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As Ethan Siegel has pointed out, the paper is more speculation than science , but it has generated a lot of buzz in the media, often with sad laments over our lonely fate. But how bad would it actually be? Suppose the paper is right, and Earth is the only living planet in the cosmos. A single pale blue dot in a sea of empty dark. What would that really mean? To begin with, we wouldn't have to worry about alien invasions. No big eyed monsters wanting to have us for supper, no alien shapeshifters trying to replace government leaders, no red scare from Mars.

Human history is filled with violent attacks of one civilization against another, and so we've extended our history into hypothetical interstellar conflicts. But while these tales can make for exciting stories, they aren't very realistic. The energy, heavy metals and water necessary for a civilization like ours are plentiful in the universe, so there's no reason for aliens to target Earth. We might imagine an alien culture so xenophobic that it tries to extinguish any competition, but such a species would likely self destruct through aggressive infighting.

A more realistic existential threat could come merely from contact with another intelligent species.

Alone in the Universe | American Scientist

Imagine if we connected with a species , years more advanced than our own. One that had solved the challenges of violence and ecological collapse. One that could travel between stars as easily as we take a train. Humanity would be forever in the shadow of almost god-like creatures, and our culture would become either a reflection of or reaction to their civilization.

In our own history we see languages die, and cultural traditions fade. 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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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. As they migrate, such planets would presumably disrupt small bodies that might have been in the habitable zone, although one might remain there subsequently.


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Severe orbital variations do not preclude liquid water but could inhibit the development of biological systems. An eccentric orbit also increases the likelihood that in a system of similar planets, one occasionally might be chaotically disrupted. Of the extrasolar planets currently known with confirmed and published orbital parameters, only 11—2.

Figure 8. This tilt allows the north and south poles to face the Sun at different times of the year, maintaining a constant habitable temperature across the planet top. The obliquity of other planets in our system bottom , and perhaps for some extrasolar planets, varies widely. A related parameter is the obliquity of a planet, the angle between its spin axis and the axis of its orbit around its star.

No other planet in our own solar system has such a stable, much less congenial, obliquity; that of Mars seems to have varied chaotically between about 0 and 60 degrees but is currently about 25 degrees. Current models of Earth-sized planet formation suggest that high obliquity angles should be common, the result of collisions from all directions in early stages of formation, although the subsequent evolution of obliquities is less well understood. The rotation axes of stars can also be tilted with respect to the orbital plane.

Measurements of exoplanets suggest so far that stars hosting hot Jupiters also have large obliquities, probably the result of strong gravitational perturbations in these extreme systems.

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Again, the current set of observed extrasolar planets represents the tip of the iceberg; most extrasolar planetary systems could be different. However, the explanations advanced so far to explain these observed parameters are general; whatever tends to produce highly elliptical orbits, for instance, is presumably at work, in some fashion, in other, still-unknown systems. The third condition is planetary mass. A suitable planet must be massive enough to hold an atmosphere, but not so massive that plate tectonics are inhibited, because that would reduce geological processing and its crucial consequences for life.

Current estimates are that planets smaller than about 0. The frequency with which such planets occur is still not known, but the first indications from the Kepler satellite are that they are abundant: For planets in close orbits, about 13 percent are Earth-sized, and there is evidence that in larger orbits their numbers would be greater, especially in systems around smaller stars. Then there is planetary composition.

Lonely Universe - Passenger & The Wind In All This Noise

The relative abundance of the elements is not uniform throughout the galaxy, however, meaning that some regions may be incapable of hosting intelligent life. Furthermore, the need for radioactive elements means that a supernova, the primary source of radioactive elements, must have exploded in the vicinity of a suitable planet relatively recently but not so close or so recently as to be dangerous.


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The Local Bubble in which the Sun resides is thought to have been caused by supernovae explosions, and so it might be the case that some key elements present in the Sun and Earth are deficient in other regions of our 1, light-year zone. Finally, many planets may exist in open space, having been forcibly ejected from their stellar systems by multibody gravitational interactions. It is a stretch to imagine life on them developing and evolving to intelligence without the radiant energy of a stable star. Even under ideal conditions life might not develop easily.

The fact that life has not yet been created in the laboratory means that it is not trivially generated. Mars lies approximately in the habitable zone, but it hosts no civilizations. Even if life were the inevitable outcome of chemical processes on every planet with liquid water, there is no evidence that such fertility occurs quickly. On Earth life took about a billion years to form and another few billion years to produce us.

If sometimes the chemistry runs slower by a factor of two or three, or evolution is sidetracked, it may be too late: A Sunlike star would have swollen in size to fill the orbit of Earth. Believers in ETI sometimes counter that some life might evolve from noncarbon-based forms much more efficiently.

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Besides the absence of any plausible evidence for such a possibility, most agree that at a minimum intelligence requires complexity. Even if their brains are not made with DNA, such aliens must evolve over timescales long enough for comparably complex organs to mature. Figure 9. Unlikely accidents have facilitated human evolution. The chance conditions that might be essential to the rise of extraterrestrial intelligent life are a matter of speculation.

The development of intelligent life appears to require more than just planetary suitability. Consider the unlikely accidents—perhaps essential? A few billion years later, the dinosaurs, which had successfully dominated the planet for million years, were fortuitously wiped out by another, smaller asteroid so powerful it destroyed them all, yet did not kill off the mammals.

Many scientists have noted that there were roughly 15 mass extinctions, six of them catastrophic, on Earth before humans emerged on the scene, underscoring the complex, tumultuous, and perilous history of our evolution. Many other contingent conditions on Earth enabled life to thrive. Water is essential, but if Earth had much more water in its oceans, there would be no landmass for fish to crawl onto and evolve into toolmakers. Moreover, the route that evolution took was circuitous.

Evolutionary biologist Stephen J. Gould famously argued that our evolution was so random that it could probably never repeat; British paleontologist Simon Conway Morris argues that convergence of life on Earth toward humanity was inevitable, but only because of our extraordinarily perfect environment. The probabilities associated with all these biological terms are very uncertain, and astronomy provides no new evidence to evaluate them. Traditional discussions tend to imagine either that intelligence is inevitable on any approximately suitable planet, or that it is unlikely.

Readers can make their own estimates.