Twins who are 19.0 years of age leave the earth and travel to a distant planet 12.0 light-years away. Assume that the planet and earth are at rest with respect to each other. The twins depart at the same time on different spaceships. One twin travels at a speed of 0.900c, and the other twin travels at 0.500c.
(a) According to the theory of special relativity, what is the difference between their ages when they meet again at the earliest possible time?
(b) Which twin is older?