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This view has been the traditional attitude, and one typical portrayer was Percival Lowell, an astronomer famous for his advocation of canals on Mars, who wrote in his book, Mars as the Abode of Life :. Figure 2. One measure of intelligent life is the ability to communicate between stars, for instance with radio technology, as is used by the Allen Telesope Array as part of the SETI Search for Extra-Terrestrial Intelligence Institute.

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By these standards, our own civilization is only about years old. Today we know that Mars has no artificial canals, and that this assertion was unsupported wishful thinking. Nowadays this view, too, appears narrow. The single most remarkable finding of the new research on extrasolar planets is that an enormous variety of systems exist—a diverse range of often-bizarre environments that is considerably broader than had usually been imagined before the first one was discovered. More than 50 likely Earth-sized planets have been spotted so far. Earthlike planets, with signs of liquid water and a congenial atmosphere, have as yet fallen below the detection threshold, although in the next few years, with the added patience it takes to measure a few of their yearly transits, it is reasonable to think that some will be found.

It may turn out that our own solar system is average—but we know now that at least some planetary systems are unlike ours. Meanwhile, the results make it possible to improve models of planet formation, which in turn offer improved guidance on planets in general. Two clarifications are essential. First, only the existence of intelligent beings is relevant. Primitive life may yet be discovered on Mars; perhaps even multicellular animals will be found on a nearby extrasolar planet.

These revolutionary discoveries would help us reconstruct how life on Earth evolved, but unless a species is capable of conscious, independent thought and has the ability to communicate, we will still be alone—with no one to teach or learn from, no one to save us from ourselves and no one to battle against.

Intelligent life, for the purposes of this discussion, means life able to communicate between stars ; this implies having something like radio technology. Our own society, by this definition, is only about years old. If intelligent life is common in a universe that is As the physicist Enrico Fermi famously observed, however, the fact that there is no other known intelligent life indicates that the assumption is wrong—intelligent life is not common.

Figure 3. Our solar system is located at the center of the circle above , in one arm of the Milky Way galaxy. The circle depicts a range of 1, light-years from Earth, a distance over which signals could be sent and returned in human generations. The second important caveat derives from two features of the world that were unknown to Percival Lowell. The first is relativity—the fastest any signal can travel is the finite speed of light. Even if ETI is infinitesimally rare, in an infinite universe, every physically possible scenario, however bizarre, will exist.

Such possibilities may be philosophically amusing, but they are practically irrelevant. We cannot communicate with, or even directly measure anything about, this unlimited vastness because it lies beyond the cosmic horizon, the distance set by how far light can travel in the age of the universe. Waiting longer will not help: The universe is getting bigger and expanding away from us. In fact, for purposes of communication the limit is even stricter.

The universe is not simply expanding—it is accelerating outward, and Harvard astrophysicist Avi Loeb has shown that light sent from Earth today can never even catch up to galaxies whose light has taken about 10 billion years to reach us. Even though they are well within our cosmic horizon, such galaxies are forever beyond our reach and receding quickly.


Even if the universe lasts forever, any aliens there will never enjoy our stray transmissions of I Love Lucy. Figure 4. Extrasolar planets provide astrophysicists with numerous surprises.

For instance, the orbits of the six large planets in the Kepler 11 system bottom are different from those in our system top. This illustration is not to scale. The finite speed of light also sets a practical limit on closer stars.

XXVI Canary Islands Winter School of Astrophysics - CosmoCoffee

Most stars in our Milky Way galaxy, and presumably its billions of planets, are hundreds of thousands of light-years away, so it will take hundreds of thousands of years for any ETI there to see our signals, and that long again for us to receive a reply. To be alone for all practical purposes means to be without any communication—or even the knowledge that any signal is coming—for a very long time. How long before we feel such solitude? My choice is human generations; subjectively this seems like practically forever.

Because one generation corresponds to 25 years and at least one round-trip of messages is necessary , I limit the following estimates to stars closer to Earth than 1, light-years. We know a lot about the stars in this neighborhood and so we can be quantitative. If we choose to examine a smaller volume, say, that accessible within one lifetime, the chances of success go down by a factor of a million—because the number of stars is proportional to the volume of space and scales with time distance cubed—but we will have a yes-or-no answer sooner.

However, if we expand the search volume and the probabilities of success, the wait time goes up. But it is hard to imagine such an enterprise being practical. No wonder there are no signals, nor even faint traces, despite decades of looking. As Fermi argued, they are not there.

XXVI IAC Winter School of Astrophysics on “Bayesian Astrophysics”

One way to figure the odds is to use the Drake Equation, a set of multiplicative factors tracking the various phenomena thought to be necessary to get to intelligent life. It is not a mathematical formulation of a physical process, and every researcher who uses it breaks down the individual terms somewhat differently, but all estimate the same thing—the number of civilizations around today.

At its simplest, the result is a product of five terms: the number of suitable stars, the number of suitable planets around such a star, the probability of life developing on a suitable planet, the probability that life evolves to be intelligent and the typical lifetime of a civilization compared to the lifetime of its star. Figure 5. The Kepler satellite discovers extrasolar planets by measuring their passage in front of their stars in those cases where the orbits are suitably aligned. The shape of the brightness dip during the transit, and its repeatability over many orbits, signals the presence of an orbiting planet.

The new results from extrasolar-planet searches impact the second term. As more extrasolar planetary discoveries are announced, I hope this discussion will help the public to evaluate whether they might be suitable sites for intelligent beings. The other factors remain rather mysterious and are extrapolations from an example of one—life on Earth. The usual attitude is that with about 10 20 stars in the visible universe, even overestimating these factors by hundreds still leaves plenty of civilizations out there. But if we are unwilling to wait for a billion years to hear from ETI, and therefore only consider our stellar neighborhood, then small reductions matter a lot.

It is impossible to increase the chances much over these early, optimistic estimates, but it is easy indeed to make the chances very much smaller. The Sun lies in a cavity of interstellar gas, called the Local Bubble, which extends over roughly light-years. The approximate number of stars per cubic light-year here is 0. This result provides a first factor in the Drake Equation considering the distance limit that has been set, so the second term is the next to be considered. The first thousand extrasolar planets discovered were the easiest to find in part because they are either large or have orbits close enough to their stars that their multiple transits in front of their stars can be observed, confirmed and studied in a few years.

In their statistical review of 1, Kepler planetary candidates planets not yet completely confirmed that orbit in less than 50 days, University of California at Berkeley astrophysicist Andrew Howard and his team analyzed the trends they represent, including the finding that smaller planets are more abundant. There has not, however, been quite enough time to find Earthlike planets.


Indeed, most of the stars studied have no planets of any kind yet detected, but in a few more years we may know more about them. These first discoveries could represent unusual members of the family. Nonetheless, the new results have driven important refinements to models of planetary formation and evolution.

Paleontologist Peter Ward and astrophysicist Donald Brownlee of the University of Washington, among others, delineate a set of familiar conditions that planets must satisfy for intelligence to prosper, which I have bundled into four essential ones: stability, habitability and water, planetary mass and planetary composition.

Figure 6. The location of the habitable zone, the range of distances from a star where temperatures allow water to be liquid, varies depending on the mass and age of the star. If a habitable zone is too close to its star, any planet located there may become tidally locked, with one side perpetually facing the star.

Larger stars may burn out too quickly to maintain a habitable zone for enough time for life to evolve. Illustration adapted by Tom Dunne from D. Brin, , Earth. To meet the stability condition, the host star must be stable in size and radiative output for the billions of years it takes for intelligence to evolve.

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Our Sun is among the less-common types of stars. It may be hard for a planet around a small star to evolve intelligent life because small stars are cooler and their habitable zones —the range of distances where the temperatures allow water to be liquid—lie closer to the star. When a planet is in this closer region, it tends to become gravitationally tidally locked to the star, with one side perpetually facing the star.

Tidal locking keeps one face of the Moon pointing toward Earth. But then half of the planet will be in the dark and cold, and the other half at constant noon.

XXVI Canary Islands Winter School of Astrophysics

Life seems improbable in such a place, although some argue that life could develop in the zones with intermediate conditions. At the other extreme, stars more massive than the Sun are also probably unsuitable; bigger stars burn hotter and live shorter lives. Fewer than about 10 percent of all stars are in a nominally acceptable range of masses, from about 0. Figure 7. About half vary in their stellar distances by 20 percent. Figure courtesy of S. Udry and N. Santos, , Annual Review of Astronomy and Astrophysics — Another concern is that most stars have a companion star orbiting; about two-thirds of solar-type stars are binaries.

Their planets might orbit one star, or the other, or both, but these situations raise a flag because the changing gravitational influence of an orbiting companion star potentially could disrupt the long gestational period of a planet in a habitable zone.

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  8. The second condition for intelligent life, habitability and water, further explores the concept that a suitable planet must reside in the habitable zone of its star or have some other mechanism to maintain liquid water. The orbit must be stable as well, sufficiently circular or otherwise unchanging, so that it remains suitable for billions of years. It is important to stress again that technology is only just now able to detect Earth-sized planets.

    Planets are thought to form far from a star by the gradual coalescence of dust grains in a protoplanetary disk into larger and larger bodies. Once formed, these planets generally tend to migrate into closer orbits as they interact with material in the disk.