The universe is vast beyond vast.   Even a few short years ago we thought the universe was about 13-14 billion light years across.   We thought there were maybe 100 billion galaxies with an average 100 billion stars each.    We now know the universe is at least 150 billion light years across maybe much much more, possibly infinite and that each galaxy may have closer to a trillion stars.    In a few years the size of our universe has exploded and so the possibility of life however remote it might be in the Milky Way galaxy we can estimate that in the universe the probability of life is virtually 1.0.

In fact, scientists postulate that if the universe is very large there are so many galaxies there is another you on some planet similar to Earth composed of virtually the same characteristics.  This is a variation on the many worlds theory of quantum mechanics but different in which an infinite variety of you’s is around.  So, don’t worry about making a mistake.  Some other version of you is doing it right.     This has been called Quantum immortality for the quantum version of parallel universes.

The distance to another galaxy is so large that it is impossible using any technology we can even imagine to traverse these distances so a lot of what we see in the night sky is no different than the celestial sphere for all practical purposes that the Greeks posited because for all the imagined possibilities of exploration we might have there is zero probability of ever having any interaction with them unless the laws of physics change in a major way.   For practical purposes we must limit our thinking down to the part of our galaxy we can possibly observe in detail and have any hope of communicating or traveling to.   This puts very strict limit on the number of stars and planets that is relevant from the point of view of finding life.

So, let’s confine ourselves as Drake did to the much much much smaller Milky Way galaxy. Over the last 30 years or so we have gathered a lot of information on suns and what solar systems look like in general.   So, let’s look at some of the information we have on what are the possibilities for exploring in any way we can.

First let’s look at how far we might be able to go or see with our technology today and in the future so we know how big our practical “relevant” universe is.    Here is a rough estimate of how far and how long we could travel with different technologies.

Technology Based Capabilities Todays Tech 2100 Tech Forever
Light Years Light Years Light Years
Detectable Signals 10,000 160,000 15,000,000,000
We can Transmit to them 1,000 10,000 10,000,000
We can send an automated probe 5 17 200
We can go as humans 0 5 17


We are talking a minimum of 4+ light years to the closest star.   So, the minimum distance is 5 light years to the closest stars.  Voyager 1 is now 134 AU (Feb 2016) after 38 years it is traveling at 17km/sec.  Light is 300k km/sec or Voyager is going at 1/15th the speed of light.  Pretty impressive.  134AU = 10 ^ 13m = 134 * 8 light minutes = 2.5 light hours.  In other words voyager after 38 years is only 1/15,000th of the way to Alpha Centauri if it were heading in that direction, however at its current velocity if it were going that way it would only take another 60 years to get there.  However, Voyager is not going to Alpha Centauri but to a star called AC793888 which is about 17 light years away.   It will come close to but not enter that star system, will fly by and be deflecting into orbit with our sun around the Milky Way center and travel forever essentially undisturbed somewhere in space.  It’s thermonuclear power supply is expected to last only another 10 or 20 years.  The amazing thing is Voyager was able to pass through the rim of our solar system at 121 AU and is now in full “interstellar space.”   It was able to shed light on the boundary of our solar system which is where particles streaming from the sun form a wall against the interstellar medium and the velocities match and cancel.    In a similar way the Earths magnetic field reaches a balance with cosmic rays at about 250,000 miles from the Earth a little farther than the moon.    After this distance travelers from Earth must face the full brunt of the Sun and interstellar cosmic rays.

It’s important to understand that the fastest humans have ever traveled in space is roughly 30,000 miles per hour.   By comparison Voyager 1 is now traveling 4 million miles/hour.   This 30,000mph may seem like a high speed but it the speed of light is 22,000 times faster so if we could maintain our fastest speed to date it would take 100,000 years to reach the CLOSEST star system.  If we could go as fast as Voyager you can see we could get there in 60+ years.

Fortunately, we can beat our current highest speed pretty easily if we tried.  Obviously to make travel to star systems we would need to go at least 1,000 times faster than the fastest we have ever gone before.   That may seem crazy but we have technology which theoretically could achieve speeds on that order of magnitude that we could build today.  This gives you an idea of how vast space is that if we can go 1,000 times faster than we ever have before we will get to the closest star system in 100 years.    We likely need to find a way to go 10,000 to 100,000 times faster than we have today.  That’s harder but also doable theoretically.

We may be able to send an automated probe today to the closest stars.   We can send an electromagnetic signal today to a star possibly up to 1,000 light years away if we tried really hard and we have lots more room for improvement in our telescopes as well.   We may be able to detect some form of signal potentially up to a 100,000 light years away.   These are swags with some assumptions I won’t go into detail at this time but they underlie the basic assumptions of how far out we can imagine looking and visiting or communicating with.

By the year 2100 we might improve things substantially and reach 17 light years away with automated probes.   To do so would require developing technology that could easily go a large fraction of the speed of light.  It seems very possible we could do this by the end of the century.  If we can do that we might be able to sustain travel for decades to get ourselves physically to star system 5 light years away.

After 2100 and what we ultimately might be able to do is pure speculation.  It depends on the evolution of our understanding of physics.  The good news is that this is less limiting than one might guess.   There is a lot of uncertainty about underlying basics of reality to the extent I think we can say we don’t honestly know what is possible.    The more we learn about the real world the more perplexing things we find and the more it seems as if we are only at the beginning of understanding the true possibilities.  So, we can leave that a little more flexible.  Unfortunately, there is no relevance to that for us today.  So, it makes no difference one way or another.  All we can focus on is what is possible today and soon, maybe by 2100.


There is no dramatic change in our knowledge of physics required for us to build spaceships to make the 17 light year travel.  It is simply a matter of building a number of spaceship designs that seem feasible today.   I assume over time we build amazing telescopes and other instruments in space to radically improve our ability to see and sense the stars around us.
Here is some of the technology we might use and their capabilities in terms of how many years it would take to use them to talk to stars with different distances from us.   I assume that for human travel we are only going to be motivated by travel times measured in small numbers of decades at most.  We might be willing to have a conversation with a species that was a little farther out than that.


Travel Time
Distance from Sol 5 17 50 100 250 500 160,000 Light Years
Rocket NA NA NA NA NA NA NA 1/1/1950
Ion 36 NA NA NA NA NA NA 1/1/1998
Nuclear 18 66 NA NA NA NA NA 1/1/2020
Bussard Ramjet 9 33 100 200 NA NA NA 1/1/2040
Reactionless 9 33 100 200 NA NA NA 1/1/2040
Lightspeed 5 17 50 100 250 500 160,000 1/1/2100
Space Warp/Wormholes (assume 100x speed of light) 0.045 0.165 0.5 1 2.5 5 1600 1/1/2100

Eventually in the future several technologies may make travel times better.   One way would be once we have a post out in some solar system a hundred or 200 light years away we could transmit our “selves” (a digital representation of ourselves anyway) at the speed of light to the solar system.   Once we got there we would have to be reconstituted.  This assumes this is possible   If we can’t do that then we will require development of technology that exceeds the speed of light which so far is as speculative or more than sending your consciousness over a coded signal.   So, the distance for human travel may be in the 100 light year capability frame if we can do that.

So, how many stars are within these distances from Earth?

Distance from Sol 5 17 50 100 250 500 160,000 Light Years
# of visible suns 3 74 650 4,644 260,000 2,000,000 100,000,000,000
# of all suns 3 74 1,000,000,000,000
# of our type suns (F and G) 2 8 101 815 45,629 350,991 17,549,526,270
# of planets total
# of planets in habitable zone(HZ) 17,000,000,000
# of planets actually discovered that are in HZ as of May 2016 2(Earth and Mars) +17 +74 +97 +500 16 31
# of planets discovered atmosphere (2018 we can do )
# of planets discovered bio sign gases (2018 we can do ) TBD TBD TBD TBD TBD TBD TBD

This chart shows you that there are only 3 suns within 5 light years of Earth.   It is unlikely we will find any more suns than we are aware of today.  We have not seen any planets other than the Earth in this 5 light years that is habitable to life forms, could have life.  It is extremely unlikely that any planets in these 3 suns has intelligent life but there may be additional planets and even life.  It’s possible.

Within 17 light years there is considerably more possibilities.  We know of 74 suns in this range. At least 8 or more of those suns could be of the type that would be possible to harbor planets that are habitable.   Unfortunately so far with all our efforts looking for planets we have discovered  17 additional planets (besides the Earth and Mars) in that range that are “viable” life planets that are in the right size or range to have a warm life giving environment like the Earth.   Not bad for a start.  I assume when we get into Kepler Phase II we will discover more.  (More on Kepler later).


We have discovered 3,300 planets but only a fraction are potentially livable planets using the first version of the above satellite.  It is expected that Kepler II and the James Webb telescope as well as another telescope being sent to space specifically to detect gases around remote planets will tell us a lot more about the frequency of habitable planets around stars and the number of those that show atmospheres and even gases that might indicate some form of life.  This data could change the calculations substantially in the following analysis.

Since there are at least 260,000 suns in this range from Earth there is the possibility for many thousands of habitable planets.   This is our star trek reality.  For at least the next 100 years we are going to be limited to most likely planets within about 17 light years from Earth which is the 74 stars we have seen.   We might venture to explore at least telescopically beyond that but beyond 250 light years seems pointless.  Besides the unbelievably long time period to communicate all we can hope to do with that distance is similar to what SETI is doing which is to look for long distant signs of intelligent life.  We are not limited to 250 light years for that.  We can look out thousands of light years for that but it is a fact that the farther out we look the harder and less likely we will see a signal.

Some people classify civilizations based on their total energy generation ability.   We are considered a class II civilization able to generate almost planet level energy.   Class III can generate sun level energies and Class IV can generate galaxy level energy.  Who knows if these things exist but it is useful to categorize as then we can decide if we will be able to see them with our telescopes.   If we ever get to these higher levels then we could do a lot more in terms of our ability to both transport ourselves around as well as see things.

It is clear that even talking about our galaxy is a stretch.  The galaxy is 300 times wider than we could ever ever hope to traverse.   So, when considering what percentage of the galaxy to concern ourselves with we are talking 1 : 1,000,000th of the galaxy at least as far as any useful amount of the galaxy for us to think about.

We may out of luck see things farther out than that but any communication would be a one-way essentially forever.   That may be useful from a Drake equation point of view but for any other goal it is not useful to think of anything outside this area.

Other Possibilities may be much more likely to achieve goals

It is fun to speculate on the possibilities of the far future just to orient ourselves.  It is not reasonable to expect any of these to come about any time soon.

  1. There may exist multiple worlds according to 4 different theories in 4 different ways.  It seems impossible to traverse to any of these worlds but it is theory
  2. 96% of the universe around us is invisible to us.  Could this contain life or intelligent life right here in the same room?
  3. There is a level of physics at energies far too high for us to probe that implies a universe of reality 10 to the 33 times smaller than us.   We believe the ultimate discreteness of time in the universe is 10^-43 seconds.   So, it is possible for some form of intelligence or life to exist at some ultra micro level that we can’t observe possibly operating at speeds beyond our comprehension.
  4. Ultimately, intelligence is all the same.  We may simulate it in a computer and we may find all the companionship or issues with artificial intelligence that we would ever find with regular intelligence
  5. We may accelerate the intelligence of existing living things (monkeys, dogs,…) or even create new living things eventually that are intelligent.
  6. If we learn how to copy ourselves into digital form then we may transcend the physical reality of our world and operate in a virtual world thinking, playing and possibly other civilizations have done the same thing which is why we don’t see them.

These are just some of the things that boggle the mind and give pause.

Lagrange_points_simple.svgIn this series

Part I – Goals for our desire to understand and find extraterrestrial friends

Part II – how big is our search area?

Part III – what are the problems in finding life

 Part IV the equations 

Part V  the conclusions