Are We Alone?

19 08 2009
The Earth, my home and yours.

The Earth, my home and yours. It looks lonely, doesn't it?

I recently received an email. It’s a good one, and the discussion it concerns is important.

Sam writes:

Hey guys,

I really enjoyed the Hubble post.  That was a pretty amazing video!  Between that, and seeing “District 9” this weekend, I was wondering what you thought the odds are that we are alone in the universe?




Let me preface this discussion by saying that with exactly zero evidence of extraterrestrial life, any optimism I express is the result of speculation. I think it is well grounded speculation, but it is still speculation.

Now, to address the question we first need to get more specific. When we ask, “Are we alone in the universe?” we may be asking one of two questions. The first is, “Does life exist in places other than Earth?” The second is, “Does intelligent life exist in places other than Earth?”

Let’s start with the first question, “Does life exist in places other than Earth?” My answer is yes. Here are the pieces that make me say this:

1. Liquid water is not rare. We know it exists on Earth and should exist on the surface of other planets orbiting their sun in the so-called “Goldilocks zone” – the region where the planet’s surface is neither so hot that the water boils off, nor so cold that it freezes forever. Some evidence suggests water once existed on Mars. Europa, a moon of Jupiter, is suspected of having vast liquid water oceans beneath its icy crust.

This diagram shows the "Goldilocks Zone" where the temperature is "juuust right" for liquid water to exist on the surface of a planet. Notice as the star gets brighter and hotter, the planet must be further away. Likewise if the star is colder, the planet must be closer.
This diagram shows the “Goldilocks Zone” where the temperature is “juuust right” for liquid water to exist on the surface of a planet. Notice as the star gets brighter and hotter, the planet must be further away. Likewise if the star is colder, the planet must be closer. Click for bigger.

2. The basic components of life form spontaneously from common compounds. Amino acids, nucleotides, and sugars all form spontaneously given certain conditions. We have found amino acids on meteoroids and comets, and have synthesized them from common compounds in laboratory settings without shepherding.

A diagram of the experimental apparatus used in the Miller-Urey experiment. This experiment and the many that have followed since have demonstrated that organic molecules form readily from simpler compounds.

A diagram of the experimental apparatus used in the Miller-Urey experiment. This experiment and the many that have followed since have demonstrated that organic molecules form readily from simpler compounds.

3. Plasma membranes form spontaneously from lipids in water. The membranes which are necessary to create the bounded chemical reactions we call cells will be created from commonplace interactions between nonpolar lipids and polar water molecules.

4. Planets orbiting stars are not rare. To date, 373 planets orbiting other stars (also known as exoplanets) have been found. And this is only in the decade or so since we figured out how to find them. Since an object that is not actively throwing off huge amounts of energy (like a star) is a necessary platform for life as we know it, this improves the odds.

5. And the most important part: The universe is incomprehensibly large. Modern estimates of the number of stars in the universe hover around 7 x 10^22, or seventy sextillion, total stars in the visible universe. That’s 70,000,000,000,000,000,000,000 stars. Do you know how many stars that is? That is ten times as many stars as there are grains of sand on Earth. Let me repeat that. There are ten times as many stars in the visible universe as there are grains of sand in all the Earth’s deserts and beaches. Think about that! For every grain of sand on Earth there are TEN STARS! Every grain! This is a number far beyond human comprehension. And this is just in the visible universe! All those stars from regions whose light has not yet reached our little Earth are completely unknown to us! As time passes and our instruments grow stronger, we may yet add several more zeroes to the above figure. Now, the importance of the huge number of stars is that with numbers like this, even astronomically small odds become sure bets. If a star having a planet that develops life is a 1% chance, that gives us 7 x 10^20 stars with life. If life is rarer, and only develops near .0001% on stars, that leaves us with 7 x 10^16 stars with life. 7 with 16 zeroes. Even if life is so rare and miraculous that it appears around only .0000000001% of stars, that still leaves us with 7 x 10^10 stars with life! That’s 70,000,000,000,000 stars with life! Seventy billion stars with life in the visible universe!

Because of these factors, I am very confident that life exists elsewhere in the universe. We are not alone. Elsewhere in this vast and mighty universe, I am sure there are things that divide, grow, and die. Perhaps there are even many worlds with things that crawl, run, jump, fly, or swim. And maybe there are even those with things that think.

This brings us to the next point, which is “Is there other intelligent life in the universe?” In response to this question I am much less confident about the answer. I feel good about the odds that life can and has arisen elsewhere in the universe. The components are simply too common and the possible locations too numerous for our planet to be the only site where life develops. But what about life that is intelligent, like us? I cannot answer this. I have no idea how common or easy it is for life to evolve intelligence. It took at least three and a half billion years for it to happen here. Never before humans was there a family of organisms with intelligence like ours. Not in all of the trillions of generations stretching back to those original lives that started all this. Never before us.

Still, I cannot dismiss the thought of intelligent extraterrestrial life. In a universe with seventy sextillion stars, and however many of those that develop life, even astronomically small odds stand a good chance. We know intelligent life does develop given the proper circumstances – we ourselves are evidence of this. For us to be the only intelligent life in the universe, intelligent life would have to develop around only .0000000000000000000007% of stars. If this was the case, then only one planet would develop intelligent life, and that one planet would be us. But if the odds are even slightly better than this, then we could be confident that there is intelligent life elsewhere in the universe. Other beings like us, who can think and feel and wonder. Other beings like us, who gaze at the stars and relish the wind on their skin. Other beings like us, asking questions of the heavens and wondering if there are others up there who wonder too.

Of course it must be asked, if intelligent extraterrestrial life exists, why is there no evidence?* Why haven’t we heard from them? Why haven’t the hard working men and women at SETI picked up a broadcast? Well again, I think the answer has to do with the size of the universe. Its tremendous size means that even radio waves, which travel at the speed of light, take a hundred thousand years to travel from one side of the galaxy to the other, and many millions of years to reach us from other galaxies. Even if we did one day receive a signal from a distant intelligent source, there would be no guarantee that the senders and their kind survived all those millions of years while their signals were swimming the cosmic oceans.

We may never have contact with other intelligent life. The distances and timescales involved may be simply too large for life to overcome, and scientific progress may reveal no way to circumvent the cold truth of astronomical distance. Yet this is not reason for sadness. It is still, I think, a great and wonderful thought to think that when I look up at the stars, somewhere up there, somewhere in the innumerable lights of the night sky, maybe someone is looking back.


*I am of course choosing to deny the claims of the thousands who believe they have been abducted by aliens or seen such beings zipping about the night sky or making mischief in their corn fields. There are simply too many natural, unspectacular explanations for such claims (e.g. hallucinations, hysteria, lies, memories created by psychotherapy, poor eyesight, hoaxers), and too little hard evidence, for them to be taken seriously.

P.S. Check out Nye’s program on astrobiology for some more information, including info on how we detect extrasolar planets. I really can’t recommend Nye’s programming enough. He has an excellent way of conveying information.




3 responses

20 08 2009
The Stud

You’re the best, Neil. Astrobiology rocks!

What’s your take on the Drake Equation?

20 08 2009

I think the Drake Equation is good for identifying what factors are important in determining the amount of intelligent, communicative life in the galaxy, but not very good for actually calculating the figures.

For other readers, the Drake Equation is an equation (really) thought up by Dr. Frank Drake in 1960. It looks like this (from wikipedia):

N = R* x fp x ne x fl x fi x fc x L


N is the number of civilizations in our galaxy in which communication might be possible;


R* is the average rate of star formation per year in our galaxy

fp is the fraction of those stars that have planets

ne is the average number of planets that can potentially support life per star that has planets

fℓ is the fraction of the above that actually go on to develop life at some point

fi is the fraction of the above that actually go on to develop intelligent life

fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L is the length of time such civilizations release detectable signals into space.

So you see the Drake equation covers the factors that need to be considered when we try to calculate the number of civilizations out there that we might communicate with. The problem, of course, is that we don’t know the values for any of those variables. I know you’ve read Sagan’s Varieties of Scientific Experience. A listener responded to his discussion of the Drake equation with the following criticism, one I agree with:

“I’m a wee bit skeptical at Drake’s equation. It doesn’t really indicate how much extraterrestrial life there is. All it indicates is whether the user of it is a pessimist or an optimist.”

He’s saying that since there’s not very good data that would allow us to insert accurate figures for the variables above, the equation really just measures the amount of confidence the user has in the existence of extraterrestrial life. Sagan’s response is that even with pessimistic numbers inserted, the equation still yields a significant number of communicative civilizations. The problem with that is that the numbers could be way lower than Sagan’s pessimistic estimates. We just don’t know.


21 08 2009
Sam T

Hey Neil,

Thanks for answering my question! It was a good answer that also taught me some new things about science- I had no idea there were so many stars!


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