Alpha Centauri B, otherwise known as Rigel Kentaurus B, Toliman B, Bungula B, Gliese 559 B, HD 128621 and HIP 71681, is the companion star, or secondary, of the binary system. It is a main-sequence star of spectral type K1 V, making it more an orange color than the primary star; it is slightly smaller and less luminous than the Sun. Alpha Centauri B is about 90 percent the mass of the Sun and 14 percent smaller in radius. Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare. Component B is more magnetically active than Alpha Centauri A, showing a cycle of 8.2±0.2 yr compared to 11 years for the Sun, and about half the minimum-to-peak variation in coronal luminosity of Sol.

In 2012, a planet around Alpha Centauri B was detected. After that, scientists met up to discuss an unmanned mission to the planet. Eventually, in 2048, the Bullet probe (equipped with a small robotic lander and rover) was launched and arrived at Alpha Centauri Bb (later known as Helion) in 2057.

The existence of a planet, Alpha Centauri Bc, was announced in 2013. It has an estimated orbital period of approximately 12 Earth days – smaller than that of Mercury – with a semimajor axis of 0.10 AU and an eccentricity smaller than 0.24.

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