Always a fascinating subject.

Though "...from the stars comes only silence."

Greg Easterbrook's comprehensive analysis: Are We Alone?

...Scientists assume that contact with any extraterrestrial civilizations that may exist is most likely to occur by radio; therefore SETI is primarily a vocation of radio astronomy. But because it is possible to monitor only a small portion of the vast number of space radio frequencies, those who wish to tune in the heavens must select a channel. In 1959 Philip Morrison and Giuseppe Cocconi, two physicists at Cornell University, argued that because hydrogen is the most common substance in the universe and naturally emits radio waves in a relatively noise-free portion of the radio spectrum, it would be logical to listen there for a signal from intelligent beings. Most SETI researchers have accepted this reasoning and, since the frequency of the hydroxyl radical, which combines with hydrogen to make water, is close to the hydrogen frequency, this prime search area was later dubbed "the water hole." Paul Horowitz's superanalyzer listens to several frequency bands in the water hole.

...Needless to say, there is no way to know if the waterhole frequencies, which seem to us a logical place to direct our efforts, would seem logical to anyone else. Some have suggested that SETI listen on the frequencies of tritium, an isotope used in thermonuclear bombs which is quite rare and thus is associated with technological advancement. Horowitz, frustrated by five years of cosmic silence, recently switched the Harvard receiver over to the second harmonic of hydrogen -- a frequency that has no natural counterpart, and so is free of background noise. Any signal logged there would almost have to be artificial.

...Some astronomers doubt that planets pre-dating Earth are likely, however. Most of the heavy elements necessary to form planetary core and mantle are thought to have been produced not during the Big Bang but by the detonation of supernovas: observations of supernova 1987A appear to confirm this. The primordial cosmos was rich with supernovas, but just how many had to explode before space contained enough metallic elements to forge planets is a matter of conjecture. So, too, are related issues -- for example, how much time passed before matter accumulated in our galaxy and others structured like it.

...Hart's most unnerving calculation involves what he calls the "continuously habitable zone." Had Earth spun in an orbit only five percent closer to the sun, Hart says, it would have experienced a runaway greenhouse effect, creating unbearable surface temperatures and evaporating the oceans. Venus provides evidence for this: 28 percent closer to the sun, that planet has a nearly opaque atmosphere high in carbon dioxide and 900-degree surface temperatures. And had Earth been positioned just one percent farther out, Hart believes, it would have experienced runaway glaciation, locking its surface water in ice for eternity. Recently other scientists have endorsed Hart's concept of such a zone but have proposed that it must be wider, perhaps extending as far as the orbit of Mars for an Earth-like planet.

...Carbon has a subatomic quirk that allows it to form astonishingly complicated molecules that happen to be excellent for storing detailed information such as the secrets of life. The helical strands of human DNA have more than six billion distinct molecular components, each composed of carbon assemblies that are themselves complex. Amino acids, sugars, fats, and other building blocks of organic life also rely on carbon's quirk.

Below freezing, carbon-based molecules cannot obtain the liquid water they need to operate biologically. And above a few hundred degrees Fahrenheit, the precious life-coding chains break down; eventually they burn. So carbon can probably be the chemical foundation of life only under conditions approximating Earth's.

Now, here's the rub. There are very few elements like carbon. Silicon has the same subatomic quirk, which is why science-fiction writers speculate about silicon-based life. But silicon, like carbon, could form sophisticated information-storing molecules only under a fairly narrow range of temperatures and pressures. All remaining elements with the quirk seem to be quite uncommon in the universe, compared with carbon and silicon.

"...HERE, then, are the possibilities with respect to life on other worlds:

We have company. The scientific search for extraterrestrial beings, which is mathematically unlikely to succeed in a short time, may simply not have looked in the right place yet ....

...We had company. Perhaps thinking beings are common, but so is the sorrow that civilizations acquire weapons more readily than wisdom. Life comes and goes ....

...We are alone in this galaxy. Maybe the skeptics are right: the odds against life are so fantastic that one viable planet per galaxy is the best that can be hoped for ....

...We are alone, period. Perhaps there is no sound of breathing on any other world, no matter how many stars stretch out to the barricade of existence. And there never will be ....

...We are the first. Because the universe is ancient, man assumes he must be a latecomer. We expect that other beings could be to us as the greatest teacher is to the lowliest student. But by its own measure, creation glistens with morning dew. Stars are still forming ....

Christian Science Monitor - Study: If aliens exist, they probably want to destroy us.

When considering the prospect of alien life, humankind should prepare for the worst, according to a new study: Either we're alone, or any aliens out there are acquisitive and resource-hungry, just like us.

These two unpalatable options are pretty much the only possibilities, according to the new study. That's because evolution is predictable, and alien biospheres should thus produce intelligent creatures much like us, with technological prowess and an ever-increasing need for resources.

But the fact that we haven't run across E.T. yet argues strongly for the latter possibility — that we are alone in the universe's howling void, the study suggests.

The Daily Galaxy - Humans May be the First Generation of Advanced Life in the Milky Way.

Sasselov believes that life is probably common in the universe. He said that he believes life is a natural "planetary phenomenon” that occurs easily on planets with the right conditions. "It takes a long time to do this,” Sasselov said at a 2011 Harvard conference. "It may be that we are the first generation in this galaxy.”

Though it may be hard to think of it this way, at roughly 14 billion years old, the universe is quite young, he said. The heavy elements that make up planets like Earth were not available in the early universe; instead, they are formed by the stars. Enough of these materials were available to begin forming rocky planets like Earth just 7 billion or 8 billion years ago. When one considers that it took nearly 4 billion years for intelligent life to evolve on Earth, it would perhaps not be surprising if intelligence is still rare.

If life did develop elsewhere, adds Andrew Knoll, the Fisher Professor of Natural History using the lessons of planet Earth to give an idea of what it might take to develop intelligence. "Of the three major groupings of life: bacteria, archaea, and eukaryotes," he said, "only the eukaryotes developed complex life. And even among the myriad kinds of eukaryotes, complex life arose in just a few places: animals, plants, fungi, and red and brown algae. Knoll said he believes that the rise of mobility, oxygen levels, and predation, together with its need for sophisticated sensory systems, coordinated activity, and a brain, provided the first steps toward intelligence."

Offered without additional comment.

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