten 726-8 B
Luyten 726-8 B is the cooler, dimmer star of the pair with a spectral type M6.0 Ve. The fainter companion has 10 percent of our Sun's mass, and 14 percent of its diameter. It is only some 4/100,000th as bright as our Sun. The star though is the most prominent example of a variable flare star. In fact such flare stars are called "UV Ceti variables" after Luyten 726-8 B's variable star name, UV Ceti. The star can magnify its brightness some five fold in less than a minute, before falling back down to normal luminosity within a couple minutes before flaring suddenly again after several hours. In 1952, an extreme example of the star flaring was observed, when Luyten 726-8 B increased its brightness some 75 times in a period of only 20 seconds.
There have been unconfirmed claims of two possible planets around Luyten 726-8 A, both Jupiter sized worlds but containing more mass. Using the Hubble Space Telescope, astronomers were unable to find collaborating evidence for substellar companions at this time. The distance for an Earth-like world to be comfortable, with liquid water on its surface, would be centered on 0.0071 AU from either star. At such a distance, the orbit of the hypothetical planet would be 17 hours. The distance increases to between 0.04 and 0.07 AU when accounting for the large amounts of infrared radiation, as class M red dwarfs emit most of their energy in this region of the spectrum. In either case, such a world would be tidally locked to the star. Flare stars also typically emit, in addition to bursts of light and radio waves, some 10,000 fold increase in the output of x-rays when they flare. Such regular intense solar flares would normally be lethal to life similar to that found on Earth on a planet so close to the star, making it unlikely life as we know it to exist around flare stars such as Luyten 726-8 AB.