.. gets more complicated. Two-thirds of solar type stars (M class) in our Milky Way galaxy are members of binary or multiple star systems. When two stars are orbiting close together, their planets orbit both stars. When the stars are far apart, their planets only orbit one of them.
Some problems with this situation include: Planets may not be able to form unless the stars are at least 50 AU away (1 AU = distance from the Earth to the sun) and stable orbits can only be achieved where companion stars are at less than 20 million miles apart or farther than one billion miles.3 A planet’s orbit pattern is also of concern. Earth’s orbit is very stable and only has a small degree of ellipticity. A highly elliptical orbit would cause a planet to oscillate in and out of a habitable zone. If a planet could even form in this situation, their orbits would be perturbed by varying gravity of more than one star causing ejection or falling into one of the stars. Another factor in considering the habitable zone is insolation.
Insolation is the stellar energy a planet receives. This quantity could only vary by as much as ten percent without affecting its habitability. Much less than ten percent fluctuations on Earth is what causes our climate changes during seasons. Furthermore, the insolation effect would be magnified in a binary star system due to periodic eclipse of one of the stars. Rather than focus on individual star-planet distances in reference to their habitable zones, let us choose the entire Milky Way as a basis.
The diameter of our galaxy is about 85000 light-years across. Our sun is located 25000 light-years from the center of the galaxy. Our solar system is located in a region where star density is low, which is indicative of the habitable zone of the Milky Way. Places further toward the center of the galaxy are too densely packed with stars to be in the HZ. The outer zone has a different problem: the wrong type of matter for Earth-like planets exists there, the concentration of heavy elements and rate of new star formation is too low.
Even the shape of the Milky Way is important. Our spiral shape is much preferred to elliptical since elliptical galaxies typically contain no heavy elements, little dust, little new stars, and an abundance of asteroids and comets. It has also been hypothesized that a habitable time zone exists for the creation of Earth-like systems. For two billion years after the Big Bang, carbon, oxygen, phosphorous, potassium, sodium, iron, copper, as well as uranium were not present in the universe. These elements are required (among a few others) for organic life.
Only after this time period were supernovae explosions able to produce heavy elements up to uranium. Stars forming now have fewer radioisotopes than the sun did when it formed 4.6 billion years ago. If a planet were to form around a star with fewer amounts of these isotopes, the planet’s core would not have enough radioactive heat to drive plate tectonics. Also, galaxies 30-40% older than ours seem to have more instances of being irregularly shaped, and therefore not able to contain an Earth-like system. To expand the habitable zone to a broader category, consider the entire universe. Statistically the universe is either too cold or too hot, too dense or too vacuous, too dark or too bright, or contain too little heavy elements to support Earth-like planets.
However, even though time and statistics my prove otherwise, I adamantly disagree that these findings lead to Earth being totally unique and the entire universe being devoid of intelligent life. The vast, almost incomprehensible size of the universe leads me to believe that the information that we know at this time is astronomically smaller than what we don’t know about its properties. For example, in 1961 there was a now renown conference held at the National Radio Astronomy Observatory in Green Bank, West Virginia, to discuss the question of a ‘search for extraterrestrial life’ (SETI). That gathering brought together a worldwide array of prominent astronomers and exobiologists. The conference set out with the intention of attempting to quantify, by theoretical means, the number of technically advanced extraterrestrial intelligence civilizations within the Milky Way galaxy.
The solution was an equation, now known as the Green Bank equation, though also widely referred to as the ‘Drake equation’ after Frank Drake the astronomer who proposed the core of the expression. The equation seeks to quantify the number, N which is the number of technical civilizations in our galaxy.4 The equation is as follows N = R fp np fl fi fc L where: R = mean rate of star formation in the Milky Way fp = the fraction of those stars which form planetary systems np = the number of planets in those systems which are ecologically suitable for life forms to evolve fl = the number of those planets on which life forms do actually develop fi = the number of those life forms which evolve to an intelligent form fc = the number of advanced intelligent life forms which develop the capability of interstellar radio communication L = the lifetime of those advanced technically advanced civilizations Values for most of these factors are far from being certain, but some have been estimated. In fact, only the value of L has been altered to any significant degree since the SETI conference in 1961. The estimations include: R = 10/year fp = 0.5 np = 2 fl = 1 fi fc = 0.01 L = 34 So even if these estimates vary, the number N of technical civilizations only present in our galaxy equals 3.4. If technically advanced civilizations were to exist and have lifetimes of a few thousand years then a galactic community appears a distinct possibility.
Other researchers and working groups, for example Sagan5 , have examined the question and concluded there could be 106 technologically advanced extraterrestrial civilizations in the galaxy. That’s 106 civilizations in our relatively small galaxy alone! Hubble’s Deep Field (taken near Ursa Major in the Big Dipper constellation) has found more than 1500 individual galaxies. For just this tiny region of space (about 2.6 (arc-min)2), N would equal 159,000 civilizations. In addition to these encouraging calculations, science has now proven that life could evolve on a basis other than DNA. Furthermore, life does not have to be carbon based. Life may be derived from other elements such as silicon.
Hydro-silicon molecules are possible, but since the silicon atom is much heavier than the carbon atom, the atomic bonds for silicon are only stable at much lower temperatures than for carbon (on the order of -150 F). So hydro-silicon compounds might form, for example, on the moons of the Jovian worlds where the temperatures are low.6 As of yet, the Earth has not experienced any contact from these other intelligent beings. I agree that this is a fact that does not support my ideas of the existence of such beings. However, I believe that it is possible that life as we know it may not be advanced enough, technologically speaking, to have the ability to contact extraterrestrials. Or, we presently may have the technology, but since humans have only had the ability to send detectible waves (like radio) outward from the planet for the last 100 years or so, and taking into account the great distances these waves have to travel, it is quite possible that nothing intelligent has heard us yet.
In my opinion, if indeed we are alone in this universe, wouldn’t it just be a terrible waste of space? Science.