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
Max Planck Institute for Solar System Research Research offers the PhD programme "International Max-Planck Research School (IMPRS) for Solar System Science" together with the University of Göttingen. The Solar System School offers a three-year course of study emphasizing actual research. The curriculum covers the entire area of the solar system from small bodies to the planets and the Sun. The goal is a broad, interdisciplinary, and solid scientific education, extended with courses in numerical physics, space technology, project management, scientific writing, and presentation techniques. There are always about 50 graduate students studying at the Solar System School. Two thirds are from abroad from over 30 different countries.