Friday, November 7, 2008

Jupiter--Earth's Sentry

When I observe bright planet Jupiter beaming its beauty in Earth’s night skies, I’m reminded of the story told to my earth science students each October when they assembled for their annual “star watch.” Each year I invited “Pastor Pete,” an avid student of astronomy, to begin the night session in the gym with slides and some appropriate astronomy tales. One of his favorites (and mine) was his description of Jupiter as God’s provision to protect earth from the dangerous comets and asteroids which periodically approach Earth and threaten to impact us. Jupiter’s strong gravity, generated by more than twice the mass of all the other solar system planets combined, either sweeps the comets toward itself, or, more likely, deflects them harmlessly out of our solar system. Without Jupiter, we would be impacted by these comets and asteroids more than 1000 times more frequently.

The many characteristics of our solar system, and in particular, our earth, set us uniquely apart from any of the several hundred extra-solar planets discovered around neighboring stars. So far, we are able to observe only planets in our galaxy. Our home galaxy, the Milky Way, is 100,000 light years wide and contains about 100 billion stars. There are hundreds of billions of galaxies out there in the distant reaches of the universe. It has been estimated that our universe may contain as many as 10 billion trillion planets. But unlike Carl Sagan’s judgment that there may be up to a million civilizations like earth’s in our galaxy alone, it appears that earth and its life may be unique. The chances that every one of the several hundred known necessary life-enabling parameters present on Earth and in our neighborhood could exist anywhere else in this universe is incomprehensibly remote--essentially zero. Each and every parameter must be present, some fine-tuned to an incredible degree.

The remote placement of planetary giant Jupiter in our solar system is unusual and unexpected, as is its nearly circular orbit. A highly elliptical Jupiter orbit would throw inner planets like Earth into chaotic orbits and preclude life. Earth life depends on an amazing array of “just right” characteristics, ranging from a just right atmosphere, to the presence of plentiful water, to the occurrence of a narrow temperature range to keep most of earth’s water in liquid form. Apart from earth's necessary life-friendly characteristics, the entire universe must also be fine-tuned to an unimaginable degree to make life on earth possible. In the past few decades scientists have debated the “anthropic principle,” an idea that the many characteristics of the universe seem to have been deliberately set in place to support the existence of man. Other versions of the anthropic principle are currently being debated.

I could only hope my former students were fascinated by the role of Jupiter as a sentry, similar to the friendly neighborhood crossing guard whose role is to stop traffic for the safety of the street-crossers. We could also hope that, later in the evening, their telescopic view of Jupiter through Pastor Pete’s telescope, complete with its cloud bands and four tiny moons, piqued their curiosity about the wonders of our solar system, our galaxy, and our universe.