N=R*fp*ne*fl*fi*fe*L
Most of these numbers are Unknown so it makes the entire equation impossible to churn out an answer. And there are a few flaws in the equation. The equation which you can get on Wikipedia. R is the rate of new star formation per year, fp is the fraction that have planets, ne is the average number of plants that could habour life per star with planets, fl the fraction of planets that could have life that develop life, fi is is the fraction of planets that develop life that develop civilizations, fe is the fraction of those that learn broadcast their intelligence off their planet and L is the lifespan of that civilization before they destroy themselves or die out.
Most of these numbers are not known, but a few of them are known and can provide numbers, like how many stars are born every year in our galaxy can be estimated. Thanks to Kepler, the planet finding telescope, we have an idea of how plentiful planets are. From the various plant surveys we have an idea of how many may support life. But the rest is still speculation. We have not discovered life or evidence for life on Mars yet so of the three planets in the habitable zone of our sun, only one has life, but it is theoretically possible on Mars as well as two or more of the moons of Jupiter and Saturn.
There are problems with this equation. Like a few Years ago, North America and bits of Europe went dark according to the Drake Equation. We stopped broadcasting most our television signals thought the air and began exclusively sending them through wires. It is more energy efficient to do it that way and so it costs less, but if you were watching from really far away and did not understand the warnings, you might assume there was a global war and most of the people were wiped out. If there were technological species out in the galaxy, how long before their civilizations did what we did? Not long. They might not have ever broadcast their communications through the air.
That is one problem. Another problem is that the basis of the equation is the star formation rate in the galaxy. The problem with this is that all stars could develop planets, in the habitable zone and could start life, but not all the stars would last that long. The large stars would burnout and possibly explode before life could get a foothold, other stars could allow life to begin before snuffing it out. The oldest fossils on Earth are from the first billion years, so if the star lasted less than a billion years, life may not have even formed. It took another three billion years for the explosion of life forms that we see to day to begin, so we could say that complex life would require a star to last four billion years for complex life to evolve and another five hundred million to develop intelligence, well our level of intelligence. So assuming we were fast or slow or just average should we require a star to last at least five billion years.
There are other necessities for life. A magnetic field is required unless life can only exist beneath the waves. For a magnetic field you require a couple other things, lots of iron for one. Our magnetic field is caused by the rotation of a liquid metal core, Jupiter has a magnetic core caused by the rotation of its liquid metallic hydrogen core. Mercury has a ver weak magnetic field, cause by the slow rotation of its solid metal core. Mars does not have a magnetic field, its core went solid millions of years ago and if there was life there, it died. So although there are trace metals in out bodies, we rely on planets formed from metal rich surroundings, so planets with around metal poor stars are poor candidates for life and therefore intelligent life. So we should ignore all the stars that are too old too.
The problem that I thought of was one that my astronomy teacher brought up twenty or so years ago. He stated that living near a supernova is really bad for your life. He stated that the force of a supernova could strip an atmosphere at 100 light years distant. I did some quick calculations based on a few known facts, 1) the size of the galaxy, 2) the rate of supernovas in our galaxy and 3) the volume of space affected assuming 100 LY radius of distruction. Supernovae occur about 2.5 per century. Over 500 million years that is a lot, about 125 million. The size of the galaxy in cubic light years is very, very huge, but so is a sphere with a radius of 100 LY. Divide the two numbers and you get how many supernovae would have and divide the supernovas by that number and you get the number of supernovae that occurred within 100 LY of any given location assuming even dispersion of supernovae. And the number is… 152!!
So I checked the number of 100 LY destruction and my professor may have been using hyperbole to create impact. Some scientists suggest that you need to be within 33 LY of the supernovae, so 1/3 the radius = 1/27th the volume or about 6 supernovae within that distance over 500 million year for any given point. Some suggest within 10 LY will destroy a planet, plug in those numbers and you get 15% chance that a supernova was within 10 LY of any given point. This assumes that there star densities are equal all through the galaxy; it isn't. There are more stars in the core than where we are, 500 times as many stars per given volume. So life most likely would not evolve in the center region of our galaxy and depending on how close is too close, supernovae might have rebooted the process on any given planet.
Dwarf stars, of which our Sun is one, but I mean the Red Dwarfs, the truly small stars. They have a very narrow habitable zone, because they do not produce a lot of light and therefore the planets are cooler further away and because they are smaller, the stars are less stable early in life, for several billion years they have massive coronal mass ejections and flares that could potentially wipe out potential life. After that time period, life is good around those stars for much longer than life would be around our Sun, the star would last up to ten times as long, but the magnetic field of a planet only lasts as long as the core remains liquid and the rotation rate is relatively fast — the closer to a solar mass the faster a planet will become tidally locked, like Mercury and the closer moons of Jupiter and Saturn, as well as our own moon.
So how about a new Drake Equation?
N the number of planets with life and therefore potential intelligent life
I the fraction of stars with more than the minimum metal required
M the fraction of stars between 1.25 and 0.5 solar masses
A the fraction of stars older than 4 billion years
S the chance of cosmic disaster (proximate supernovae etc.) this excludes core stars as the risk becomes too high
P the percentage chance of moons and rocky planets in the habitable zone between 0.7 and 5 Earth masses
L the chance that life will evolve, could be 100% could be less.
N=I*M*A*S*P*L
If life starts, I would think it would start to evolve. If it evolves some sort of photosynthesis would evolve, oxygen using life forms can get bigger than non-oxygen using life. The reason why I used 4 billion years old stars was to allow for the emergence of oxygen using organisms.
The size of the star, is important too. The bigger a star is, the shorter its life, but also the sooner that it grows into its next stage. What I mean is that when our sun first started up, it was not as bright as it is now and not as big either. As it aged, it got bigger and its luminosity increased. Larger stars get bigger faster and might get bigger faster than early life could evolve and that would be bad. Smaller stars have the other problem already mentioned, increased instability. The range I picked is not based on science, it is just an arbitrary number, but a good first start.
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