circa 1970: Neptune, fourth largest of the planets in our solar system. The atmosphere consists mostly of hydrogen and helium, but the presence of three per cent methane lends the planet its striking blue hue. (Photo by Hulton Archive/Getty Images)
circa 1970: Neptune, fourth largest of the planets in our solar system. The atmosphere consists mostly of hydrogen and helium, but the presence of three per cent methane lends the planet its striking blue hue. (Photo by Hulton Archive/Getty Images)