The Solar System formed 4. 6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
Had Jupiter continued to gain weight, it would have grown ever hotter and hotter, and ultimately self-sustaining, raging nuclear-fusing fires may have been ignited in its heart. This would have sent Jupiter down that long, shining stellar road to full-fledged stardom. Had this occurred, Jupiter and our Sun would have been binary stellar sisters, and we probably would not be here now to tell the story. Our planet, and its seven lovely sisters, as well as all of the moons and smaller objects dancing around our Star, would not have been able to form. However, Jupiter failed to reach stardom. After its brilliant, sparkling birth, it began to shrink. Today, Jupiter emits a mere.00001 as much radiation as our Sun, and its luminosity is only.0000001 that of our Star. Galilean Moons Of Jupiter. One dark, clear January night in 1610, Galileo Galilei climbed to the roof of his house in Padua. He looked up at the sky that was speckled with the flickering fires of a multitude of starry objects, and then aimed his small, primitive "spyglass"--which was really one of the first telescopes--up at that star-blasted sky above his home. Over the course of several such starlit, clear winter nights, Galileo discovered the four large Galilean moons that circle around the largest planet in our Sun's family, the enormous, gaseous world, Jupiter. This intriguing quartet of moons--Io, Europa, Ganymede, and Callisto--were named for four of the numerous mythic lovers of the King of the Roman gods. The discovery of a moon for Makemake may have solved one perplexing puzzle concerning this distant, icy object. Earlier infrared studies of the dwarf planet showed that while Makemake's surface is almost entirely frozen and bright, some areas seem to be warmer than other areas. Astronomers had suggested that this discrepancy may be the result of our Sun warming certain dark patches on Makemake's surface. However, unless Makemake is in a special orientation, these mysterious dark patches should cause the ice dwarf's brightness to vary substantially as it rotates. But this amount of variability has not been observed.