Solar analog (–0.21) for solar twin. 16 Cygni B is sometimes noted as twin, but is part of a triple star system and is very old for a solar twin at over 9 Gyr. Another way of defining solar twin is as a "habstar" — a star with qualities believed to be particularly hospitable to an Earth-like planet. Qualities considered include variability, mass, age, metallicity, and close companions. The requirement that the star remain on the main sequence for at least 3 Ga sets an upper limit of approximately 1.5 solar masses, corresponding to a hottest spectral type of F5 V. Such
Planet solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet. Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field. In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times