Are We Alone? Hints

Haven’t we all looked at the night sky full of stars on a clear, cloudless night and wondered with awe and inspiration if there are other living beings, just like us, in a distant galaxy, far, far away?1

This age-old question has intrigued human beings throughout history. Today, despite the great advances in telescope and satellite technologies, the search for extraterrestrial (ET) life is still in its infancy and no one has found life anywhere other than on the Earth. This does not mean, however, that the universe is devoid of life other than our own. The recent discovery of exoplanets — planets that orbit a star other than the Sun — has resulted in a rapid and substantial progress in the field of astrobiology — the scientific study of origin, evolution, distribution and future of life in the Universe — and some tantalizing hints have started to emerge.

In this article, my goal is to recount some of the recent efforts made to establish whether life exists elsewhere in the universe. A commonly used definition of life is “a self-sustaining system capable of Darwinian evolution.” Interestingly, some scientists believe that we will never fully understand what life actually is until an alternative is found elsewhere that is based on the “same DNA, metabolism, and carbon-based blueprint shared by all known life on Earth.” Finding evidence of alien life or the life that does not originate in the Earth, no matter how elementary in form it is, would undeniably represent a fundamental revolution in science. This could be as simple as a single-celled organism, or could even be the hypothetical beings found in science fiction literature who appear from civilizations far more advanced than ours. However, finding evidence of a cosmic civilization is challenging because scientists do not know what the signature of such a civilization could be like. Nevertheless, if intelligent life exists, is there a way of communicating with them? Is interstellar travel feasible? Finally, what is the future of life on Earth and beyond? These are some of the most fascinating questions that faces humanity and some of these will be discussed later in this article. But first, instead of trying to predict the make-up of a possible signature of an extraterrestrial civilization, we will take the different route initially: try to gauge what are the odds that intelligent alien life exists out there in the cosmos. This is exactly what astrophysicist Frank Drake did in 1961 and as we will see shortly, this provides amazing insights to the central questions of this article: are we alone in the universe?

Drake Equation

Frank Drake developed an empirical equation to estimate the number of advanced civilizations (N) likely to exist in the Milky Way galaxy with whom radio communication is possible. The Drake equation, as it has become known, is a seven-variable equation (Figure 1) that has no unique solution. Nevertheless, it is a generally accepted tool for the scientific community to obtain a reasonable estimate for N, but one has to make some drastic assumptions regarding the values of its variables.

The Drake equation may appear daunting at the first glance, but it is really a straightforward and clever “approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy”. Drake equation’s real worth is perhaps not in the answer it gives, but in the insights it provides, when one tries to plug in values for the various terms. The first three terms — R_*, f_p, and N_e are fairly well known to astronomers, thanks to the recent advances in telescope and satellite technologies. For example, R_*, the average rate of star formation per year in our galaxy, is about 1 star per year, contrary to Drake’s originally suspected ten in 1960 based on the data available at that time. Similarly, the estimates for f_p, or the fraction of those stars with planets, is pretty well known now: 0.4 or 40%. Finally, the current estimate for N_e, the average number of those planets that may develop an ecosystem, is between 0.5 and 2, contrary to Drake’s original estimate of 2.

Figure 1: Drake Equation. Credit: SETI Institute web site

However, trying to estimate values for the last four terms of the equation gets trickier as much more research and data are needed to provide reasonable answers. f_l, the fraction of those planets that succeed in developing life, is completely unknown. Is life inevitable, or is it a lucky anomaly? This is a hotly debated philosophical question as we shall see soon, but Drake believed that formation of life is inescapable under suitable conditions; he set f_l to 1.

Far more difficult to answer is the question, what fraction of those planets develop intelligent life, f_i. Drake put down 0.01, but this really is anybody’s guess and modern science can do no better. It is possible that Drake could be considerably off the mark here as some people argue, based on the fact that only one species evolved to be intelligent (possessing interstellar communication technologies) out of the millions found on the Earth.

Also unknown is the value of the penultimate term, f_c, or the fraction of those planets with intelligent life that develop interstellar communication capabilities. Drake guessed 0.01 or 1%. But nobody really knows for sure until other alien civilizations are detected.

The last term, L, the average length of time such civilizations survive and continue to send communications, is perhaps the most interesting and we will defer discussions about it until later. Nevertheless, using his own equation, Frank Drake’s current estimate for communicative civilizations in our Milky Way Galaxy comes out to be around 10,000. But your guess may be as good as his! In fact, you can use Drake’s equation to make your own estimate by visiting the PBS website.

Drake Equation Revisited

With the discovery of exoplanets — we will discuss exoplanets later in this article — more precise estimates of some of the factors in Drake equation were attempted and several improvements of the equation were proposed over the years. Here I will mention one recent approach by Frank and Sullivan (Frank & Sullivan, 2016), who showed how to completely eliminate the least known term L — how long civilizations might survive — by simply expanding the equation. This allowed them to answer the ‘cosmic archeological question’: ‘Have they ever existed?’, rather than the answer that the Drake equation gives to the usual question of narrower scope, ‘Do they exist now?’”

That still did not get rid of the huge uncertainties in f_i, the fraction of those habitable planets where advanced life could evolve. Rather than trying to guess the value of f_i, Frank and Sullivan proceeded to calculate the odds against it. As a result, a lower limit on the probability that technological species have ever evolved anywhere other than on Earth emerged. How did they achieve this? In their own words, they were able to segregate the “newly measured astrophysical factors from the fully unconstrained biotechnical ones” in Drake’s equation, with the help of the new data obtained from NASA’s Kepler satellite and other searches.

The cosmic archeological question is of particular scientific and philosophical consequence according to Frank and Sullivan. The answer they got is remarkable: humanity is not the only technological intelligence that has evolved in the Universe, provided the probability that a planet in the habitable zones (the region surrounding the host star in which water can remain in its liquid state and where planets could support life) develops a technological species is astonishingly low (\sim 10^{-24}).

Fermi Paradox

A couple of decades before Drake did his work, in 1939, Enrico Fermi, the Italian-born American physicists and Nobel Laureate, one of the chief architects of the nuclear age, asked a question which has since become famously known as Fermi’s paradox: “Where are the extra-terrestrials?” This deceptively simple question is actually based on astute reasoning at a time when astronomers did not have powerful telescopes and satellite technologies at their disposal. Consider this — the Milky Way galaxy is made of billions of stars. The diameter of the galactic disk is 100,000 light years across, rotating once in every 200 million years around the galactic core. The Kepler satellite2 data suggests that there are 2\time 10^{22} stars in the universe, 20% of which have planets that reside in habitable zones with atmospheric temperatures suitable for life to thrive. If the same proportions of planets exist in our galaxy, thousands, or may be millions, of these planets could support life. Our Galaxy is between 10 to 15 billion years old, so some of these planets could house civilizations much older than ours. It is then not difficult to imagine that the spacecrafts built by these advanced civilizations could have swarmed the entire Galaxy, even if they are capable of traveling at only 1% of the speed of light. But there is no evidence of any alien intelligence so far. So, where is everybody?

Reality Check

There are two schools of thoughts and the answer depends on who you ask.

Planetary scientists Carl Sagan and William I. Newman argue it would be improbable for life not to exist somewhere other than the Earth. Scientists who share this view are known as Contact Optimists. Their assertion is rooted in the Copernican principle of mediocrity which states that Earth does not occupy a unique place in the Universe — it is an ordinary planet orbiting an average star in an ordinary galaxy. Also, there is nothing special about life on Earth. Life may have sprung independently at multiple places across the universe, 10–17 million years after the Big Bang, which occurred 13.8 billion years ago. Alternatively, life could have originated in some habitable planets (in our Solar System or outside) and spread via meteoroids. Complex organic molecules could also have been formed in the protoplanetary disk of dust grains surrounding the Sun before the Earth was even formed. Flora and fauna would eventually develop on worlds capable of spawning life; some planets will at least occasionally produce intelligent species through the process of natural selection. But spreading of civilization is a slow process, interstellar travel is difficult if not impossible, and that societies destroy each other before they can reach us (more on this later).

Then there are the proponents of the Rare Earth Hypothesis such as Peter Ward and Donald Brownlee of University of Washington, who believe that complex life is extremely rare in the universe. In their book, Rare Earth (Ward & Boownlee, 2003), the scientists argue that the essential ingredients for life are: a terrestrial planet with tectonic plates located in the habitable zone of a suitable star, a large moon, magnetic field, and oxygen. A massive planet like Jupiter is also necessary as a gatekeeper to deflect killer asteroids. Microbial life may be common in the universe but for complex intelligent life to develop and prosper, an extraordinary set of unlikely circumstances is crucially needed. Life on Earth evolved unusually fast and can be regarded as a cosmic aberration. The evidence gathered from other planets in our Solar System suggests that the chances of life developing in an average Earth like rocky world is exceedingly low.

Ward and Brownlee also reason that advanced civilizations are prone to self-destruct. Such a sentiment is shared by the renowned British theoretical physicist Stephen Hawking, who believes that life on Earth is at a risk of being destroyed by a disaster, such as global climate change, an unexpected nuclear holocaust, a global pandemic or a genetically engineered virus, or even the growing threat of artificial intelligence. The risk is cumulative. Hawking, speaking at Oxford University Union, explained “Although the chance of a disaster to planet Earth in a given year may be quite low, it adds up over time, and becomes a near certainty in the next thousand or ten thousand years. By that time, we should have spread out into space, and to other stars, so a disaster on Earth would not mean the end of the human race.” It is, therefore, not unreasonable to expect that an advanced civilization will face similar existential risks. In fact, the last term in the Drake equation, L, the average length of time such civilizations survive and continue to send communications, is very hard to guess as we have just one example to work with.

Hawking’s view of the future of life on Earth may sound apocalyptic. But the reality is, even if humankind survives all natural disasters, or all self-induced catastrophes, even random celestial events such as impacts by comets or asteroids, life on Earth is not perpetual. It will eventually come to an end with the death of its Sun. In its dying phase in a few billion years, the Sun will become a red giant, absorbing its nearest planets Mercury, Venus, and Earth. But Earth will become uninhabitable much sooner than that. Its oceans will evaporate making the planet a scorched, lifeless wasteland before being finally engulfed by the Sun.


The question then is, how does one establish communication with these extraterrestrial civilizations? The most promising scheme is to search for electromagnetic signals (radio or light) in the sky — beacons that could be “beamed” toward Earth from alien worlds. This can be an inadvertent act (similar to the way humans leak television and radar signals into space) or a deliberate one. Sending signals (called ETI signals) is a low-cost alternative to interstellar travel, because messages travel at the speed of light. By today’s technology standards, interstellar travel could take centuries, even millennia, and the energy requirement for such a long journey is astronomical.

There are many scientists who have dedicated their lives and careers to look for such signals and their projects are known as the search for extraterrestrial intelligence (SETI). SETI projects presently are funded by private donors in the United States but during the mid-1970s NASA became an active participant in SETI but trepidations about wasteful government spending led Congress to terminate these programs in 1993.

None of the SETI experiments have corroborated any ETI signals so far, although many false positives have surfaced and subsequently eliminated on further analyses. In 1977, scientists at Ohio State University discovered the famous Wow signal but ensuing observations failed to find this signal again. Consequently, the Wow signal lost its claim as fabricated by an alien civilization.

The discovery of cosmic objects called pulsars has an interesting history where electromagnetic signals from these objects were initially misdiagnosed as ETI signals. Pulsars3 are neutron stars that emit regular pulses of radio waves and other electromagnetic radiation at rates of up to one thousand pulses per second. Interestingly, the researchers initially could not rule out the possibility that their pulsating signals could be sent by intelligent beings from another world, which eventually proved to be a false. But the research led to a Ph.D. and a coveted Nobel Prize in physics4.

Clearly, discovery of cosmic intelligence is the Holy Grail of science. As Stephen Hawking pointed out “…there is no bigger question…. A universe full of technological civilizations is a very different place from one with only us.” Even the most of the ardent skeptics agree that it is important to know whether there is life beyond the Earth, and interestingly, nearly two-thirds of Americans believe that some form of alien life exists somewhere in the universe. Although none has been found so far, it does not mean there is no hope at all.

We will now discuss two recent discoveries in the field of astronomy that has caused much excitement and opened up new areas of research. The phenomena I am about to describe now are most likely produced by perfectly natural astrophysical phenomena — something that no theory has conjured so far. But for the time being they are unexplained and have aroused much interest among the SETI researchers around the world.

Boyajian’s Star

In 2016, the discovery of an extraordinary star, KIC 8462852, was announced. The star was observed by Kepler during its prime mission and first noticed by citizen scientists as part of the Planet Hunters project. Unofficially known as “Tabby’s Star” or “Boyajian’s Star” (Boyajian, et al., 2016), the star did something that was never seen before — its brightness dimmed at irregular intervals (up to 22% of its brightness), with variable timescales on the order of days.

Further analyses confirmed that the dipping signals in the data were astrophysical in origin, not caused by instrumental errors nor were they artifacts of data processing. Could a black hole5 be the cause? Unlikely. A nearby black hole’s massive gravity would cause the star to wobble, but such characteristic quivering has not been observed. Besides, the black hole would act like a lens, actually brightening the light detected by Earth-based telescopes. Spots like Sunspots? Unlikely. Spots on the Sun do cause drops in brightness, but the extent of dimming in Boyajian’s Star is much more than that. Family of light blocking comets? The comet swarm hypothesis is plausible for some short-term dips, but very unlikely for long lasting dimming that astronomers witness on the Earth.

Interestingly, some scientists have attributed the dimming as an artifact of star-light occlusion by an ‘alien megastructure’ such as a “Dyson sphere”6 orbiting the Boyajian’s Star. But exotic explanations should be the “explanation of last resort” after every other possibility has been ruled out (Wright, Cartier, Zhao, Jontof-Hutter, & Ford, 2015). The problem with an alien explanation is, we do not know how to model the nature of artificial structures or the likelihood of detecting them. Meanwhile Boyajian’s star is receiving a great deal of scrutiny from various observatories around the world, because, according to Wikipedia, “it remains an outstanding SETI target because natural explanations have yet to fully explain the dimming phenomenon”.

Fast Radio Bursts (FRB)

In 2007, Duncan Lorimer and his colleagues (Lorimer, Bailes, McLaughlin, Narkevic, & Crawford, 2007) at West Virginia University discovered an unusual signal buried in the 2001 historical archives of the Parkes radio telescope in Australia. The signal was in the form of a 5-millisecond radio burst that arrived on 24 August 2001 from an unknown source in deep space, billions of light years away. But no more subsequent bursts appeared and the initial excitement faded. However, general acceptance of the Lorimer bursts, also known as the “fast radio bursts” (FRBs), came after similar bursts were noticed by observers using the Arecibo radio telescope in Puerto Rico (Spitler, et al., 2014). FRBs have a characteristic dispersion — the high-frequency waves arriving in the detectors a few hundred milliseconds before the low-frequency ones, just like the way frequencies change in a slide whistle, where the high notes turn into low in a few thousandths of a second. Interestingly, the Arecibo signal, called FRB 121102, was found to repeat; its unusual nature prompted astronomers to study it further using the Very Large Array (VLA) in New Mexico. Eventually other radio arrays were able to pick up more FRB’s from the sky.

So, the question is, what are the FRBs? The brevity of the signal duration imply that the source is most likely a compact object that discharges an enormous amount of energy — a stellar-mass black hole, or a neutron star with diameter no more than a few hundred kilometers. But nobody knows for sure, and a long list of alternate sources have been proposed. They range from merging black holes, flares on magnetars7, to even ETs (see Appendix I to see why)!

Seth Shostak, the Senior Astronomer at the SETI Institute has this to say about FRBs: “… given the strange radio signature of FRBs, it’s tempting to wonder if they could be screeches transmitted by intelligent beings. That’s not impossible: The raw ingredients for life, including habitable planets, were certainly in place many billions of years ago.” He then warns: “You can cook up an alien explanation for just about any sort of signal you discover. Extraterrestrials have been given credit for pulsars, quasars, and lots of other odd celestial behavior. But while extraterrestrials are easy to profile, they are — and should be — hard to convict. There’s not been a single case in which aliens were responsible for any new cosmic mystery8.”

Promising planetary finds

We now turn to the more realistic possibility in the near term — to predict or actually find the existence of any life form by analyzing the gases in the exoplanet atmosphere. Remember, astronomers can only “see” and measure the colors of light from the orbiting exoplanet’s star. Some of this light is absorbed by exoplanet’s atmospheric molecules. These absorption features serve as unique “signatures” of the type and quantity of molecules present in the exoplanet’s atmosphere as different molecules absorb different colors of light.

The necessary ingredient for life, as we know it, is water. Recent observations by planetary probes together with ground and space based telescopes have shown that water is commonplace throughout our Solar System and in the Milky Way galaxy. Oceans of liquid water were detected beneath the icy crusts of two of Jupiter’s moons, Europa and Ganymede. Liquid water was also detected in Saturn’s moons Enceladus and Titan. Water appears to be in direct contact with the rocky seabed in all of these moons. Billions of years ago, Mars was also covered by oceans. The seasonal dark streaks observed today on the Red Planet’s surface may be caused by salty flowing water. All of this raises the possibility that complex chemical reactions, that eventually spawn life, are in play.

Observations by NASA’s Kepler space telescope suggest that majority of stars in the sky have planets, many of which may be habitable. Indeed, Kepler’s data has shown that rocky worlds like Earth and Mars are probably more abundant throughout the galaxy than gas giants such as Saturn and Jupiter. Many more await discovery in the coming years. And that’s just in our Milky Way galaxy. Who knows, some of them could even harbor intelligent life and awaiting discovery as our detection techniques and scientific thinking progress with time.

The goal of exoplanet hunters such as Sara Seager of the Massachusetts Institute of Technology is to detect temperate, Earth-like exoplanets that are appropriate for atmospheric characterization. Inspired by Drake’s equation, Seager has recently proposed her own equation9 to focus simply on the question of whether any alien life is present, not necessarily the technologically advanced kind. Instead of trying to assess the chances of finding radio capable civilizations, as Drake’s equation was designed to do, her new equation evaluates the chances of detecting signs of life on exoplanets by signs of biosignature gases. According to her calculations, two inhabited planets could reasonably come to light during the next decade.

Why is it important to focus on exoplanets? Because “our current understanding of life’s origin on Earth indicates that given a suitable environment and sufficient time, life will develop on other planets”10 as well. Thus, it is important to identify these Earth-like exoplanets in the hunt for alien life. Many scientists and philosophers had intuitively assumed planets existed beyond our solar system. But it took a while to identify them. That started happening in the mid-1990s, only when our technology and scientific thinking became sufficiently mature. In fact, since 2009, NASA has been hunting for Earth-like planets that could potentially harbor life. At the time of writing this article, NASA’s Kepler space telescope has helped identify 20 most Earth-like worlds among more than 4,000 exoplanet candidates. The number is growing steadily every day. These Earth-like exoplanets are located inside the habitable zones (also known as Goldilocks Zone11) of their Sun-like stars — signifying the likelihood of liquid water splashing on their rocky surfaces.

A great example of a planetary system that is providing humanity with its first opportunities at discovering evidence of biology beyond our Solar system, was discovered recently by a team of international scientists using a network of ground and space based telescopes. The system, consisting of at least seven Earth-sized planets orbiting a red dwarf star called TRAPPIST-1, is 12 times less massive than the Sun and only slightly larger than Jupiter, lies in the constellation of Aquarius, located 39 light years away from us. If you want to know more about the seven worlds of TRAPPIST-1, click here.

The search for exoplanets will receive an immense boost after the Transiting Exoplanet Satellite Survey (TESS) and the James Webb Space Telescope (JWST) are launched around 2018. Their state-of-the-art design is particularly suitable for finding and characterizing Earth-like planets orbiting small stars. Specifically, TESS will be used to identify the rocky planets and the JWST to observe the planetary atmospheres during their transits or during secondary eclipses. If everything goes according to plan, scientists should be able to infer signs of life on those planets. Perhaps finding signatures of life outside our Solar System may not after all be so farfetched in the next decades. So, stay tuned.

Interstellar Travel

With the discovery of exoplanets, the question that naturally comes to mind is, will we be able to visit some of these exotic worlds one day? Scientists and astronomers do not expect this to happen any time soon because of the enormous distances that separate us from the stars. According to Special Relativity, no usable information can travel faster than the speed of light, and hence it would take centuries, even millennia, to travel between stars. Today’s fastest rocket would take 30,000 years to reach the Alpha Centauri which is the star system closest to us, being 4.37 light years away.

However, there is real hope now of sending unmanned probes that could transmit pictures and other information back to the Earth within a generation.

A team of astronomers working with the European Southern Observatory’s (ESO) 3.6-meter telescope at La Silla, Chile, along with other telescopes around the world, reported discovery of a planet orbiting our Sun’s nearest neighbor, Proxima Centauri. Proxima Centauri is a red dwarf star located in the constellation Centaurus and only 4.23 light-years away from the Sun. Its planet, dubbed Proxima b (Anglada-Escudé, et al., 2016), is at least 1.3 times the mass of the Earth and orbits its parent star with a period of 11.2 days and maintaining an average distance which is 20 times closer than Earth is from the Sun. Despite the close proximity to its parent star, Proxima b actually rides in the crucial habitable zone. This is because Proxima Centauri is cooler, with luminosity only 0.15% of the Sun, which means that its planet is warm and can probably support liquid water on its surface. But Proxima b is likely to be tidally locked with its parent star with the result that one of its sides perpetually faces its star, just as the moon does to Earth. And as a result of its closeness to its parent star, Proxima b could be blasted with stellar flares which are detrimental for life to evolve.

Nevertheless, the discovery of Proxima b has opened up the thrilling possibility that one day humankind will be able to send a probe to this exoplanet. In fact, this is what Stephen Hawking and Yuri Milner’s ambitious Breakthrough Starshot project12 is aiming to do — send nanocrafts (ultra-light unmanned space craft that travels at 20% of the speed of light) within a generation to our nearest star system, the Alpha Centauri13. Their mission: “Seek scientific evidence of life beyond Earth, and encourage public debate from a planetary perspective.” Light from an Earth based phased array of lasers will propel the nanocrafts attached with light sails to 20% the speed of light. During the quick flyby, the nanocrafts will take pictures of Proxima b and make other measurements. The collected data will be beamed back to the Earth, reaching the Earth in 4.23 years.

Is humanity eternally destined to belong only to the Earth? Or will we be able to reach the stars one day and even build colonies elsewhere in our galaxy? Humanity must continue to venture into space for the future of humanity. The journey has just begun.

  1. This essay is an adaptation of my blog post “Are We Alone?” The original article can be found here.
  2. The Kepler spacecraft is a space observatory launched by NASA on March 7, 2009 to discover Earth-size planets orbiting other stars. Kepler was very successful at finding exoplanets, but its extended mission in 2013 was marred by failures in two of the four reaction wheels when the telescope could not be pointed accurately. In November 2013, a new mission plan named K2 “Second Light” was proposed and in early 2014, the spacecraft underwent successful testing for the K2 mission. On December 18, 2014, NASA announced that the K2 mission had detected its first confirmed exoplanet, a super-Earth named HIP 116454 b. The Kepler mission has so far found 4,696 candidate exoplanets, 2,331 of which are confirmed. The number of confirmed exoplanets, less than twice the size of the Earth, and riding in the habitable zone, is 21. The K2 mission, on the other hand, has detected 458 candidate exoplanets of which 173 are confirmed.
  3. A dead, collapsed star consisting mostly of neutrons and is only about 20 kilometers in diameter.
  4. Antony Hewish was awarded (jointly with fellow astronomer Sir Martin Ryle) the 1974 Nobel Prize in Physics for his work in identifying pulsars as a new class of stars. His student, Jocelyn Bell, was ignored by the Swedish Nobel committee in spite of her stellar contributions. She obtained her Ph.D. degree from the University of Cambridge in 1969. Pulsars appeared in the appendix of her dissertation.
  5. An astronomical object whose immense gravitational field entraps everything, even light, that gets too close (closer than the black hole’s event horizon).
  6. A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures most or all of its power output. Freeman Dyson, the English-born theoretical physicist and mathematician most famous for his work in quantum electrodynamics came up with the concept of the megastructure bearing his name. Earth’s non-renewable sources, such as fossil and nuclear fuels, will be exhausted in the not-to-distant future and to sustain the future growth, the human race (or for that matter, any alien civilization) will need to capture much more of the Sun’s light. It could do this with a Dyson Sphere.
  7. Type of neutron star with an extremely powerful magnetic field. The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.
  8. Quote taken from the online article “Blame it on the Aliens” by Seth Shostak, the Senior Astronomer at the SETI Institute.
  9. The Seager equation looks like this:
    N =N_*\cdot F_Q\cdot F_{HZ}\cdot F_O\cdot F_L\cdot F_S
    N = the number of planets with detectable signs of life
    N_* = the number of stars observed
    F_Q = the fraction of stars that are quiet
    F_{HZ} = the fraction of stars with rocky planets in the habitable zone
    F_O = the fraction of those planets that can be observed
    F_L = the fraction that have life
    F_S = the fraction on which life produces a detectable signature gas.
  10. Quote taken from the SETI Institute web site.
  11. The region around a star in which temperatures would allow a planet to have liquid water.
  12. The Breakthrough Initiatives were founded in 2015 by Yuri and Julia Milner to explore the Universe. It is a $100 million research and engineering program aiming to demonstrate proof of concept for a new technology, enabling ultra-light unmanned space flight at 20% of the speed of light. Stephen Hawking, along with luminaries Lord Martin Rees, Ann Druyan, and Frank Drake participated in the launch of project “Starshot” for Interstellar Space Exploration. The program is led by Pete Worden, the former director of NASA AMES Research Center, and advised by a committee of renowned scientists and engineers. The board consists of Stephen Hawking, Yuri Milner, and Mark Zuckerberg.
  13. Alpha Centauri is a three-star system and the sun’s closest stellar neighbor. Alpha Centauri A and Alpha Centauri B are the two main stars that form a binary pair. Their average distance from the Earth is about 4.3 light-years. The third star, Proxima Centauri, is about 4.23 light-years away from Earth; thus, it is our closest star other than the sun.

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