Far away in this context means anything outside of our own solar system. If we start with 'nearby' stuff first then we are really talking about exoplanets. As you found on the homework there are a lot of these already and there are a solid handful that are earthlike. Given the discussion about life on moons around Jovian planets that opens up even more possibilities. Turns out that one significant effort to understand the opportunities for life on exoplanets is based right here in the northwest. The Virtual Planetary Laboratory lives at the University of Washington and approaches the question of life on exoplanets from a range of perspectives and has a lot of software and presentations that you can explore.

 

Drake Equation:

As we open up the scope of this question even further we quickly come to the Drake Equation which was proposed as a way of understanding the probability of detecting signs of intelligent life from within our own galaxy. Frank Drake was looking for ways to use the Arecibo Radio Telescope, which is our most sensitive listening device, to see if radio transmissions from other civilizations could be detected. The Arecibo Radio Telescope has had a long life but it has been in the news recently due to long term damage that has led to the closure of many projects including the search for signals from other civilizations.

 

 

We will explore this equation as presented by SETI since that is Dr Frank Drake's legacy at some level. Here is a modification of Drakes original equation.

Drake Equation (modified): N = N_g N_s f_H n_H f_L f_I f_T L

N_g - number of galaxies in 'universe'

N_s - number of stars in typical galaxy

f_H - fraction of stars that have habitable planets

n_H - number of habitable moons in a solar system

f_L - fraction of habitable planets the have life

f_I - fraction of life that is 'intelligent'

f_T - fraction of intelligent life that creates technology

L - lifetime, in years of that civilization

As you move from left to right in this equation you will find that we have less and less certainty about what values to use in the equation. Let's look at the first 5 where we may have some sense of the values. This will give us a sense of how many planets out there have some form of life.

Just Life: N = N_g N_s f_H n_H f_L

The number of galaxies in the universe is in the range of 200 10⁹ (= N_g). A large galaxy like ours has 100 10⁹ (=N_s) stars. Some galaxies have only 100 10⁶ stars. The average is somewhere in between. Take your pick. At this point we see that nearly every star we see has some planets around it (f_H = 1). At the moment it looks like about 1 in 1000 has an earthlike planet. To be a little cautious we could say 1 in 10,000. In our own solar system there are a number of moons that seem like life is possible. If that were to prove to be true then n_H might be anywhere between 1 and 10. In this list f_L is the first value we really have only speculation to help us with. Perhaps, if the conditions are appropriate, life will always happen. Perhaps, even in the best of circumstances, life only happen 1 in a million times. Let's look at both possibilities.

Just The Milky Way: N_{possible planets/moons} = N_s f_H n_H

Starting with the part we perhaps understand reasonably well for our own galaxy we would get

N_{possible planets/moons} = 100 10⁹ stars x 1 10^-4 possible planets/star x 2 moons/planet = 20 10⁶

20 million planets or moons where life might arise just in our galaxy. If life is easy to get started ( f_L = 1) then there is a lot of life out there. If life is any form is very unlikely ( f_L = 1^. 10^-6 ) we might well be alone in our galaxy. Never mind the question of whether that life is intelligent in some sense and detectable by us. If you consider the 200 10⁹ galaxies in the universe it gets to be a lot of chances for life to get started. Very unlikely that we would notice it outside of our own galaxy.

This is where it becomes clear that we need better information. The estimate for the number of places where life has orginated in the Milky Way galaxy ranges from 20 to 20 10⁶ depending on how easily life gets started. As we explore our own solar system we may get a better sense for this part of the Drake equation. If there is life in some form in multiple places in our solar system it will make it seem very likely that life, in some form, exists elsewhere in our galaxy.

Considering the Whole Universe:

If we consider the whole universe and not just our own galaxy then we get to multiply the numbers above by 200 10⁹ which makes it feel likely that, even if life is pretty rare, that it has started at other places in the universe.

But would we notice?

If that feels vague to you you haven't seen anything yet. If we now ask about 'intelligent' life that might produce a technology that would allow us to detect it's presence we get a whole other level of uncertainty. We only have the earth to look at for now and even here it's hard to decide if 'intelligent' life has occured 1x or many times. As we learn more and more about other species on this earth it feels more and more difficult to assert that humans are the only intelligent creature. We appear to be the only creature which has created what we currently recognize as 'technology' that could be detected from a distance but that's a separate question. This makes it tough to decide what numbers to put in for f_I and f_T. As we explore other parts of the solar system we may discover life in a lot of places or nowhere. It's going to take some time to develop a better sense of what fractions to put in here.

The last big question is how long do technological civilizations last. Again we only have one example and it hasn't lasted very long yet (a few hundred years at best). Given that it takes hundreds of thousands of years for signals just to cross our own galaxy it seems not so surprising we haven't detected any signs of other technologies like ours - yet.

Super Aliens:

Another feature of this overall discussion is sometimes the assumption that other intelligent techological life will have figured out how to travel faster than the speed of light and could be all around us (the Star Trek effect). This is an interesting discussion since it presumes that, in the future, we will find ways to violate what we now think of as fundamental rules of the universe. The argument goes something like this. The technology we have now would have been inconceivable to humans just 500 or 1000 years ago. Why would we not expect the same from our future selves? We can't imagine how to move faster than the speed of light but you can't prove that something like this isn't possible. Such advancements have happened over and over again in history so why would they stop now? It's good argument and I don't know if there is a good way to respond to it. IF you feel like the Drake equation insists that there are other techological civilizations out there then the absence of contact with them suggests faster than light hasn't happened - or has it? Now we're starting to get weird:)

Life Moving Through Space:

An important consideration in all of these discussions is the question of whether life has to come into being independently on every possible planet. For a long time we believed that life would not survive in space in any sense. We now have evidence that some forms of life, including fairly complex life like tardigrades, can survive for significant periods of time in space. This suggests that it may be possible for life to be carried from planet to planet and possibly from star to star. This opens up the possibility that life can be 'seeded' onto other planets with welcoming environments. Here's an image of a tardigrade (TED video about these remarkably tough creatures).

Recently Frank and Sullivan have proposed a different way to think about this question. Here is a summary of their work from a NASA article. We'll walk through this to understand its differences from the Drake Equation.

FInally, to put a different spin on it here is Eric Seigel's cautionary perspective on the whole thing. If you want to dig deeper this paper by Anders Sandberg does a good job of carefully exploring the range of possibilities based on our best current understanding. Both Eric and Anders conclude that it is quite reasonable to expect that we might be alone in our galaxy given what we currently know.

Here's a final TED talk that explores the possibility of a more lonely universe.....

What Makes This Cool!

I would want to point out that in your lifetimes we are likely to learn a bit more about the possibility of life in different environments as well as different kinds of life. If it were to turn out that there is life on every moon in our solar systemt that has liquid water on it somewhere that would change everything. If we find conclusive signs of past or current life on Mars or Venus that would also change the calculation. If we look everywhere in our solar system and find nothing that resembles life that will also be incredibly interesting and important for our understanding of the Drake equation. You will almost certainly get to see this over the next 50-100 years. I envy you!!

So .... what do you think now?

Perhaps more importantly, do you feel like you understand what we humans currently do and don't know about the universe around us?

HW: Alone?

Why do we now argue that f_H is relatively large (meaning in the range 0.5 to 1.) based on your explorations of the exoplanet database?

HW: Alone?

If we find some form of life (even fossilized) on Mars, or one of the moons of Jupiter, which factor in the Drake equation will be affected? Explain your reasoning.

HW: Alone?

What is your personal desire for how the Drake equation turns out? Do you want to find other life in our galaxy or do you hope that we are unique in our own galaxy? Explain your reasoning.