In our search for life beyond Earth, it is only natural to begin within our own Solar System. The closest star system to ours is some four light years away. Even if we had the technology to travel at the speed of light, it would still be cumbersome to begin our search beyond the boundary of the Kuiper belt. Nevertheless, the worlds contained within our Solar System offer us much to explore and ponder. Due to their similarity to Earth, many have focused on the other three terrestrial planets as possibilities for extant life. Until recently, few have considered life on the other planets, the gas giants.
If we travel beyond the asteroid belt, the first planet we encounter is the king of all the planets, Jupiter. Just one of several gas giant planets, Jupiter has been the center of much speculation about life beyond Earth. In general, the physical conditions on the gas giant planets seem pretty unlikely to have formed life. Jupiter, for example, is composed primarily of the same things as the Sun, hydrogen and helium. There are small amounts of sulfur, ammonia, oxygen, and water. Furthermore, the temperatures and pressures on Jupiter are very extreme. The temperature can get as high as 10,000 degrees! In addition, the pressures can get up to three million times as the pressure on Earth at sea level. Jupiter does not have a solid surface, either. If you were to parachute through the atmosphere of Jupiter and try and land on the surface, you would find yourself descending through hydrogen and helium gas that would increase in pressure as you descended. Eventually pressures would be so great that your body would be crushed. Deep inside of Jupiter the hydrogen and helium cease to be gases and, due to the high pressures, become liquid instead. With such extreme conditions, and no solid surface to serve as a cradle of life, it seems impossible that life could have started on a gas giant planet.
Carl Sagan and Ed Salpeter at Cornell University conjectured that organic chemicals in Jupiter's atmosphere could have reacted and eventually produced aerial, floating life forms in the lower atmosphere. These life forms would be like little gas bags, adjusting their own air pressure as they waft through Jupiter's turbulent atmosphere. Critics of this theory contend that the updrafts and down drafts in the atmosphere would be so extreme that they could carry life forms and nutrients to levels that would be too hot or too cold to allow for life. (It’s possible they were being tongue in cheek, even though the paper was published in a reputable journal.) In fact, early results from a parachute probe into Jupiter suggested that there were far fewer organic molecules than originally expected, causing some to question the ability for life to have formed at all. As for Saturn, Uranus, and Neptune, there is too little information to know. However, with the Cassini-Huygens Mission, we have gained a wealth of new information about Saturn and its moons.
The pessimism about life on gas giant planets is balanced by the consciousness that we still know very little about the complete range of physical conditions that can support life processes on Earth, let alone extraterrestrial environments. At times it might be easy or just convenient to assume that life requires an Earth-like environment with liquid water, oxygen, carbon, and moderate temperatures. But with no general theory of biology and no sense of the physical bounds on biology, we just cannot be sure. Our ongoing exploration of Earth has revealed some extremely inhospitable environments that indeed have very robust life forms! Consequently, we should not limit our conjectures about life to terrestrial examples.
Recently, there has been talk about the possibility for life beyond the asteroid belt, but not on gas giants. With our increasing capacity to study planetary bodies with greater precision, interest has increased with regard to the moons of gas giant planets. In particular, Io and Europa, moons of Jupiter, and Titan, a moon of Saturn, have taken lead roles in the field of astrobiology. Each of these moons has very unique conditions that in one way or another may contribute to our understanding of the origins and evolution of life in the universe.