A couple of years ago the eminent theoretical physicist Stephen Hawking warned that a human encounter with intelligent extraterrestrial beings could be dangerous for us, sort of similar to the way Europeans were dangerous to ancient Native Americans, “which didn’t turn out very well for the Native Americans.” In hindsight, it would not have been a good idea for the Native Americans to go searching for Europeans.

Still, there is a profound natural fascination for the idea that we are not alone. In the last 60 years or so, our collective thinking about the possibility of alien life has evolved from a state of dismissive skepticism to a sense of probability. Hawking himself has indicated the possibility of extraterrestrial life is perfectly plausible. There are so many stars in the observable universe — 100 billion or more galaxies containing an average of, let’s say, 100 billion stars each — that it seems less likely we’re unique, and more likely there are planets like ours with life forms. Astrobiologists nowadays construct detailed theories of how, where and when life — not necessarily intelligent, but any life — might arise in places other than Earth. The SETI Institute (“search for extraterrestrial intelligence”), which was founded in 1984 and is run by serious scientists, actively searches for intelligent extraterrestrial life by sending out and listening for coherent signals. It’s one of many such projects in the past 50 years, some associated with Harvard and NASA.

If intelligent life is common, the physicist Enrico Fermi asked in about 1950, then why haven’t we seen them yet? This was a hard question to answer, and became known in SETI studies as the Fermi Paradox, or in some lexicons the Great Silence. There are five basic replies to the question, and maybe a sixth:

1. Life in the universe is extremely rare. We are indeed alone.

2. Extraterrestrial civilizations destroy themselves before they can travel to the stars.

3. The extraterrestrials just haven’t gotten here yet, the same as we haven’t gotten there yet.

4. They’re already here and hiding, or possibly spying on us. (This is called the “zoo hypothesis.”)

5. They’ve already been here and gone, but we are not yet capable of recognizing their traces.

These hypothetical replies to the Fermi Paradox all assume intelligent extraterrestrials will evolve technologies similar to ours. So let me throw in one more possibility:

6. Material technology as we have developed it is an anomaly; the ETs do not have spaceships, radios or jeejahs, and we are looking for the wrong kinds of communications.

The best hope right now for finding extraterrestrial life is in our own solar system — not intelligent life, but more likely microbial life, or signs of ancient microbes now extinct. At the moment, the most likely host worlds are Mars, Jupiter’s moon Europa, and Saturn’s moons Titan and Enceladus. Titan is in a sort of frozen early-Earth state with a thick atmosphere that theoretically might support a form of hydrocarbon-metabolizing life. Enceladus also has an atmosphere, and both apparently have subsurface oceans of water, which the astrobiologists generally consider an essential component for the emergence of life. Europa, also cold, also has a vast internal water ocean.

On Mars the situation is different because its climate, while mostly colder than here, is still a lot more Earth-like. A lot of water apparently is locked up in ice in Mars’ surface, and there are strong geological signs that liquid water once flowed there. Because of the ice and sometimes possibly melted water in places, it’s thought that microbes could be living in the sand. The Viking spacecraft tested for signs in 1977, but the results were inconclusive. Just around midnight Aug. 6, the Mars Science Laboratory mission is scheduled to land with the main scientific objective of assessing Gale Crater as a potential habitat for life. It might find chemistry that indicates life could exist, or it could find ancient Martian microbe fossils, or in the most unexpected scenario, it might find living microbes.

Some NASA researchers argue Martian fossils have already been found in a meteorite that broke off Mars about 4 million years ago and fell to Earth in Antarctica. There is disagreement about whether the forms seen in the rock were actually once-living beings, but the researchers have answered their scientific critics cogently — though not conclusively — several times.

These are not signs of intelligent life. But conclusive indications that any life arose elsewhere than Earth would be spectacular to the imagination. Our whole understanding of who we are and where we are would suddenly unfold into another dimension. Of course, it might well be another dimension of uncertainty, and it could in the end, in a myriad of completely different ways, be dangerous. But really, when was life ever safe? Let’s find those alien Europeans. Maybe the astrobiologists’ work will help us live profitably with them. Or fend them off.

Dana Wilde’s collection of writings on the stars, planets, space and time, “Nebulae: A Backyard Cosmography,” is available from Booklocker.com in electronic and paperback formats. More information is available at www.dwildepress.net/nebulae.