What do you believe? Do you believe in fate? In love? In God? Do you believe in evolution? Global warming? The big bang? 

Our beliefs — those things we hold to be true — are a central part of what defines us. They shape our lives in ways seen and unseen. They form a foundation for our ethics, values, and even our political views.

There is a popular idea among scientists that belief is not a part of science. One does not believe in evolution, one understands evolution, as if the mere comprehension of natural selection ensures one’s acceptance of evolution. If you don’t believe in evolution, you simply don’t understand it. But that’s nonsense. One can understand a concept without accepting its validity, and people can and do choose not to believe in evolution. People believe in creationism. People believe the Earth is flat. They believe there is a divine creator, or that there is no god. Those beliefs are a part of their identity, and we cannot simply declare their beliefs to be invalid. The central freedom anyone has is a freedom of thought.

The reluctance to speak of belief in science stems, I think in part, from the fact that that it is often used by trolls and the like to paint science as a religion. If scientists believe in evolution then it is no different than a belief in the Holy Trinity or the Great Pumpkin, and can be dismissed as mere dogma. In this view all beliefs are statements of faith, made piously in the absence of evidence. Blessed are they that have not seen and yet believe, as Christ admonishes doubting Thomas. Thus, changing one’s belief is a sign of weakness. It demonstrates a tragic loss of faith.

But there are central beliefs (tenets if you prefer) of scientific adherents. A belief that the cosmos has (at least in part) an objective reality, and that humans have the ability to understand that reality, though incomplete it may be. A belief that, despite its many flaws, the scientific method of observation and experimentation allows us to build a confluence of evidence that brings to light an emergent truth. These are not controversial beliefs, and they are held by scientists all over the world, whether they be atheist or devout, and regardless of their political persuasion. Thus, evolution, global warming, and black holes are a part of that emergent truth. Like most scientists I believe them to be true, but it is a conditional belief, supported by the scientific evidence we currently have.

With the recent March on Science this weekend, there has been a great deal of discussion about science and politics. Is science inherently political? Should it be? Or should it strive to be neutral? Individually, scientists can be politically active, and many loudly proclaim their views. As debates over the science march and related issues have demonstrated, even scientists don’t agree on their politics. But one thing they do agree upon is that the cosmos has an objective reality, and humanity is best served when we listen to what that reality teaches us. To my mind, our political discussions should start with those lessons. We should start with a recognition of the scientific evidence we currently have. If we hold that to be common ground, our political debates will still be fierce, but they will lead to the betterment of us all.

At least that is what I believe.

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More than 3 billion years ago, life appeared on planet Earth. We aren’t entirely sure how life arose on Earth, but a common idea is that it first formed around hydrothermal vents in our planet’s young oceans. These vents provide not only the thermal energy necessary to sustain life as we know it, they also provide a rich source of chemical compounds useful in forming living organisms. Even today deep sea hydrothermal vents are rich with a diversity of life. If such vents were the home of Earth’s tree of life, similar vents on other worlds might foster life as well. 

We normally think of possible alien life as existing on Earth-like worlds, with warm sunlight and firm ground, but life could actually be more common around the icy moons of a cold gas planet. That’s true even if we limit ourselves to life similar to terrestrial life, which is carbon based and dependent upon plentiful water.

Water on Europa and Earth compared. Credit: Kevin Hand, Jack Cook, Howard Perlman.

For one thing, water is far more common in the outer solar system than on Earth. Jupiter’s moon Europa, for example, has much more water than Earth. Ganymede, another Jovian moon, has about 70% more liquid water than Earth. Water is an incredibly common molecule in the solar system, and without the intense heat of the Sun to drive water away from these worlds, they keep much of their water. If these worlds were small planets, their water could have frozen into ice long ago (although even lonely Pluto hints of having liquid water beneath its surface).  But these are moons orbiting a massive planet, and the gravitational stress upon them helps to keep their interior warm. We know that life could survive in the oceans of these worlds, but could it arise?

New evidence suggests that it’s possible, at least for Saturn’s moon Enceladus. We’ve known that Enceladus has liquid water in its interior for a while. Water geysers have been seen erupting from the moon’s surface. But in October of 2015 the Cassini spacecraft flew directly through some of these plumes, giving the probe direct access to their composition. Chemical analysis found high levels of H₂, which is likely produced by hydrothermal vents within its interior. In other words, the interior ocean of Enceladus is quite similar to the early oceans of Earth.

Even if the conditions are right for life to form on Enceladus, it might not have had the time. We aren’t sure how old the moon is. It’s surface is relatively young, though that could be due to its thermal activity. Some computer models of the orbital dynamics of Saturn’s moons imply that all the moons closer to Saturn than Titan could be quite young, and Enceladus could be only 100 million years old. But other icy moons could have similar hydrothermal vents and are much older. Europa, for example, is about the same age as Earth and has liquid water. If it has vents similar to those on Enceladus, life could have formed on Europa long ago.

Paper: J. Hunter Waite, et al. Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes. Science Vol. 356, Issue 6334, pp. 155-159 DOI: 10.1126/science.aai8703 (2017)

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I haven’t been writing as many posts of late. Part of this is due to the usual time constraints of work and family, and part of it is due to a new large project that’s been ramping up recently. But part of it is due to the limitations of my own writing. 

I can’t say too much about the new project, but one of its central goals is to convey some of the emotions behind scientific discovery.  The sublime beauty of a theory, or the numinous sense of cosmic scale. It’s hard to do, particularly if you want to move beyond the trite passages that are used far too often. So I’m having to learn new ways of expressing ideas, which is a challenge. It’s so much easier to fall into the old pattern of simply explaining ideas and reasoning.

As we’ve all seen recently, our emotional reaction to facts and ideas is often more powerful than the facts and ideas themselves. We are not just thinking creatures but feeling ones, and sometimes we need to plant emotional hooks before we can convey scientific understanding. We also have to be careful not to overuse an emotional ploy, we fall into the trap of equating feelings and facts. There’s a balance that can be reached where scientific knowledge and emotional connections can reinforce each other. Some science writers achieve that balance masterfully, while others such as myself only aspire to find a balance of their own.

Long story short, I’m focusing a bit more on learning and a bit less on writing at the moment, so I beg your patience as I strive to overcome the tyranny of words.

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There’s a lot of potentially habitable worlds in the Universe, and yet we haven’t found any evidence of intelligent civilizations other than our own. Why is that? Lot’s of ideas have been proposed, such as the idea that aliens are being intentionally silent, or that intelligent life kills itself off in a short time. But another idea is simply that we’re the first civilization to appear. Someone has to be first, so why not us? 

Most stars are actually much smaller than our Sun.

It’s generally thought that the existence of intelligent life should become more likely over time. As the Universe evolves, more heavy elements are created and become available, and stellar systems with heavy elements (like our solar system) are more likely to form.  Life also takes time to arise and evolve, and over time it has a greater chance of achieving the complexity necessary for intelligence. So it seems reasonable that the odds of sentient life increase with cosmic age. Of course, after trillions of years star production will have died off, and even small red dwarfs will start to cool and fade, meaning that the likelihood of life arising at that point is basically zero. So somewhere between the big bang and the ends of time there should be a period of time where intelligent life is most likely to evolve.

A new paper looks at just when this “peak sentience” might occur. In this work they formulate an equation calculating the probability for life to form on a potentially habitable planet in a particular volume of space. It’s similar to the Drake equation, and includes similar factors such as the number of stars, and the number of habitable planets, but looks at how the overall probability changes over time. All things being equal (and only assuming life similar to that on Earth) the equation predicts that life is most likely to arise about 10 trillion years from now around small red dwarfs. In the grand scheme of things, the appearance of life on Earth occurred quite early, so we might just be the first civilization to arise.

All that said, there are reasons not to take this work too seriously. Key to the conclusion is the idea that all things are equal. Specifically that potentially habitable planets around small red dwarfs are just as likely to have life than Earth-like planets around Sun-like stars. That skews the data a bit, because small red dwarfs are much, much more common than stars like our Sun. But red dwarfs are also known to have large solar flares that could seriously harm any life on a close planet, and red dwarfs are so cool that habitable worlds would need to be very close to the star. So close that they would likely be tidally locked, with one side always facing toward the star. It’s quite likely that red dwarfs aren’t very life friendly, so they really shouldn’t be included in the tally. If you just include Sun-like stars, then the peak occurs roughly around now, which would mean life on Earth could be rather typical, and arose at a pretty typical time. So this work doesn’t answer the question of where life is out there as much as it raises an interesting question about the origin of life over time.

Still, it’s fun to imagine that trillions of years from now an alien species might find remnants of a great intergalactic civilization they refer to as the first ones, never knowing that we called ourselves human.

Paper: Abraham Loeb, et al. Relative Likelihood for Life as a Function of Cosmic Time. arXiv:1606.08448 [astro-ph.CO] (2016)

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After the Big Bang when the Universe was a dense fireball it began to cool. It now has an average temperature of about 3 K, but there was a time when it had a temperature of about 273 K and 373 K. In other words, the average temperature of the cosmos was just the right temperature for liquid water to exist. Since liquid water is necessary for life as we know it, this raises an interesting question. Could life have arisen in the early Universe? 

This period is known as the habitable epoch of the early Universe, and it existed between 10 to 17 million years after the Big Bang. While it was the right temperature for liquid water during that time, that doesn’t mean that liquid water existed. The first elements of the Universe were primarily hydrogen and helium. To produce oxygen necessary for water, very young large stars would have needed to fuse oxygen in their core, then exploded as a supernova within the first 10 million years of the Universe. If such a thing did occur, it would have been extremely rare. Forming water from oxygen and hydrogen is pretty easy in space, but there would need to be sufficient water and other matter for it to gather in liquid form, rather than vapor.

Then there is the issue of time. The epoch itself only spans 7 million years, which isn’t nearly long enough for complex life to evolve (at least if Earth is a reasonable example). Add to this the fact that life also needs other elements like carbon and nitrogen in addition to water, and it doesn’t look particularly likely.  It is, however, and interesting example of how life might have arisen in ways we wouldn’t expect. We think of life as evolving around a typical star when the Universe was already billions of years old, but in the earliest cosmological moments it’s possible that life, uh, found a way to arise.

In a recent paper presenting the idea, the purpose was not to argue that such early life was likely, but rather as a discussion of the anthropic principle.  The anthropic principle comes in many forms, but one of the more controversial versions argues that if the various parameters of the cosmos were different then life wouldn’t arise. It’s almost as if the structure of the Universe was specifically tweaked for life to exist. But if life could arise in the early Universe, in period radically different from the present Universe, then it shows that life isn’t as delicate as we might think. A different set a cosmic parameters could allow for life to arise in radically different ways.

While I don’t think it’s likely life appeared just a few million years after the Big Bang, it is an interesting idea. It’s also a great example of why we shouldn’t presume that the story of life on Earth is the only story life could have.

Paper: Abraham Loeb. The Habitable Epoch of the Early Universe. International Journal of Astrobiology 13 (4): 337–339 (2014) doi:10.1017/S1473550414000196

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There’s been a flurry of discussion about academic diversity recently. This time its about the lack of conservatives in universities, but it could just as easily be about gender, or ethnic and cultural backgrounds, or economic diversity. But why should we care about diversity at all? Isn’t science supposed to be a meritocracy? 

On some level science is a meritocracy. If we think of science as a competition of ideas and theories, then the best and most useful theory should (and usually does) come out on top. Even the most cherished and time-tested model is discarded if new evidence and new ideas leads to a better one. Our process of peer review is designed to filter out weak models and weak evidence. This merit-based approach has been wildly successful.

But while science strives to be fair and unbiased in its testing of ideas, the process is colored by the fact that scientists are human. We all approach the world with a perspective created by our personal experiences, and those experiences are deeply shaped by our socioeconomic, gender, racial and cultural heritage. No amount of scientific training will change that fact. While we can use scientific methods to filter the good scientific ideas from the bad, the origin of those ideas is still deeply dependent on the human equation. Quite simply, the wider we cast our net, the better science and all of us will be served.

Diversity is not a game.

Unfortunately much of the argument about diversity (pro and con) seems to treat diversity as a game of Pokemon, where the goal is to “catch ’em all.” People who are considered “diverse” are hired for their status, then pushed into the sidelines until they are needed for a Pokebattle. Suddenly a wild committee appears! Hispanic woman I choose you! When diversity is treated as a checkbox it is worse than useless. Not only do you not cast a wider net of perspectives and ideas, you reinforce the view that diversity is worthless. “We hired three racial minorities and they failed to succeed. Typical.”

In order for diversity to succeed we have to connect to a wider diversity of people and perspectives. We need to be challenged by ideas very different from our own, and we need to listen. While increasing the diversity of scientists can allow these kinds of connections to flourish, we should be careful not to focus on satisfying checkboxes. Instead we should focus on ways to build wider connections, and provide opportunities for people with a wide range of backgrounds to flourish. The more we do that, the better science will become in the long run.

It won’t be easy, and it won’t improve overnight. But over time we can learn to listen to new perspectives, to challenge them and have them challenge us. Because the human equation of science is complex, subtle and beautiful, if only we take the time to see it.

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With the addition of more than a thousand new exoplanets, we have also added to the list of “potentially habitable” planets in the galaxy. 

Potentially habitable in this case simply means potentially rocky planets with surface temperatures that could allow for liquid water. Such planets are often called “Earth-like,” though that’s a bit misleading. Venus is very different from Earth, and yet if it were an exoplanet it would be considered highly “Earth-like.” Even the term “potentially habitable” is a bit misleading. Some of the larger, super-Earth mass planets are potentially rocky, but could also resemble Neptune more than Earth. The potential for liquid water is also a bit fuzzy. Both Venus and Mars are within the range of solar distance that could allow for liquid water, but neither of them have liquid surface water. The category also excludes bodies that could harbor life in different ways. Some warm-Jupiter planets could have large moons within the habitable zone of a star, while more distant planets and moons might have liquid water due to internal heating. Europa and Ganymede, for example, have oceans of water under an icy crust, which could be warm enough to sustain life.

That said, there are currently about 40 potentially habitable exoplanets. Plotted on a map of our galaxy, as seen above, you’ll see a few scattered across the sky. But the bulk of them are in the upper left region of the map. That’s where the Kepler spacecraft has been looking for planets, and that’s where the vast majority of planets have been found. It’s that small portion of the sky that gives a reasonable idea of just how many habitable planets there might be. We have every reason to presume that other regions of the sky along the Milky Way would have a similar density of planets. If you imagine that level of planets across the entire Milky Way, you’d see the likely potential of habitable worlds.

It’s not likely that all of those worlds harbor life. It may be that only a small fraction do, or perhaps even none. At this point there are too many variables we don’t understand about the origin of life on a potentially habitable world. But it’s now clear that the potential for life is extraordinarily high. There are warm worlds similar in mass to Earth, and there are a lot of them.

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A new study looked at public opinion of scientists in the U.S. They found that while scientists are perceived as being honest, they are also seen as robotic and lacking emotion. From my own experience I can certainly understand why. 

Of all the characters in the Marvel universe, the one I most strongly see in myself is Bruce Banner. Talented, but shy and withdrawn, and always wary of his emotions, lest they let loose the Hulk within. I’m willing to bet I’m not the only scientist who feels this way. Feelings are understandable, but science requires evidence beyond personal experience. Upon the altar of science we place the cold equations, and let the data judge. We’re wary of fallible emotions lest they lead us away from objectivity. You can see this in scientific papers, and even in my own blog writing. Stick to the facts, and don’t let emotion show.

Of course there is a deep emotional component to my work.  I have stood under the Milky Way, moved to tears by its beauty. I’ve felt awe in the sublime elegance of a mathematical theory, and joy in scientific success. Some of the deepest emotional experiences I’ve had were fueled by scientific pursuits. It is those emotions that drive me to pursue science, not cold objectivity. Without an emotional impact I wouldn’t be a scientist. I’m just more comfortable when those emotions are hidden, partly due to my own personality and partly due to my scientific training. Even writing this post makes me a bit uncomfortable.

By hiding our emotional side, scientists not only promote the stereotype of being cold an amoral, they also lose a powerful tool. The power of the Hulk can create havoc, but it can also save the world. Likewise, our emotions can motivate us to do good. Rage at the social inequalities within our institutions, joy in the success of our colleagues, empathy to leave the world better than we found it. Emotions can push us beyond our comfort zone, and encourage us to improve our community. And by showing our emotional side we can better connect with the public, and make science more welcoming. We do a disservice to science when we perpetuate the robotic stereotype.

I don’t know that I’ll ever be comfortable expressing my emotions publicly, but in the future I’ll try to present more of them in my writing. Because the scientific community and society as a whole will be better off if occasionally we unleash the beast within.

Paper: Bastiaan T. Rutjens , Steven J. Heine. The Immoral Landscape? Scientists Are Associated with Violations of Morality. PLoS ONE 11(4): e0152798. doi:10.1371/journal.pone.0152798 (2016)

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1) You live on a rock
Your home is a roughly spherical rock that orbits a star at about 100,000 miles per hour. Most of the surface is covered with a layer of water to produce lakes and oceans, and surrounding it all is a layer of air so thin that if the Earth were the size of an apple the atmosphere would be thinner than the apple’s skin. That’s home, and so far it’s been home for every single human that has ever lived. 

2) You are made of stardust
Sure it’s a bit cliche, but that doesn’t make it any less amazing. The carbon, nitrogen and oxygen in your body was created in the heart of a star long before our solar system formed. The most common elements in your body are also the most common elements in the galaxy. As a result you embody the history of our solar system.

3) Part of you is as old as time
While the heavier elements in your body were created in the hearts of stars, the hydrogen in your body formed about three minutes after the big bang. But the protons in your body formed about a millionth of a second after the big bang. Some of the protons that formed in the earliest moment of the Universe are in your body today.

4) You are the center of the Universe
Often it’s said that the Universe has no center. The big bang didn’t begin a particular point as a great explosion. All of the Universe (even space and time) formed together, which means where you are right now was once at the heart of the big bang. So you really are at the center of the Universe. Of course the same can be said about any other point in the Universe.

5) You are moving at 360 kilometers per second
The Universe is filled with a sea of microwave energy known as the cosmic microwave background (CMB). It is the thermal remnant of the big bang. The wavelengths we see are almost the same in all direction, but interestingly one side of the Universe has slightly longer wavelengths than the other. This is because our solar system is moving through space, causing the CMB to be slightly redshifted behind us and blueshifted ahead of us. It turns out that we are moving through space at about 360 km/s, which is surprisingly fast.

6) You are bombarded by neutrinos
Neutrinos are created in the core of our Sun through nuclear fusion. They are also created in the cores of distant stars. Since neutrinos don’t interact strongly with other matter, they tend to stream through things without interacting. And there are billions upon billions of them. At any given moment there are about 100 billion solar neutrinos streaming through every centimeter of your body. At night they travel through the entire Earth to reach you.

7) You are hotter than the Sun
The Sun has a much higher temperature than you. Even its surface is nearly 6,000 Kelvin, compared to your measly 310 Kelvin. But per volume you generate more heat than the Sun. That’s a bit misleading, since you generate heat through most of your volume through chemical reactions, but the Sun only generates heat in its core through nuclear fusion. Still, it’s a cool fact.

8) Everything is attracted to you
The universal law of gravity states that every mass is attracted to every other mass. When you step on a scale in the morning, the weight you measure is not just your weight in the Earth’s gravitational field, it is also the Earth’s weight in your gravitational field. Gravitationally you are pulling every slightly on everything around you. Everything is gravitationally attracted to you, even though that attraction is usually too small to notice.

9) Your experience of time is unique
Contrary to popular belief there is no cosmic clock that determines time in the Universe. According to relativity, there is no universal “now.” Instead, what constitutes the present depends upon your particular position and motion, and anyone with a different position and motion experiences a slightly different rate of time. You have a “now” that is uniquely yours.

10) There is only one of you
The Universe is mind-bogglingly huge. It’s estimated that there are 100 billion galaxies in the visible universe alone. That’s more than 10 galaxies for every man, woman and child on Earth. Those galaxies might have an average of about 100 billion stars. Around most of those stars might be tens of planets. Despite that vast diversity, the odds of your combination of DNA and your experiences coming together by chance is so astronomically tiny that there can only be one you in the entire cosmos.

The Universe is 93 billion light years across with billions upon billions of potentially habitable worlds, and yet there’s no one else quite like you.

The post Ten Amazing Things Astrophysics Says About You appeared first on One Universe at a Time.

We like marking time by the Sun. Its rising and setting marks a day, and its path along the ecliptic marks a year. The solar year seems to be our favorite marking of time. Its cycle follows the seasons, and so we have lots of annual celebrations, including our own special day. Of course there are lots of other ways we could mark our time.

We could note the seasons of the Moon, shifting from new moon to full and back to new over about 29.5 days.  For young children such a clock is sometimes used, but as we get older a lunar month seems too short to take much notice. Of course the lunar cycle is a bit out of sync with the solar year, but every 19 years the Moon and Sun come back into sync. This Metonic cycle could be used as a basis of celebration, but nearly two decades seems too long for a personal celebration.

We could mark time by the Soros cycle, which follows the pattern of eclipses. While eclipses happen 4 times a year, the relative alignments of the Earth, Moon and Sun cycle through a pattern of about 18 years. But since each eclipse isn’t seen from everywhere on Earth, such a pattern isn’t particularly commemorative.

If we lived longer we might celebrate other cosmic cycles, such as the 1,400 year Sothic cycle of ancient Egypt, which marked the rising of Sirius just before sunrise on the first day of the year. Or the Great Year marking the 25,800 years it takes for the rotational axis of the Earth to make one complete precession. Or perhaps the galactic year marking the Sun’s 250 million year journey around the galaxy.

But our lives span an intermediate time between mayflies and immortals, and so the journey of the Earth around the Sun marks our cycles of time.

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Recently the star KIC 8462852 (aka Tabby’s Star) has made news again because of its strange behavior. Possible explanations for its varying brightness (such as comets) don’t seem to fit the observational data, which has some speculating that the star’s behavior could be explained by the presence of an alien civilization. While many astronomers admit that is a possibility, they don’t think aliens are the likely cause. For one, mysterious behavior is not enough to conclude the cause is aliens. For another, the likelihood that an alien civilization actually exists is still a matter of some debate. 

The odds of an alien civilization coexisting with humans is often calculated by the Drake equation. It was first proposed by Frank Drake in 1961. Simply take the rate at which stars form in our galaxy and multiply it by the fraction of stars with planets, the average number of planets per star that could support life, the fraction of those that actually develop life, the fraction of life bearing planets that develop civilization, the fraction of civilizations that have detectible signals, and finally the length of time a civilization might last. Crunch the numbers and you have the number of civilizations in our galaxy capable of communicating with us.

When Drake first proposed the equation, the values for each term were largely unknown, but we now have good estimates for many of them. We know that most stars have planets, and the odds of a potentially habitable planet is actually quite high, possibly as high as 100 billion in our galaxy alone. Unfortunately the really important factors of the Drake equation are still completely unknown. On how many potentially habitable planets does life actually arise? How many of those give rise to civilizations? How long does a typical civilization last? No idea. Depending on the answer to those questions the number of civilizations in our galaxy could range from hundreds of thousands to only one.

The equation was never intended to give an absolute number, though it is often used that way. There are also alternatives such as the Sara Seager’s equation, which focuses on our ability to detect civilizations indirectly rather than requiring active communication. Just because an alien civilization is quiet, that doesn’t mean we can’t see evidence for them. Seager’s approach is to focus on stable red dwarf stars with known potentially habitable worlds. Since red dwarf stars are by far the most common, the odds that we’d find alien life near such a star is higher. She then focuses on planets that transit their home star from our vantage point and are near enough that we have a chance of observing the effects of the planet’s atmosphere on the star’s light. She estimates that there might be two inhabited worlds might be detectable in the next ten years. Of course this presumes that life forms readily on a habitable planet and survives billions of years, which might not be the case.

Even NASA has toyed with the idea of large space habitats. Credit: Don Davis.

What make’s Tabby’s Star particularly interesting is that it hints at being evidence of an artificial structure the size of a solar system, such as a Dyson sphere, which is something only highly advanced civilizations could create. Of course the big underlying assumption here is that the more advanced a civilization is, the more likely it will build such a structure. The idea was first presented by Nikolai Kardashev in 1964, who proposed a classification of civilizations based upon their energy use. Type I civilizations harness the resources of their home planet, such as humans today. Type II harness almost the full energy of their home star, possibly through technology such as Dyson spheres. Species within the Star Trek universe would typically be Type II. Type III are civilizations that can harness the energy of an entire galaxy, such as the Asgard of the Stargate universe. Carl Sagan later generalized the Kardashev scale to a logarithmic function of energy use, and estimated that we were at about 0.7.

The Kardashev scale presumes that more advanced civilizations will necessarily demand more energy. Humans have so far lent credence to this idea, since our modern global civilization consumes much more energy than earlier agrarian civilizations. If our human population and demands for technological convenience grows, we will likely expand out into the solar system with a continued rise in energy consumption. But such a future is not guaranteed. It’s also possible that we will instead reach a stable and sustainable population level, and combined with increasing energy efficiency our energy consumption may flatten. Technological civilizations may stabilize at type I rather than continuing up the scale.

That’s the real challenge with calculating the odds. Everything we’ve pinned down so far point to a good chance that life forms on planets across the Universe, but there’s still too many unknowns.

The post What Are The Odds That Aliens Exist? appeared first on One Universe at a Time.

By some estimates there could be more than 100 billion potentially habitable planets in our galaxy alone. While “potentially habitable” does not mean a planet is inhabited, if life arose on even a fraction of those worlds there should be millions if not billions of inhabited planets in the Milky Way. So why haven’t we seen evidence of them? 

Not counting the (slim) chance of an alien megastructure around KIC 8462852, the galaxy seems remarkably quiet. Projects such as SETI have scanned the heavens for radio signals, and found none thus far. Infrared sky surveys haven’t found thermal signatures of any alien civilization. This apparent silence given that extraterrestrial life should be common is often referred to as Fermi’s paradox. Lots of solutions have been proposed to address the paradox: perhaps they are intentionally being quiet; perhaps we just happen to live in a particularly barren corner of the galaxy; perhaps they lost interest space exploration and instead watch reality television.

An alternative explanation is that life may be far more rare than we think. Perhaps the odds of life arising on a planet are so unusual that out of 100 billion possible worlds, only one has given rise to life. The problem with this idea is that we know life arose quite early on Earth. On a geologic scale, as soon as Earth cooled enough to be potentially habitable we starting seeing life. We also know that the building blocks of life such as amino acids are found in comets and asteroids. That would seem to imply that life forms readily on habitable worlds. On Earth life eventually gave rise to a technological civilization, so why wouldn’t that happen on some other worlds as well?

Life may play a necessary role in keeping a planet habitable. Credit: Chopra and Lineweaver.

A new paper in Astrobiology proposes a solution. Perhaps life does arise on most potentially habitable worlds, but doesn’t get a strong enough foothold to keep the planet habitable for billions of years. Their idea is known as the Gaian bottleneck. Life may form readily on a habitable world, but life as we know it requires an atmosphere containing volatiles such as water, ammonia and methane. These are common molecules, but their abundances can be thrown out of whack by geologic processes. For example, Mars and Venus were both warm and wet in their early history, with plenty of the ingredients necessary for life. But rising carbon dioxide levels pushed Venus toward a runaway greenhouse effect, while Mars lost its atmospheric water and became cold and dry. Of the three potentially habitable worlds in our solar system, only Earth gave life a strong foothold, eventually leading to a rise of atmospheric oxygen and other volatiles through biological processes.

The authors argue that life plays a central role in maintaining a habitable world. Living organisms keep the molecules necessary for life in the system as it were, so the more life you have on a planet the more habitable the planet is. As a result, there could be a certain critical mass necessary for life to survive long term. Planets that break through the Gaian bottleneck can sustain life for billions of years, but if it’s rare for life to break through the bottleneck, the Universe may be filled with living planets that soon become inhospitable.

The Universe may be silent because planetary extinction is the rule rather than the exception.

Paper: Chopra, Aditya and Lineweaver, Charles H. The Case for a Gaian Bottleneck: The Biology of Habitability Astrobiology. January 2016, 16(1): 7-22. doi:10.1089/ast.2015.1387.

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