28. Interstellar Travel, SETI, and the Rarity of Life

Transcript: Speculation about the possibility and implications of life in the universe dates back a long time. Over two thousand years ago, Plutarch wrote about the possibility of other worlds with their own races of men and beasts. By the eleventh century in Europe, the Catholic church had declared the idea of plurality of worlds, other worlds possibly with life on them, to be heretical, and in 1609, the mystic Giordano Bruno was burned at the stake in Rome for writing about life in the universe beyond the Earth. The first attempt to communicate was hypothesized in 1820 by the German mathematician Karl Friedrich Gauss. He wrote about the possibility of carving the forests of Siberia into a geometric shape of the Pythagorean triangle. In the 1840s, Joseph von Littrow wrote up the idea of creating trenches in the Sahara desert in geometric shapes and filling the trenches with gasoline and lighting them to be viewed from far away. Neither of these projects was ever funded.

Transcript: However unlikely a successful outcome a priori, SETI is based on the premise that science is an empirical endeavor. Rather than philosophizing about the possibility or prospect of life in the universe, we should look. Rather than wondering if people are communicating with us from afar, we should listen. So the rebuttal to skeptics is that we must do the experiment. In this decision making, we have decided that photons are cheaper and faster than rockets, at least with current technologies. Humans have also thrown their messages in a bottle out into space in the form of the Voyager and Pioneer spacecrafts which were laden with the images and sounds of Earth and are now drifting in outer space. Meanwhile, regardless of our intentioned efforts to communicate, we have been betraying our existence for over fifty years by the steady leakage of radio and TV signals into space. Someone looking and listening from afar could realize that there had been intelligent civilization on Earth simply by the creation and leakage of electromagnetic signals from our radio and TV transmitters. SETI is based on the premise that life in the universe exists and has become sophisticated enough to communicate somewhere, but it’s hard to say that it’s a true science because it’s impossible to estimate the probability of success of this experiment.

Transcript: It is very difficult to avoid anthropocentric thinking when considering the issue of life in the universe. Anthropocentric thinking means considering the subject in terms of human origins, human perceptions, human values, and human culture. When we think of science fiction, it’s often the case that aliens look something like us. They are bipedal, or they appear to be mammals or somewhat similar to us in some way. Life in the universe could be so strange as to be unrecognizable. Perhaps the biggest challenge, if life in the universe were discovered and we found that our biology is not unique, would be to the conventional religions of the world. In the Judeo-Christian faith, Christ appears in the image of humans and dies for their sins. What is the situation for such a religion when it’s found that intelligent aliens exist? Do they have souls? Were they saved? What is the role of world religions when we find intelligent life elsewhere in the universe? People have barely begun to consider seriously this prospect, but we’re faced with it because the universe is a vast place, and it’s quite likely that intelligent life may exist somewhere.

Transcript: One of the standard conceits of science fiction is that humans, when they develop the capability for starships and interstellar travel, will meet another set of civilizations with whom we can communicate and share our cultural values and our civilization histories. The truth is that the time scale issue makes such a very unlikely outcome. To illustrate this, consider the evolution of three prototypical Earths. Imagine that the evolution of life and complexity is somehow key to the rate of evolution governed perhaps by the distance to the Sun, which is the energy source. Two identical Earths, one of which evolved at a rate one percent slower than the other, would yield outcomes that after four and a half billion years were forty-five million years different. So on one Earth there would be humans having developed from primates, but on the other Earth it would only be twenty million years after the dinosaurs became extinct and no primates or apes existed. Now imagine the timescale difference was ten percent. After four and a half billion years, one Earth would be the present day Earth, and the other would be four hundred and fifty million years previous. That's a time when the oceans had only just begun to contain complex life. Dinosaurs, mammals did not exist, nor birds, an utterly different world. And if the timescale was thirty percent different, not a huge percentage difference, one Earth would be like the present day Earth, and the other Earth would not even contain multicelled organisms. It would be utterly different and alien from our world, and yet it is the same world just different in time by only thirty percent. This gives us a sense with our own planet of the vastly different outcomes that could happen as cosmic time unfolds and evolution plays its role.

Transcript: One of the arguments that we are not alone in the universe and that there are other intelligent civilizations is based on what’s called the timescale consideration. Remember that there has been life of some kind on Earth for about four billion years. But only in the last couple of million years has the human species existed, and only in the past couple of hundred years has technology existed and the possibility for space travel. This is a tiny fraction in the history of the human species and an even tinier fraction of the history of life on Earth or the age of the universe. Thus, if civilizations are eventually developing the capability for space travel and traveling around the galaxy, on average any civilization we might encounter or that might exist will be essentially be infinitely advanced compared to us, that is to say hundreds of thousands or millions of years more advanced than we are. As Arthur C. Clarke, the science fiction writer said, “Any sufficiently advanced civilization is indistinguishable from magic.” So one of the profound issues of thinking about life in the universe is that if we have only just developed the capability for technology and for space travel, and we encounter it elsewhere in the universe, we will be looking at people far more advanced than us.

Transcript: The Fermi Paradox starts with the simple question, where are they? It’s based on the huge number of potential sites for life and the large amount of time in the history of the universe for intelligent civilizations that can travel in space to have developed. There is no answer to the Fermi paradox, but thinking about possible answers enlightens us into the role of the universe and life itself. Here are some possible solutions. We might be alone or so alone as to be isolated in time and space if intelligent life is rare. Alternatively, we might not be alone. There may be other intelligent life forms, but the technical and energetic difficulties of space travel could make it an unusual outcome. Another possibility is that intelligent civilizations self-destruct before they ever get the capability for interstellar travel. This of course may be our outcome still. Another possibility is that intelligent civilizations exist, but they are hiding from us. This is called the Zoo Hypothesis. Yet another possibility is that they exist, but they don't care because we are insufficiently advanced to interesting to them. That's a sobering prospect, and in truth we have no logical way of deciding between these and many more answers to the question, where are they?

Transcript: The idea of travel into space leads naturally to the concept of galactic civilizations. It’s been the history of humans on this planet to explore their evolutionary world, to radiate into every niche of the planet to try and understand it. We’ve only had space travel for fifty years, and it’s sobering to think that it’s thirty-five years since we’ve been back to the Moon. But in the long term, over hundreds or thousands of years, it’s likely that humans will live in space and explore beyond the solar system. If we can do this with barely a hundred years of space travel, than what would the capabilities be of a civilization that had been traveling in space for hundreds of thousands or even tens of millions of years? We can imagine that it would only take an advanced civilization a few hundred thousand years to explore the entire Milky Way. They could colonize it in that way, traveling with sentient probes. They need not travel the large distances. Once they’d radiated once through the Milky Way, they could set up sentient probes to gather information from nearby intelligences and act as repositories of knowledge in the form of a galactic network of information. This is all utter speculation. We have no idea whether or not exploration at this level is a cultural imperative of ours, not shared by other intelligent civilizations should they exist elsewhere in the galaxy.

Transcript: Johnny von Neumann, the father of modern computing, was thinking about the Fermi Paradox, the prospect that with life spread through the galaxy there must be intelligent civilizations that would presumably colonize the galaxy. He came up with the idea of what is called a von Neumann machine. If it sounds speculative, realize that it’s only a modest extrapolation of current technology. A von Neumann machine is a self replicating probe which travels from our solar system to nearby solar systems, mines asteroids to create replicas of itself, and then travels onwards spawning through the galaxy and propagating and sending back the information to the home planet, Earth. This sounds difficult, but we are probably only thirty to forty years away from producing such a technology ourselves. Even traveling at a tenth of the velocity of light, it would take only a hundred thousand years for such a set of hypothetical probes to propagate through the entire galaxy and send back information to the home planet. Given that the Milky Way has existed for ten billion years, it seems implausible that some intelligent life form would not have already done this if they had the capability and so should have colonized the galaxy. Von Neumann machines bring into sharp focus the Fermi question, where are they?

Transcript: In the 1940s, Enrico Fermi, who was a Nobel Prize winning physicist and developer of the first working fission reactor, was sitting with his colleagues talking about philosophical issues when he asked the following question. “Where are they?” His premise was that it’s so likely that life must have developed elsewhere in the Milky Way, that the probability of their not being intelligent civilizations must be very low. Therefore, they must exist, but since they haven’t visited, where are they? This seemed to Fermi a logical paradox, and in fact the logical paradox of what’s called the Fermi question has been asked over and over again by people speculating about life in the universe. The trouble with the paradox is the premise behind it which is that intelligent, communicable civilizations exist or that if civilizations exist, they will communicate. Of course it’s possible that communication is a cultural issue rather than a biological imperative, or it’s possible that communication is not a feasible option because civilizations are so isolated in time and space. The trouble is there are many reasons to explain the absence of communication, and it’s difficult to decide between them on purely logical grounds.

Transcript: As an analogy for the Drake equation where independent probabilities combine to reduce an initially large number to a small number, consider the following hypothetical situation. Imagine the number of students in a large university, say, forty thousand. Now imagine what fraction of those students are women. It’ll be roughly fifty percent which brings you down to twenty thousand. Imagine the fraction of those that are econ majors. That's probably about one percent given the large number of majors at a university. At this point you reduce the number to two hundred. Imagine the fraction of those women who are economics majors who have red hair. It’s perhaps one-tenth. Two hundred goes down to twenty. Imagine the fraction of those red haired women who are economics majors who at any given time are in class. That's probably about ten percent, and twenty has been reduced to two. And imagine the fraction at any given time that are reading a magazine. Maybe it’s ten percent or twenty percent, but at this point you are down essentially to one person or less. So it’s possible starting from forty thousand to reduce yourself to essentially nobody who satisfies all the conditions of being a woman, an economics major, having red hair, sitting in class, and reading a magazine. However, if the starting number was larger, say, perhaps the total number of students in the United States, more like twenty million, then each of the numbers has to be multiplied by five hundred, and at this point it is possible, however rare, that there are people who satisfy all of those contingent probabilities. This is the issue of the Drake equation where starting with a huge number of potential sites for life, even if all the joint probabilities of reaching intelligent, communicable civilizations are combined together, it still may be possible that the outcome occurs.

Transcript: Photons are the fastest thing there is. Light and other forms of electromagnetic communication travel at three hundred thousand kilometers per second, vastly out seeding the capabilities of any type of rocket that we know or can imagine. As an example of their swiftness, consider the distance covered in one year in various other forms of travel compared to the number of years needed to cover that same distance that photons travel in only one year. A human, with a speed of maybe 0.01 kilometers per second, could travel fifty thousand kilometers in a year but would need two hundred million years to travel the same distance that a photon travels in one year. The fastest car, traveling at a speed of about 0.05 kilometers per second, travels in one year a distance of a million kilometers but would need ten million years to travel the distance that a photon travels in a year. A jet aircraft with a speed of maybe three hundred meters per second can travel ten million kilometers in a year but would need a million years to travel the distance that light travels in a year. The Voyager spacecraft, the fastest spacecraft we’ve made, with a speed of twelve kilometers per second, would travel four hundred million kilometers in a year but would still need twenty-five thousand years to travel the distance that light can travel in a year, and a fusion powered spacecraft, that does not yet exist but with a speed of three thousand kilometers per second, 0.1 percent of the speed of light, would travel in a year a hundred billion kilometers, but still would need a hundred years to travel the distance that a photon travels in one year.

Transcript: Science fiction writers have long dreamed of various ways of cheating the constraints of time and space. One popular device in science fiction and on TV is the idea of warp drive, travel at speeds faster than the speed of light. However, Einstein’s theory of relativity, a good physical theory of the universe, says that the speed of light is an absolute limit. It takes an infinite amount of energy to accelerate anything, even a microscopic particle, to the speed of light. Hypothetically, tachyons or faster than light particles may exist, but no one’s yet observed them. As far as we know, warp drive is not possible. Another possible device is a wormhole, a hole in space and time that connects to another part of the universe, potentially allowing instantaneous travel over large regions of space. Wormholes are also truly speculative. We may develop the technology to understand and manipulate space-time in this way, but at the moment it's a purely theoretical idea. Finally, it’s possible that suspended animation may be achieved for humans, allowing humans to construct space arcs where large groups of people go into suspended animation allowing their biological clocks to slow drastically or even stop entirely, enough for a journey to the stars. Again, this is based on biological mechanisms that we do not understand at the moment, and nobody knows if such suspended animation is possible in practice.

Transcript: As an example of the advantages of relativistic space travel, or travel close to the velocity of light, consider a hypothetical journey to a star that's five lightyears distant. Several hundred stars are this close or closer. At the limit of modern chemical rockets, a speed of five one-thousandths of a percent of the velocity of light, the roundtrip journey would take a hundred thousand years. At one-tenth the velocity of light, the limit of an ion engine or a solar sail, the trip would take fifty years. At a half the velocity of light, the trip would take ten years as seen from the Earth and only eight years as seen from someone traveling on the spacecraft. At ninety percent of the velocity of light, the trip would take six years or two and a half years as seen by someone traveling on the spacecraft. And at 99.9 percent of the velocity of light, due to relativistic effects, as seen from the Earth the trip would take five years, but as seen by somebody traveling on the spacecraft, the roundtrip would only take a few months. These are the advantages of relativistic travel which also lead to the twin paradox where twins separated, and one traveling at relativistic speed to the stars would age far less than the twin that remained home on Earth.

Transcript: Visionaries have imagined leapfrogging over current or even planned technologies to the forms of rocket propulsion that can reach relativistic speeds, rocket speeds close to that of light. One form of energy release is matter-antimatter annihilation. Antimatter is rare on Earth. It naturally doesn't occur, but it can be created in the lab and potentially in a rocket engine. Matter-antimatter annihilation liberates the entire mc2 of trapped energy in matter with an efficiency that’s more than a hundred times that of nuclear fusion within the Sun. A second technology is a ramjet which would gather diffuse hydrogen from the interstellar medium itself as the rocket traveled. Imagine a huge scoop many kilometers across gathering the diffuse hydrogen. The hydrogen would then be fused to helium within the rocket to create the energy source. Using either of these technologies, neither of which have ever been prototyped, it would be possible to travel to the stars in times much less than a human lifetime.

Transcript: Conventional rocket systems, even those that rely on nuclear energy, tend to rely on the principle of burning a certain amount of fuel, accelerating the rocket initially, and then having it coast to its destination. Other ideas for propulsions systems, some of which have been tested or prototyped but none of which have been put into large scale operation, involve steadier or more gradual acceleration towards the distant target. In an ion engine, a tiny amount of fuel is used continuously to slowly accelerate the rocket. The ion jet and the ion engine use a small amount of fuel but an enormous muzzle velocity or ejection velocity to create the acceleration. NASA tested this technology in a small way with the Deep Space 1 probe, and in principle it could reach speeds of a few percent of the velocity of light. Solar sails are another possible technology. In this idea, a large reflective sheet is suspended in space, and the pressure from photons is used to accelerate the spacecraft. Because the sheet is reflective, the photons bounce off and transfer their momentum to the spacecraft, gradually accelerating it. The problem with solar sails is that as the distance from the Sun increases, the effectiveness of solar pressure diminishes, and so solar sail powered aircraft would almost certainly have to be assisted by powerful lasers beamed out from Earth as the energy source. The problem would then remain as to how to slow down that rocket when it reached its destination.

Transcript: Rocket scientists have long been aware that fusion or fission are more efficient energy sources than chemical energy for rockets, and so there has been much experimentation but no totally viable design that’s currently used for any propulsion system. Of course, the environmental climate is such that people are not too happy about having nuclear reactors in space. The energy source is however much more efficient. Gaining energy from E = mc2, even with less than one percent efficiency as happens in the Sun, would mean that the equivalent of one kilogram of fuel was the equivalent of the energy from all the gasoline used in the United States in a year. This is a fantastically efficient way of powering a rocket. Project Rover in the 1960s speculated and built some prototypes for engines that could be used in rockets and demonstrated that they were three to five times better than chemical rockets, but the project was shut down in 1973. Also in the 1960s, project Orion developed plans to send bombs from behind a spacecraft and use the explosions to sequentially push and accelerate the spacecraft to large speeds. No prototype was ever built. In Britain in the 1970s, Project Daedalus developed the idea of injecting pellets of deuterium and helium 3 and then fusing them with electron beams. A prototype on paper of this rocket could have reached Barnard’s Star at a distance of six lightyears in roughly fifty years. Project Daedalus was never funded, and no prototype was ever built.

Transcript: The difficulty of space travel to the stars with conventional rockets is illustrated by the best examples of our technologies so far. The Saturn V rocket, the largest ever built, that took the astronauts to the Moon burnt a mixture of kerosene and liquid oxygen, but it took a rocket the size of a fourteen story building to launch a small, cramped payload with three astronauts to the Moon. Four manmade spacecraft have left the solar system: Pioneer 10 and 11 and Voyager 1 and 2. They are at distances of several billion miles now beyond the orbit of Pluto, but even though they are the fastest and most distant manmade objects humans have ever created, and that they have left the solar system, Pioneer 10 would take about a hundred thousand years to reach the equivalent distance to Alpha Centauri. It’s not actually headed in the direction of Alpha Centauri. It’ll take a few million years before it reaches the vicinity of Aldebaran, a red giant. The enormity of sending a civilization or a large group of people into space is illustrated by a hypothetical example of launching a starship with, say, five thousand people on it. The weight or mass would be a hundred million kilograms. To travel to ten percent of the velocity of light, necessary for interstellar travel, the energy requirement would be ten to the power twenty-three Joules, a hundred times the world’s annual energy usage. Even if we developed an energy source a million times more cost effective than chemical energy sources in terms of dollars per kilowatt hour, the cost of such a mission would be several trillion dollars.

Transcript: The fundamental limitation of space travel for our civilization is connected with the vastness of space and the difficulty of finding efficient energy sources for raising a payload out of the Earth’s gravity and into interstellar space. The entire history of the space age, only about fifty years, is that of chemical rockets. The Saturn V is nothing much more than a large firecracker. Chemical fuel has been used for almost every space age advance, including getting to the Moon, and chemical energy is an energy source that's a thousand or ten thousand times less efficient than fission or fusion as energy sources. Humans have not yet harnessed the most efficient energy sources that they might need to travel to the stars. The fundamental issue is in raising a payload to the velocity of eleven kilometers per second or twenty-five thousand miles per hour. Giving it this much kinetic energy gives an idea of what the energy requirement is in terms of half times the mass times the velocity squared. Conventional rockets need a mass ratio, that is the ratio of payload to fuel, that's one part in forty which sounds impossible. The use of multistage rockets lowers this fraction to one in three or one in four, but the energy requirements to raise large payloads, even into Earth orbit, are stupendous.

Transcript: Large brains and the possibility for communication occurred after a relatively long time of evolution of life on Earth, only after about four billion years of natural selection. It’s interesting to note that for many species on Earth, there is a strict linear ratio between body size and brain size. It’s called the encephalization ratio. It indicates that although humans are unusual in the complexity and sophistication of their brain function, they are not extraordinary in terms of brain size relative to body size. For example, the human brain has a mass of about one kilogram compared to a body mass of a hundred kilograms, but it’s part of a linear chain of species involving primates and mammals, eventually down to birds. The ratio is similar to birds which have brains of mass a few grams and bodies of mass a few hundred grams. This encephalization ratio among all the species on Earth implies that many species may develop large brains given sufficient evolutionary time.

Transcript: The fastest way to communicate through space is with electromagnetic waves traveling at three hundred thousand kilometers per second. But space is vast, and so if civilizations in the Milky Way are rare, then communication becomes a real issue. Civilizations, if sufficiently rare, can be isolated in time and space. In terms of the Drake equation, if the average civilization lasts less than a few thousand years, then on average the distance to the next nearest civilization is a few thousand lightyears. Thus it’s impossible to exchange signals during the lifetime of the civilization. If the pessimists are correct and the number of civilizations in the Milky Way is extremely low, then it’s totally impossible to exchange signals. The universe is vast. There is little prospect of exchanging signals with distant galaxies because the light travel times are tens or hundreds of millions of years.

Transcript: Even though there’s no way to decide logically or with available evidence whether the optimistic or pessimistic views of outcomes from the Drake equation is valid, we can follow the implications of the lifetime of civilizations. Under the optimistic assumptions for SETI, the number of intelligent, communicable civilizations is roughly equal to the lifetime of the civilization in years. Civilizations scattered through the Milky Way galaxy become isolated in time and space. They can communicate only with light signals or other electromagnetic waves. If a civilization on average lives about a hundred years, then the typical distance to the next nearest civilization is a thousand lightyears. Thus, the civilization will be dead before it has time to exchange a return signal with a neighboring civilization. If, on the other hand, the civilization lasts a million years, the average distance between such civilizations in the Milky Way is only a few lightyears. Thus there is plenty of time for them to exchange round trip signals with nearby civilizations. Thus, communicability of civilizations is related to their longevity and their isolation in space within the Milky Way.

Transcript: Astronomers who are pessimistic about the prospect of intelligent, communicable civilizations in the Milky Way tend to take low values for the factors in the Drake equation. For example, they assume that not every solar star has habitable planets, and that the number of habitable planets per typical solar system is less than one, also that life does not evolve on every habitable planet, and that even if life exists, it does not always end up as intelligent or with the capability for technology. Using pessimistic or low values for the factors in the Drake equation, the end result is something like the number of communicable civilizations equals ten to the minus nine or ten to the minus six times the lifetime. Thus, even for a very long-lived civilization in the Milky Way galaxy, we may be unique as the only intelligent civilization.

Transcript: Astronomers who are optimistic about the prospect of intelligent civilizations in the universe tend to take high values for the numbers in the Drake equation. For example, assuming that the fraction of stars with planets is close to a hundred percent, that there is at least one habitable planet per star, and that when a habitable planet exists not only does life develop but intelligent life develops eventually, then intelligent life with technology will naturally want to communicate through the universe. With optimistic values for the factors in the Drake equation, the end result is that the number of intelligent, communicable civilizations is roughly equal to L, the lifetime of a civilization. Thus, if civilizations live ten thousand years, there may be about ten thousand in the Milky Way galaxy. If they live a million years, there are a million civilizations in the Milky Way galaxy, and so on. Dr. Christopher D. Impey, Professor of Astronomy, University of Arizona

Transcript: The final factor in the Drake equation is a time given in years, the average lifetime of a civilization in the intelligent and communicable state. In other words, we are interested not just in the creation and existence of civilizations but their duration, and not just the duration of a civilization of intelligent creatures but their duration in a state where they can communicate in space. Human civilizations are about ten thousand years old, but we’ve only had the technology for space travel and communication through space for about fifty years. Thus, an optimist may say that civilizations, once they develop, have the duration of thousands of years to do the enterprise of space travel and space communication. A pessimist might note that within a few dozen years of human technology developing bombs, we were on the verge of blowing ourselves up. So perhaps civilizations do not endure, and perhaps this number is a small number of years. We have no idea what the true number is in a cosmic sense.

Transcript: The sixth factor in the Drake equation is the fraction of planets that have intelligent life where technology develops, in particular the technology that allows for communication through space or for space travel. As with the previous few factors, we are completely uncertain as to how to estimate this number. Again, we have only ourselves to go on. We know that in one case, on Earth, intelligence did lead to technology and the ability to explore space and receive and send electromagnetic signals. However, we share the planet with other intelligent species such as orcas and other apes that have never developed such technologies and might never even given sufficient time. Optimists might place this fraction at a high level if intelligence beyond a level of ours necessarily leads to technology, but if it’s not an inevitable consequence of technology, the fraction could be much lower. Once again, we simply do not know.

Transcript: The fifth factor in the Drake equation is the fraction of habitable planets where life exists where intelligence also develops. In other words, how likely is it that intelligence evolves given the formation of life itself? Once again, we have no scientific way to answer this question. We know that on Earth, life evolved intelligence. We also know that it took quite a long time to do so, several billion years. Optimists would say that intelligence is a natural adaptive outcome of natural selection and biological evolution and might place this fraction as high as unity. However, pessimists would say that intelligence was an outcome only for a handful of species among hundreds of millions on Earth, and even so it took a very long time to develop. Given less time, say, perhaps because of a shorter lived main sequence star, life may never develop intelligence. So, we are uncertain about this factor.

Transcript: The fourth factor in the Drake equation is the fraction of habitable planets on which life actually develops. This is the first factor we reach in the Drake equation where the answer is simply not known based on scientific investigation because we only have the example of life on Earth to go by. In other words, given a habitable planet, one on which liquid water could or does exist, which is in the habitable zone of any kind of star, does life always develop or not? Optimists would place this fraction at one, saying that life is an inevitable given suitable conditions, but the pessimists might place a much lower number on this fraction of, say, one percent. In truth, we do not know.

Transcript: The second factor in the Drake equation is the fraction of stars that have planets around them. This is a pure astronomical number. We will know the answer to this within a few decades at the most, and we are getting our first indications of what this fraction may be already. Astronomers have only so far been successful in looking for large planets, more like Jupiter and Saturn than the Earth, Mars, or Venus. The techniques are improving all the time. At the moment, the fraction of stars that are like the Sun with planets that are like Jupiter or Saturn is twenty or thirty percent. Given the sensitivity of the techniques this number could increase, but it’s probably not a hundred percent. On the other hand, the fraction of stars that have Earth-like planets is unknown. We do not have sensitive enough techniques to determine this number. Typically, an upper bound to this number may be a hundred percent. Earth-like planets could be ubiquitous, and a lower bound is typically ten percent or maybe five percent.

Transcript: The first factor in the Drake equation is the current rate of star formation in the Milky Way. Stars are being born, and stars die all the time. At any given time in the history of the Milky Way, it's the rate of star formation that's related to the number of sites for planets to form and life to potentially develop. In the Milky Way with roughly forty billion stars and an age of roughly ten billion years, the ratio of these two numbers is four. Given that the rate of star formation has increased over the time span of the Milky Way, the maximum bound on this number may be ten. So this number is known within a factor of two, but bear in mind that this accommodates the formation of all stars in the Milky Way, regardless of their likelihood of life forming around them or planets forming around them.