The Fermi Paradox - Rare Earth Hypothesis Part 1

Well it seems my first thread on the Fermi Paradox generated some interest. Not sure if it's because people here on ATS want to talk about it, or just need a break from all the politics. In any case, it's good to see.

This thread will concentrate on one of the "solutions" to the Fermi Paradox. In this case the Rare Earth Hypothesis.

For those who do not know what the Fermi Paradox is, you can click on this link here to the intro thread and read the OP for it.

Before we get into this solution, I would like to make sure all understand: The Fermi Paradox is nothing more than a question. The original question is "Where are they?", which is basically asking: If the universe is so vast with so many planets and stars, where are all the aliens at?"

It's not something that tries to prove that intelligent alien life does not exist. It's really nothing more than a question everyone should ask, especially if you do believe that intelligent life is out there: Where are they?

Also: all opinions are welcome here. If you disagree with someone, that's fine, just make sure you all keep it civil. I'm sure we can debate things without the need to get nasty with each other. If you feel the urge to be that way, please go to the Political Mud Pit forum. Keep it out of here. If you can't convince each other, simply agree to disagree. Considering how speculative much of this can be, there's a good chance of not being able to prove someone wrong anyways.

So let's dig in!





The Rare Earth Solution, Part 1.

Basically this says that the reason we do not see a bunch of aliens flying around, sending out signals from their home worlds or stopping by Earth for a pizza, is because "Earth Like" planets are rare.

Now, there's two parts to this: Earth Like planets that can support life as we know it, and Earth Like planets that developed intelligent life.

This thread is going to cover the first part. After all, for a tool using intelligent race, we most likely need a "Earth Like" planet. Having some strange alien creatures that evolved inside a gas giant or on a world completely covered with oceans where they can't develope fire for smelting metal, are more than likely going to be disappointing for our tool using advanced civilization that can broadcast signals or fly out into space.

Before we go into "Earth Like" planets, we need to take a look at a couple of things first. That is our galaxy and stars.

Our galaxy, the Milky Way, has somewhere between 100 billion to 400 billion stars in it. However, every galaxy has what we call a habitable zone also, just like each star has one. For stars the zone is based upon temperatures: too far out you're too cold. Too close in and you're too hot.



For galaxies the way it works is: Too far out, you have much older metal poor stars. Too close to the core and you're having to deal with very large amounts of radiation and too many stars close together, where stars going nova or super nova will effect many star systems.

Okay, so we don't want to be too close to the core of a galaxy....what's this about "metal poor" stars?

Well, stars have several classifications, most of which deal with their size, temperature, life span, etc. However there is a classification for their age too. We basically put them in three categories:

"Population 1, 2 and 3" stars.

Population 3 stars are the oldest stars. These are stars that formed closer to the beginning of the universe. When we look at their spectrum, we find that they have hardly any heavier elements like metal in them. This is hardly surprising since it was the stars themselves that forged our heavier elements.

Population 2 stars are old stars that formed after Population 3 stars, but have a little bit more metal in them...but are still very low in that content.

Population 3 stars are the newer stars. Our sun is considered one of these, and have a much higher metal content.

So why is this important? It's important because the planets around a star are normally made up of the same thing as the star they orbit. Which means planets around Population 3 and 2 stars are either not going to be rocky, or if rocky will be very, very low in heavier elements like metal.

The lack of these metals means that even if alien life that becomes intelligent develops on these planets, they will not have the heavier elements available to them to build technologies, especially anything that can go out into space.

So having a star in "just the right place" is important, and because of this, it lowers the number of stars in a galaxy where intelligent life as we know it might exist.



Stellar Classification.



Yes, that's right. The stars. You can have a world that's an exact duplicate of our Earth, but if it's around the wrong star, then we have a problem.

Let's look at our star. Obviously it's been a great star for Earth to have since we live on a planet teaming with a large diversity of life.
First, let us look at the classification of stars. Astronomers use letters to classify them. In order they are M, K, G, F, B, and O.

There is a further classification with a number next to that letter. You can have a G type star, but they range from G0 to G9. Those numbers are a bit less important than the letters for the purpose of our discussion here.

The reason the star is so important is because of how a star behaves, how long it lasts. Those are two very, very important factors.
For example, M type stars are red in color and have low surface temps and long lifespans. O type stars are blue and are very, very hot and tend to have very short lifespans.

When we look at the population of star types, we find that there are many, many more M, K and G type stars than there are of the others. The most populus stars that we know of are Red Dwarfs. The reason this is important is because in general, the hotter the star, the shorter it's life span.

Small red dwar stars have life spans measured in hundreds of billions of years. In fact: none of these stars have died yet. The universe itself is too young for them to of used up all their fuel.
Massive blue super giants have life spans less than 100 million years. From what we know of our sun and Earth, it took several billion years for life to forum and for tool using humans to come about.

If your star has a life span of less than 100 million years, it means any planets around it have no chance and developing life as we know it long enough to evolve into a advanced tool using species. From what we know of life here on Earth, that process took billions of years.

The other thing we need is a stable star. Having a star that has a bad temper (throws off massive amounts of itself through flares and CMEs) is not a good thing to have. The sun is a very stable star for the most part.

So, what we are seeing is:

1) We need a star to be in the right place in the galaxy and staying there.
2) Stars that are high in metals like Population 3 stars.
3) Stars that last long enough and are stable enough for life on planets around it to evolve.



Continued in the next post:

Next: we need planets.....we need rocky planets. Just a few decades ago, we were not even sure if there were planets around any other stars. Since that time, we have discovered thousands of them. In fact: it's more common for a star to have planets than for stars to not have planets. So that is good news.

However, we need to have some rocky planets if we hope to find that "Earth Like" planet.

Why? Why can't it be a gas giant or water world? Why not a frozen moon around a gas giant with a ocean underneath miles thick ice?

Well, it may certainly be possible for some sort of life to develope in these places. In fact, planetary scientist think it might even be possible for air born bacteria to exist in certain places in the atmosphere of Venus, and I can't think of a more hostile planet for life to try and develop on!

But, from what we know of our own planet, if you want a tool using species that is going to advance in technology, you're going to need a rocky planet that has metals. Being able to harness fire to smelt metals and build tools was a crucial step for our species. We know dolphins here on Earth are very intelligent.....but being stuck in water, they'll never be able to harness fire and build tools that way. Neither could some blimp like creature that say evolved and lives in the upper cloud deck of a Jupiter like world.



Now, the next thing we need is for one of those rocky worlds to be in the Habital Zone, or that Green Zone. This is an area around a star that if a planet orbits inside that area, and it has both an atmosphere and water, that water will be able to stay a liquid.

Why does it have to be water? Well, again, we are having to base what we know about life here on Earth. We know that water here on Earth played an important role for life to develop. We also know that water is a very, very common thing in the universe. We see it everywhere.....because oxygen likes to bind with hydrogen. In fact, the hydrogen and oxygen that make up water, are part of the 4 most common elements in the universe: CHON or Carbon, Hydrogen, Oxygen and Nitrogen.

CHON pretty much describes what you, me, other people, animals, insects, and plants are all made of. So it stands to reason: if we, living organisms that are also intelligent tool using beings are made of CHON, it's a very good chance that is what will make up other life out there in the universe. Doesn't mean it HAS to be that way. It's just that reasoning says it increases the chances of life being out there.

Anything else?

Oh yes.....plenty of other things. First, let's talk about that green zone and the rocky planet:



That rocky planet can not have a eccentric orbit that takes it out of the green zone. It's orbit needs to be as circular as possible to where it spends all it's time in the green zone. If instead it has an orbit where it spends half it's time outside of that green zone, we've got a big problem, the water either boils away or freezes solid (along with parts of the atmosphere in some cases).



We also need the planet to spin on an axis that doesn't swing around madly. Or rather this would be a very ideal thing. Having a planet who's spin axis move around all over the place will cause dramatic climate change all over the planet. If this happens, it could inhibit life getting started, or developing into anything considered advanced, as the life that develops there may not be able to adjust to climate change that happens too fast and too extreme.

But....Earth! Earth has had all sorts of climatic changes! Ice Ages!

This is true. But most of those changes took millions of years to happen. A spin axis that moves around and changes could happen in only 10,000 years, which is a eye blink on the evolutionary scale.

Size: we also do not want the planet too big or too small.



Too big or too massive and it's gravitational acceleration becomes so great that it becomes impossible to conventional chemical rockets to get anything in orbit and inventing chemical rockets for space flight would be an important step. However, because life there could still become intelligent and tool using, I won't count this. They might not figure out a way to leave their planet, but they may still develope technology that can send signals out into space.

Too small: I do have to count this one though. Too small and you have a couple of issues. One is that if it's too small, it may not be able to retain it's atmosphere. Loose the air, you'll loose any surface water. The other thing is the planet's core. It will cool too quickly and from what we understand about the Earth magnetic field that protects it from both the sun's radiation and cosmic radiation, and core that cools off too quickly would make that magnetic field go away. That would be bad for any life trying to evolve on that planet.

An oversized moon: Not really going to go into depth with this because the ideas around this are changing. For the longest time it's been thought that a planet more than likely needed a moon to help keep it anchored and help generate tides. However recently it's being more thought that this is not as critical as we have thought in the past. One could argue that it is important for an alien civilization to have a moon as an important step to achieve space flight because getting to your moon is a much easier first step than to get to another planet. However, again, you could have a technilogically advanced alien species that doesn't go out into space.

There are a few other things, like outside interference: a nearby star going super nova, passing black hole or stellar mass, asteroid or planetary collisions. However we'll skip these since they fall into another category, the universe trying to kill you off.



So, we have our list now of things to give chance for life to develope and to develope into an intelligent tool using species:

1) Must be in the right place in the galaxy.
2) Must be a metal rich star
3) Must be a long lived stable star.
4) Must have a rocky planet.
5) Must have a rocky planet that stays in the star's hability zone all the time.
6) Planet must have a pretty stable spin axis
7) Planet must not be too small so it keeps it air and magnetic field.

Okay, so what does that mean?

Well, the next step is to place probabilities on each of these things. Those probabilities are based on what we know about our galaxy, stars and the planets we have found out there. So, in essence, it actuall can come down to opinion. You can be quite generous with your statistics and odds, or very strict.

We won't go into the math for this here in this thread, as it is something you can always look up if you're interested in it. However here is what it break down to:

Generous probabilites makes it at 100 to 1000 "Earth Like" worlds in our galaxy alone that could support life as we know it and possibly develope into intelligent life that is tool using like us or more advanced.

The very strict version of those probabilites put the odds at 1 to 2.5 million.......which is not even 1 planet in every galaxy. Yikes!

I personally tend to lean toward the generous probabilites. Maybe it's because I try to be optimistic all the time, maybe it's because I like the idea of hundreds if not thousands of worlds in our galaxy that can harbor life as we know it.



Continued in the next post:

Okay, that concludes part one of the Rare Earth Hypothesis, which mainly dealt with how rare is having a "Earth Like" planet.

Part two, which will be a different thread, deals with intelligence. You have worlds with life on them....but how often does that life become intelligent enough to start using tools and advance in technology? I'm not sure if that will be the very next thread. It's kind of fun to jump around with this subject. Some of my favorites are The Universe Is Out To Get Everyone, and the Communications Problem.

That's it for this thread. Have a great day, and hopefully you won't be pulled over by the Judoon of the Shadow Proclimation....

a reply to: eriktheawful

Outstanding again.

As I stated in your other thread I lean towards this hypothesis myself.

The metallicity of stars would preclude, in my opinion, life from forming in older systems and the requirement of having to be in the habitable zone, along with the my belief that intelligence is not some sort of end game for evolution makes me gravitate towards this 'modified' rare earth stance.

The surest sign that intelligent life exists elsewhere in the universe is that it has never tried to contact us.
Bill Watterson

I believe alien life is quite common in the universe, although intelligent life is less so. Some say it has yet to appear on planet Earth.
Stephen Hawking

Intelligent Life Quotes

Drake's equation was only meant as an example formula of factoring the chance of intelligent life in the universe.
Today , it has been modernized to include as many factors as science thinks logical
We may very well be alone
Lets not waste it.

a reply to: eriktheawful

I am at work right now getting payroll ready but as soon as I get home I'm diving into this. Threads like this are the reason I have read ATS for so long! Thank you and great job, keep them coming please!

a reply to: eriktheawful

Everyone is wrong..

Given the endless size of the universe / multiverse

It is like a fish describing the taste of red wine.

a reply to: Gothmog

We may very well be alone Lets not waste it.

Just in the past June issue of Astronomy magazine, which I love by the way, Scientists now think there could be as many as 2 Trillion galaxies in the known universe. I don't think there is anything uique about our earth at all, and am firmly in the camp of we will find many earth type planets out there, in fact we seem to be, and some will have intelligent life on them. May not be life as we know it either.. Probably won't be to close, but I feel its just a matter of time...before we have found it.

a reply to: eriktheawful

I think it really just comes down to time and spacez..

We have to be in the right place in space, AT THE RIGHT TIME..

And with as much time and space as are on the table.. it's a needle in a monumental haystack.

a reply to: eriktheawful

Damn, this series is going to be really good. This is a lot of work, so thanks for taking the time to put these together.

This is what ATS needs.

Just flagging for a later read.

Glad y'all are enjoying it.

I won't be posting a lot today as my wife and son who've been away for a week are coming back home today, then I'll be with them at some friends.

But I will check in later tonight.

Do you think that the habitable planet must have rotation like our 24 hours? This would keep it from burning up on one side and freezing in the shadow.

To get around that maybe an intelligence would go underground on a planet that does not rotate?

Also, should there be some radiation filtered through the atmosphere to begin mutations? I think that is one possibility of human development, to have had a mutation in our distant past that gave us the beginning spark of intelligence. (some may dispute that we are intelligent).

Man, this one of the best series of posts ever on ATS!

a reply to: eriktheawful
No 7 - Statistics, the universe is massive

Lets assume we can travel between solar systems in 1 day using a FTL drive. We explore the solar system for life and take 1 day. Now even if there are a million civilisations in our galaxy the odds on stumbling on one in our FTL ship before the crew dies of old age is very small. The odds are 1 in 400,000 per jump with 7000 jumps in a lifetime of travelling.

So there is no way of touring successfully. The only way an alien could make itself known to another alien civilisation is they purposely went there. This means knowing there is somebody to visit. This means getting a signal that has travelled faster then light................... ahhhhhhh.

I would suspect if there is any method of FTL comms it would require a purpose built transmitter and receiver.....ahhhh.

So its not a case of nobody out there its a case of millions of needles in an earth sized haystack.

a reply to: NightFlight

That's something that we'll talk about in part two of the Rare Earth Hypothesis.

A planet's rotation period is pretty important to life as we know it here on Earth.

Imagine a tidally locked Earth with the sun: one side always facing it, the other side in constant night. You can imagine what the climate would be like at those two points, pretty extreme. Most scientist think that you'd have only a narrow strip along the terminus where you could have life that could survive.

Smaller space for life, means less life. Less life means less diversity, and life that evolves on a planet like that might have issues being able to evolve into intelligence.

A planet with a very fast rotation might have issues too with extreme weather and high winds, which could have a dramatic effect on how things evolve.

Does it have to be a 24 hour period? I'd say no.....a much larger Earth like planet, a 24 hour period would be quite a fast rotation.

If we look at our own solar system, only Earth and Mars have that close to 24 hour period.

The huge planets in our system rotate very fast:

Neptune - 16 hours
Uranus - 17 hours
Saturn - 11 hours
Jupiter - 10 hours

While the other rocky planets are slow:

Mercury - 60 days
Venus - 119 days

a reply to: yorkshirelad

Most likely a better post when we get to the Communications and Contact thread.

This thread was looking at how rare or not rare Earth Like planets are.

Loving the thread. I am curious though. Even you mentioned the line," A planets rotation is pretty important to life as we know it."

Isn't that where the key would make or could make the difference? "Life as WE know it." I mean aren't we also just one of all the equations? I would think the possibility of Life we don't know would be greater than what we do know wouldn't it?

I assume this theory is from the standpoint of we evolved from a single cell basically?

originally posted by: eriktheawful
The original question is "Where are they?", which is basically asking: If the universe is so vast with so many planets and stars, where are all the aliens at?"

The speed of light is part of the issue. I couldn't see myself jetting off to some planet 12 light years away to explore it ... just to find out it's about as interesting as Mars ... and then jetting back for however X-many-years it takes for the return trip.

The signal issue is also pretty simple. It takes something like the power of a star to get an intact signal across the Vast Vast reaches of space. When I listen to the 'smart guys' talk about this, I believe them when they say, "We're about 10,000 years away from that capability ... and there's no telling what we, ourselves, will sound like after that period of time has passed. We'll all be cyborgs by then."

To me all I needed was the Hubble deep field. A tiny little seemingly empty patch of sky that revealed thousands and thousands of galaxies. Extrapolate that to the rest of the sky and it seems there are infinite possibilities for not only life but intelligent life to have developed.

I think that we tend to look for things that have to be the exact way we developed and that approach may be a little short sighted. We may never know for sure because of the impossibly vast distances. But I hope we can eventually make our way into interstellar space and beyond.

This is the TINY patch of sky,


That revealed all of these galaxies:


There is definitely intelligent life out there. Where or how they devoloped is anyone's guess. Probably way to far away for us to ever discover.

Take the water world example, could an intelligent water species not harness underwater volcanoes for melting of metals or making tools?

My point is the amount of possibilities are endless and I don't think it has to be just like earth for it to happen.

well.. our current understanding is the universe is 13 billion years old.. but it could be more..

maybe intelligent life just died out and humans are the last survivors

The "rare Earth" is the answer to "why they are not here." That's the premise of this thread. We needn't go into other issues here. The issue is "The Rare Earth."

1. It may be a function of time. Earth is rare TODAY. One of the points people like to make about our existence is that we are just a small blip in the history of the Universe. Using this tack even if Sentient Civilizations are common, the chances of them being common AT THE SAME TIME are small, therefore that could explain why "they aren't here." Rare Earth planets are not rare, but it's rare for them to be support a Sentient Civilization at the same time.

2. It may be simply that it takes this long (14 billion years) for a Sentient Civilization to rise. Yes, if the Universe is 14 billion years old, and Earth is 4.5 billion years old, from our limited perspective that's a really long time, but the fact is, since the Big Bang the Universe has gone through an evolution. At first there were no stars at all. Then stars coalesced out of the "gas and stuff" that were the remanents of the Big Bang. Those first stars actually made the advanced elements beyond hydrogen and helium. They were (and are) "element factories." in fact, we are made up of star stuff because those elements have been disseminated throughout the Universe over time--and it took time to do so.

So the Earth's age at 4.5 billion is about a third of the age of the Universe at 14 billion. That's a pretty large percentage, about a third. So the Earth has also undergone evolution from pre-Cambrian times until now. It had to do fairly major things like develop an atmosphere, create oceans, foster one-celled organisms, get to multi-cellular, grow some feet and lungs on those early creatures, get them out on land, and start experimenting. It took awhile and was more than six days. So Earth got cranking on dinosaurs and then a damn asteroid or comet came along and wiped out all that work. So now, after 4.5 billion Earth years and 14 billion Universe years, finally, there is a Sentient Civilization that thinks about the Universe and wonders if there is anyone else out there. And that's why "they aren't here." It's because we're the first. Somebody has to be the first. What if it's us? What if it actually does "take this long?"

3. Now even if we're the first, it may be we are the first batch. Given the billions of galaxies, it may be there are several "Rare Earths," even if it's once per galaxy. So let's pretend even that there are several per galaxy. How would they ever know even to look our way? Even if they are doing what we are doing and can detect several planets around our Sun, there's no way we know of to say the third planet is inhabited. They may be able to say, "Hey, it's in the goldilocks zone!" but that's about it. To think they could pick up our radio signals is, I think, a fanciful idea. We've gone digital in about 100 years, and radio waves dissipate. Nobody else is going to get a recognizable signal from us, the movie "Contact" notwithstanding. So given that context, it is obvious why "they aren't here." Even if they are contemporary to us, they don't know we are here at all.

All these possibilities have one premise, and that is in keeping with the "Rare Earth" one way or another. The idea is that even if the Universe is "teeming with life," the chance that it is teeming with Sentient Civilizations that are predisposed to want to see us and are akin to our own is rare, and we shouldn't be surprised that "they aren't here."