A Star That Failed
More massive than all the other planets combined, Jupiter dominates the planetary system. The giant revealed by Voyager is a gas planet of great complexity; its atmosphere is in constant motion, driven by heat escaping from a glowing interior as well as by sunlight absorbed from above. Energetic atomic particles stream around it, caught in a magnetic field that reaches out nearly 10 million kilometers into the surrounding space, embracing the seven inner satellites. From its deep interior through its seething clouds out to its pulsating magnetosphere, Jupiter is a place where forces of incredible energy contend.
At its birth, Jupiter shone like a star. The energy released by infalling material from the solar nebula heated its interior, and the larger it grew the hotter it became. Theorists calculate that when the nebular material was finally exhausted, Jupiter had a diameter more than ten times its present one, a central temperature of about 50 000 K, and a luminosity about one percent as great as that of the Sun today.
At this early stage, Jupiter rivaled the Sun. Had it been perhaps 70 times more massive than it was, it would have continued to contract and increase in temperature, until self-sustaining nuclear reactions could ignite in its interior. If this had happened, the Sun would have been a double star, and the Earth and the other planets might not have formed. However, Jupiter did not make it as a star; after a brief flash of glory, it began to cool.
At first Jupiter continued to collapse. Within the first ten million years of its life, the planet was reduced to nearly its present size, with only a few percent additional shrinkage during the past 4.5 billion years. The luminosity also dropped as internal heat was carried to the surface by convection and radiated away to space. After a million years Jupiter emitted only one-hundred thousandth as much radiation as the Sun, and today its luminosity is only one-ten billionth of the Sun’s.
Jupiter’s internal energy, although small by stellar standards, has important effects on the planet. About 10¹⁷ watts of power, comparable to that received by Jupiter from the Sun, reach the surface from the still-luminous interior. The central temperature is still thought to be about 30 000 K, sufficient to maintain the interior in a molten state. Scientists generally agree that Jupiter is an entirely fluid planet, with no solid core whatever.