If an alien civilization wanted to study planet Earth, how might they do it? They could use powerful telescopes to measure the physical characteristics of our planet, or they could listen for signals from our TV and radio broadcasts. These are things we are doing in our search for alien civilizations. But a really advanced alien civilization might try something a bit more ambitious, such as an actual mission to Earth. One way to do this would be to build a probe within an asteroid, and send it on a journey across the stars. The asteroid could shelter the probe during it’s long trip through interstellar space. Once it arrived in our solar system, the probe could gather detailed information about Earth and the solar system. It might even try to communicate with humans by beaming a radio signal in Earth’s direction. Such an alien probe would look a lot like the recently discovered asteroid Oumuamua, which is why the Breakthrough Listen project wants to study it.

Oumuamua was discovered in October by the Pan-STARRS 1 telescope. Unlike any other asteroid, Oumuamua has an interstellar orbit. It is moving through our solar system so quickly that could not have originated in our solar system. Based on its trajectory, it came to our solar system from the general direction of the star Vega. Coincidentally, Vega is star aliens first communicated with humans from in Carl Sagan’s novel Contact. In addition to being the first confirmed interstellar object to enter our solar system, it also has a highly unusual cigar shape, with a length about 10 times longer than its width. Add to this the fact that Oumuamua made a relatively close approach to Earth, within 15 million miles of our planet, and it begins to look a bit alien.

Odds are this asteroid is just a chance visitor to our system. We’ve known that some asteroids can escape the solar system through close flybys of large planets like Jupiter, so it makes sense that asteroids from other star systems could travel between the stars. Such interstellar visitors might be rare, but they don’t require aliens to send them on their way. But Breakthrough Listen is interested in finding evidence of alien civilizations, no matter how long the odds. So when Oumuamua was discovered, it made for a promising target.

Oumuamua is currently about 2 astronomical units away from Earth. About twice as far as the Earth is from the Sun. That’s still much closer than many of the probes we’ve sent into space, such as Cassini and New Horizons. If it is an alien probe sending radio transmissions we should be capable of detecting them. So Breakthrough Listen will use the Green Bank Telescope to look for any evidence of alien technology, searching across four radio bands, from 1 to 12 GHz, for a total time of about 10 hours. If an alien probe wants to be detected, that’s a good frequency range to search.

Just to be clear, the odds of Breakthrough Listen finding anything are really slim. Studies so far haven’t found anything that would imply an artificial origin. But even if Breakthrough Listen doesn’t find anything, their observations will add to those we already have, and help us better understand asteroids that are rare, but natural, alien visitors.

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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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Despite our best efforts, we have thus far failed to find any signal from an alien civilization. There have been a few odd detections that are hard to explain, but nothing definitively alien. Of course to be successful, we not only have to be listening, but the aliens have to be transmitting. It turns out that transmitting a message across light years is extremely challenging, and would take a great deal of power to succeed. Is it really likely that an alien civilization would try to transmit  a message in all directions? Perhaps they might just focus on stars where they know life exists. 

It turns out that detecting life in another star system isn’t too challenging for an advanced civilization. In fact we are reaching the point with our own technology that we’ll soon be up to the task. The easiest method would be to observe a planet as it transits its star. In this way we can not only determine the planet’s size, we can also see how starlight is absorbed by the planet’s atmosphere. From this we can determine the composition of the atmosphere. Alien astronomers could see Earth’s atmosphere is rich in oxygen and water vapor, and might conclude that life is probable on our world. Knowing that, they might send a signal in our direction. So perhaps we should focus our SETI efforts on stars that could see Earth transit the Sun. It’s an interesting idea that’s worth considering.

Even now we can imagine ways of observing exolife that doesn’t require a planetary transit. One could imagine that a truly advanced civilization would know we exist even if Earth doesn’t transit the Sun from their vantage point. But with the universe so strangely quiet up to this point, what have we got to lose?

Paper: Shmuel Nussinov. Some Comments on Possible Preferred Directions for the SETI Search. arXiv:0903.1628 [astro-ph.EP] (2009)

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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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Alien civilizations are out there. At least that’s what many people believe. Finding those aliens is the goal of a new $100m project spanning 10 years. The Breakthrough Listen project, funded by Russian billionaire Yuri Milner, will use radio and optical observatories to listen for messages from alien civilizations.

While we have no direct evidence of life on other worlds, there is indirect evidence to support the idea of alien life. By some estimates there are about 8 to 20 billion potentially habitable Earth-like worlds in our galaxy alone. Of course potentially habitable doesn’t mean inhabited, and simple life such as bacteria isn’t likely to build a transmitter capable of communicating across the stars. Just how rare or common intelligent life is in the universe remains a mystery. But even conservative estimates put it in the realm of possibility. Some have even argued that intelligent life is so probable that it’s observed absence paints a cautionary tale for our own civilization.

Even with an influx of $100m, the search for extraterrestrial intelligence is not without its challenges. While it’s a common trope to imagine our TV and radio signals being picked up by advanced aliens, the reality is that our typical transmissions barely carry to the nearest stars. For us to be able to detect an alien message, those aliens must be intentionally broadcasting a message to us. But we also need to be listening, and that’s where this project comes in.

Breakthrough Listen will pay for thousands of hours of radio telescope time at the Green Bank Telescope in West Virginia and the Parkes Observatory in New South Wales, Australia. The Automated Planet Finder Telescope at Lick Observatory in California will also look for modulated laser transmission in the optical range. The project will survey the million closest stars to Earth, and it will scan the center of our Galaxy and the entire galactic plane. The project will search for messages from the 100 closest galaxies, though the transmission power of intergalactic communication is difficult to fathom.

Success of the project is a long shot, but even if we don’t discover alien messages the project will generate a wealth of observational data that will be freely available to the public.

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In the movie Contact, astronomers receive a radio signal from the star Vega. Buried within the signal is a broadcast of Hitler’s speech for the opening of the 1936 Olympic games. The television signal had made the 25 light year journey to Vega, which let the aliens know we’re here. The idea that our television and radio signals are gradually reaching ever more distant stars is a popular one, but in reality things aren’t so simple.

The reach of TV signals. Credit: Abstruse Goose

The opening ceremony of the 1936 Olympics was the first major television signal at a frequency high enough to penetrate Earth’s ionosphere. From there you could calculate that any star within about 80 light years of Earth could detect our presence. There’s even a website that shows which TV shows might be reaching potentially habitable worlds. But the problem with this idea is that it isn’t good enough for the signal to reach a distant star, it also needs to be powerful and clear enough to be detectable.

For example, the most distant human-made object is Voyager I, which has a transmission power of about 23 Watts, and is still detectable by radio telescopes 125 AU away. Proxima Centauri, the closest star to the Sun, is about 2,200 times more distant. Since the strength of a light signal decreases with distance following the inverse square relation, one would need a transmission power of more than 110 million Watts to transmit a signal to Proxima Centauri with the strength of Voyager to Earth. Current TV broadcasts (at least in the States) is limited to around 5 million Watts for UHF stations, and many stations aren’t nearly that powerful.

One might argue that an advanced alien civilization would surely have more advanced detectors than we currently have, so a weaker signal isn’t a huge problem. However the television signals we transmit aren’t targeted at space. Some of the signal does leak out into space, but they aren’t specifically aimed at a stellar target the way Voyager I’s signal is aimed at Earth. They also lack a clear mechanism for how transform the signal to an image. On Earth this works by implementing a specific standard, which any alien civilization would need to reverse engineer to really watch TV. On top of that, there is the problem of scattering and absorption of the signal by interstellar gas and dust. This can diminish the power and distort the signal. Even if aliens could detect our signals, they might still confuse it with background noise.

A color-coded version of the Arecibo message.

That doesn’t mean it’s impossible to communicate between stars. It just means that communication would require an intentional effort on both sides. If you really want to communicate with aliens, you need to make sure your signal is both clear and readable. To make it stand out among all the electromagnetic noise in the universe, you’d want to choose a wavelength were things are relatively quiet. One good region is known as the water hole, which spans a range from 18 to 21 cm. Hydrogen (H) emits at about 21 cm, and hydroxyl (HO) has a strong emission at about 18 cm. Together they can form water, hence the name for the quite gap in between. You also need to make your signal easy to recognize as an artificial signal. In Contact the aliens did this by transmitting a series of prime numbers.

In 1974 humanity made its most famous effort to send a signal to the stars. It was a radio transmission sent from the Arecibo observatory, and consisted of 1,679 binary digits, lasting three minutes. Since 1,679 is the product of the primes 23 and 73, the bits can be arranged into an image of those dimensions. There have been other efforts to send messages to the stars, but they haven’t been as powerful or as simple.

Beyond a few light years, our leaky TV broadcasts are likely undetectable. As we’ve switched to digital television and lower transmission powers they’ve become even harder to detect.  Any aliens looking for us will have to rely on other bits of evidence, such as the indication of water in our atmosphere or chlorophyl on Earth’s surface, just as we will strive to detect such things on distant worlds. Either way, the first message received won’t be a complex text of information. It will simply be a recognition of life on another world.

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The Big Ear radio telescope was built in 1963 to survey the sky for extragalactic radio sources. It is a type of radio telescope that uses the rotation of the Earth to make observations. The receiver of the radio telescope can be moved north or south, but to view things east and west you have to wait for the Earth to rotate in the right direction. This type of telescope is perfect for making sky surveys, because you can point it in a particular declination (the sky version of latitude), and then let the Earth’s motion move you through a full circle of sky over the course of a day.

After the sky survey was completed in 1971, and funding for the telescope ended soon after. In 1973 it was funded by the SETI project to search for possible signals of extraterrestrial intelligence. Then in 1977 the telescope observed a very unusual signal. It is now known as the Wow! signal, and you can see the original printout of it in the figure above. The numbers represent the strength of the signal compared to the average background radio noise. The scale goes from 1 to 9, then A to Z. The U in the figure means at its most intense it was 30 times stronger than the background noise.

The information we can gather from this signal is fairly limited due to the way Big Ear gathered data. It had two “feed horns” that pointed at slightly different directions. These funneled the radio signals into a detector. Given the way the detector processed these signals, it was only possible to determine if a signal was detected by one or both of them. The detector measured the signals once a second at 50 different frequencies. Every 10 seconds they were averaged to give the intensity, and the result was printed as seen in the figure below. This might seem pretty crude, but it was one of the earliest automated astronomical systems. Big Ear was designed so it could be turned on and left to collect large amounts of data that could be analyzed later.

Still, even with those limitations, the Wow! signal has some very interesting properties. As the figure shows, the signal starts off quiet, reaches a maximum in the middle, then dies down again. It lasts for about 72 seconds, which is exactly how long a particular patch of sky would be in focus due to the rotation of the Earth. This means the signal appeared to move with the sky, so it wasn’t likely to be of terrestrial origin. When you look at the raw data, it seems the actual intensity of the signal was constant.

Another interesting property was that it was only detected by one of the feed horns. This means it came from a narrow region of the sky, rather than a wide area. It was also only detected at one frequency, about 1420 MHz (or a wavelength of 21 cm). This frequency happens to be known as the hydrogen line because it is a frequency produced by atomic hydrogen. It is also a frequency that isn’t easily blocked by gas and dust in the galaxy, which makes it the perfect frequency for sending signals over stellar distances.

So it seems the Wow! signal was a non-terrestrial, very intense signal at the 21 cm hydrogen line that lasted for at least 72 seconds. The Wow! signal originated from a direction in the constellation Sagittarius. There is a main sequence star 120 light years away in that general direction, known as Tau Sagittarii, but there’s nothing particularly unusual about that star. There’s no clear source that could have caused such a signal.

So what was it? The short answer is we don’t know. The signal has all the properties we would expect from an extraterrestrial intelligence. High intensity, specific frequency, long lasting. It is exactly the type of signal SETI was looking for. But since Big Ear only made one measurement a second we have no way of determining if it had any detailed structure. It might have just been an intense energy burst. As far as we know, such a burst should have been detected at multiple frequencies, but who knows. It might also have been a terrestrial signal that reflected off something in space, although 1420 MHz is a restricted frequency, so nothing on Earth should have been broadcasting at that frequency.

But the signal was never observed again. Multiple observations have been made of that region of space over the years, but no hint of the signal has ever been found. If it really were an intentional signal from an extraterrestrial intelligence, then one would expect it to be repeated multiple times. Since it has never been repeated it is most likely that it was a reflected signal from Earth (say, from a military source) or some kind of radio interference. Without further observations it’s really a big mystery.

For some astronomical observations, all you can say is Wow!

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It’s estimated that there are between 8 and 20 billion potentially habitable Earth-like worlds within our galaxy alone.  Those are just the ones that orbit Sun-like stars.  If you add in stars like red dwarfs, the number of potentially habitable planets rises to over 40 billion.  Of course that is just the ones within our galaxy.  There are about 100 billion galaxies within the observable universe.  That’s a lot of potential for other intelligent species, but so far none of them have made contact with us.  Just why is a bit of a mystery.

One solution is that no other intelligent species has contacted us because there are no other intelligent species besides ours.  It is possible that the appearance of intelligent life in the universe is so extraordinarily rare that we are the only such species in the entire universe.  It’s also possible that intelligent life is rather common, and there is some other reason that motivates them to avoid us.  Either way, it would be helpful if we had an idea of just how likely or rare intelligent life actually is.  One way to estimate this is through an equation known as the Drake equation.

The Drake equation was originally proposed at the first SETI conference by Frank Drake in 1961 as a way to stimulate discussion.  Drake did not intend it as a prediction of the correct value, but more as a “what if” to consider.  The equation itself is basically a product of the rate at which stars form in our galaxy, how many stars have planets, how many planets they typically have, what fraction are habitable, what fraction of habitable planets form life, how many form intelligent life, then civilizations, and how long those civilizations last.

There are possibly billions of habitable worlds in our galaxy alone. Credit: NASA/JPL–Caltech/R Hurt (SSC–Caltech)

When Drake first proposed the equation, most of the values in the equation were largely unknown.  We now have data on several of them.   We know that about 7 new stars form in our galaxy each year, virtually all main sequence stars are likely to have planets, and they likely have more than 1 planet.  There are about 60 billion Sun-like stars in our galaxy, and it’s estimated that about 15% – 30% of those stars have planets in their habitable zone.  From that we get a value of 8 to 20 billion potentially habitable Earth-like planets.

The rest of the Drake equation remains pretty speculative.  If we suppose there are 10 billion potential Earths, how many of them actually have life?  We only have one example of life arising on a planet, and drawing conclusions from a sample size of 1 is iffy at best.  But if we assume Earth is fairly typical, then perhaps 10% of these worlds could have possessed life for at least a billion years.  That would mean there is life on about 100 million planets.

Of these, what fraction will give rise to an intelligent species?  Intelligent life arose on Earth, so it’s possible that most planets with life will give rise to intelligence.  Or it could be that intelligence is just a fluke.  Again, we only have one example.  This is perhaps the most speculative aspect of the Drake equation.  Estimates range from nearly 100% to almost none.  Of those with intelligent life, how many can communicate across the stars?  Again, it’s anybody’s guess.  So there could be as many as 100 million civilizations, or as few as 1.

Carl Sagan saw the Drake equation as a cautionary tale. Credit: Cosmos.

The last part of the equation is about how robust civilizations are.  When they arise do they last for a million years, or do they collapse within centuries?  Our own civilization is relatively young.  We’ve only had the potential to communicate across stellar distances for a few decades.  How much longer will our civilization last?  That’s a good question.

Carl Sagan saw the lack of communication with other intelligent species as a cautionary tale.  If upwards of 100 million planets have life, and the rise of intelligent life is common, then the reason we haven’t heard from our alien neighbors could be because civilizations are fragile.  Perhaps just as they develop the tools to reach the stars they also develop the tools of their annihilation.  Perhaps we should view the silent stars not as a mystery, but as a warning.

For now it is still largely speculation.  Planets are common, and potentially habitable planets seem to be common, but we just don’t have any hard evidence for more than that.

One thing, however is certain.  Either we are alone in the universe, or we are not.  Either case is deeply profound.

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