Some of the oldest stars in the Universe, those that formed just after the Big Bang, are still happily burning away. And I don’t mean that they’ve long since been snuffed out and their light merely continues to travel towards us; I mean they’re actually still burning right now. Cool red stars can have a life expectancy of a trillion years. Small hot stars can burn out in just a few million.
You see, the Universe is only about 14½ billion years old, and many stars will last much longer than that. Our own Sun has been burning for 4½ billion years, and although it has grown about 11% in diameter since it was born, it has another five billion years ahead of it without any significant changes. And it is a rather unremarkable, main-sequence white star; though it is brighter than most other stars (66-75% of all stars are dimmer than good old Sol). There are lots of others that are cooler and bigger (but not too big) that have enough fuel to go on for many times the life expectancy of our pet nuclear furnace.
Of the 70 sextillion (7×1022) stars that currently exist in the universe as we know it, the vast majority (except the very first to form out of pure hydrogen before any heavier elements had yet been created) will have planets. To put that in perspective, that number is 22,000,000,000,000 (22 trillion) times the U.S. National budget, which currently stands at $3,500,000,000,000 (3½ trillion). It’s an incomprehensibly large number.
Maybe you can get a grasp on the scale if I say that there are 7.5×1018 grains of sand on our planet. And, if you could imagine every single one of them, there would still be 10,000 times more stars in the Universe.
Why is this important?
What it all means is that according to our current theories of stellar evolution, virtually every star has planets. The few “pristine” stars that still exist in our galaxy (meaning more than 13 billion years old and composed primarily of hydrogen) are few and far between. That leaves us more than 100 billion (up to 400 billion, depending on the estimate) candidate-stars just in our galaxy. If only one in 10,000 stars had an Earth-like planet, that would still leave over one million candidate-stars that might have oxygen based, water-dependent life-as-we-know-it. Those odds are simply too big to ignore.
Why do we think there is life out there?
I’m sure part of the explanation is that the Universe is so vast and we simply don’t want to be alone. On the other hand, our experience shows us that wherever it is possible for life to exist, it does.
We have seen crab-like creatures and tube worms living lightless, miles beneath the ocean’s surface, in otherwise poisonous clouds of volcanic outgassing on the seabed, at near fatal temperatures; we’ve discovered microscopic bacteria living under the top couple of millimeters of stones in the Arctic, using minute quantities of sunlight to digest the rock; or other colonies living on the bottom layer of a sheet of ice and making their living photosynthetically. Life is pervasive.
…and then there is the Math
Frank Drake (b. 1930- ) is a radio astronomer and astrophysicist who vehemently believes that the universe is plenty large enough to contain other living beings. He went so far as to arrange a 1961 conference amongst his peers which he called SETI, or the search for extraterrestrial intelligence, and in order to stimulate discussion, created an equation to postulate the likelihood of extraterrestrial life in the Universe. Of course it was merely a tool, and not an attempt to prove that alien life existed.
The formula looks like this:
N = R* ∙ ƒP ∙ ne ∙ ƒl ∙ ƒi ∙ ƒc ∙ L where:
- N = number of civilizations in the Milky Way
- R* = rate per year of stellar formation where Earth-like planets could exist
- ƒP = number of stars with planets
- ne = number of potential life-bearing moons or planets per star
- ƒl = is the number of planets that develop life
- ƒi = those planets with intelligent life
- ƒc = civilisations revealing technological signatures
- L = the duration of technological signatures we could recognize.
Obviously the equation contains a lot of assumptive values, and errors in any of them would seriously jeopardize the results of the equation. At best it might help steer one’s reasoning in a logical direction; the equation itself would be in no way useful for discovering non-radio (primitive) civilizations, or life forms.
To find N we can plug in some “knowns” based on astronomical knowledge and experience within our own Solar System. For example, every year the Milky Way galaxy creates seven new stars on average, so R* = 7. We can also say that virtually every star has planets, so ƒP =1. Using our own Solar System as an example we can say that ne = 5, since we have the Earth plus Mars, and the moons Ganymede, Titan, and Enceladus as potential life-bearing bodies with liquid water or sub-surface oceans. Our own experience here on Earth suggests that life always develops and if there’s any possibility so ƒl might be 100% or “1” as well.
The equation might look like this then: N = 7 ∙ 1 ∙ 5 ∙ 1 ∙ ƒi ∙ ƒc ∙ L… Ah, those last three elements! They may very well remain intractable for our lifetimes. We have no way of knowing the fraction of planets that will develop intelligent life (ƒi), if any of those civilizations will reveal technological signatures (ƒc), or how long each of those civilizations will endure (L).
Like Mr. Drake however, I prefer to remain optimistic. His ambition to explore this question now permits us say with certainty that our galaxy is not teaming with easily detectable radio signals. This is something we couldn’t say 55 years ago when he started this adventure.
Our technology and capability has steadily improved in all this time. We now have more computational ability in our mobile phones than they had in their most massive computers in the 1960s. The SETI@Home project allows all sorts of individual computer users to donate computational power to the project so that we can analyze all that data. If an answer is to be had, we are now closer to it than we have ever been. Click that link; download the software; join the fun! You may go down in history as a co-discoverer of the first alien signal from another civilization. Do it!