Stargazing scientists have recently begun to focus on the prospect of encountering intelligent extraterrestrials, and the more they think about it the more they realize the first meeting probably won’t be with little green men in flying saucers.

What aliens might look like is a growing question among astrobiologists, who are increasingly conjuring up creatures more Lilliputian than mega-brained or reptilian.

“The intriguing possibility is they are, in fact, here, but we just don’t know it,” said Andrew Fraknoi, the emeritus chairman of the astronomy department at Foothill College who recently taught a course on aliens at the University of San Francisco’s Fromm Institute and believes space aliens could very well be microscopic or unrecognizable as a life-form.

Fraknoi is on the board of the Search for Extraterrestrial Intelligence, known as the SETI Institute, based in Mountain View, where questions about alien civilizations are often discussed. He has long speculated that members of a civilization billions of years old might by now have evolved into a mechanical-biological mix, like a robot with a brain, capable of living for thousands of years as they travel through space.

But it is also possible, he said, that advanced civilizations would have sent into space thousands of tiny canisters holding the germs of life programmed to incubate and grow when they encounter suitable conditions around a star.

“In all the mathematical models, a species that started early in the history of the galaxy and had the will and resources to diffuse could by now have filled many parts of the galaxy with its artifacts or biological spores,” Fraknoi said.

The otherworldly speculation comes after the recent discovery of two interstellar objects zipping past Earth prompted a surge of interest among scientists in space travel and alien civilizations.

A spinning, red, cigar-shaped object called 1I/Oumuamua was spotted in 2017, followed by the sighting last year of a comet named 2I/Borisov. They were the first verified sightings in human history of objects speeding by from outside our solar system.

The objects, by their very existence, brought home to many astronomers the reality that rocks or vessels potentially carrying biological spores from other solar systems could actually reach Earth.

The notion got a major boost from Avi Loeb, the chair of Harvard University’s astronomy department. He co-wrote a scientific paper suggesting that Oumuamua’s odd, elongated shape and peculiar nongravitational acceleration could mean it is a mechanical probe — a light sail driven by sunshine — sent by an alien civilization.

The object, first spotted by the University of Hawaii’s Institute for Astronomy, was, by all accounts, strange. Observations from Earth as it shot past the sun on Sept. 9, 2017, at a speed of 196,000 mph showed that it was slowly spinning, like a bottle on its side, and that it was missing the tail of gas or dust that would signify a comet.

Astronomers around the world immediately attacked Loeb’s hypothesis, and a subsequent study published in Nature Astronomy last year concluded that Oumuamua was a rocky conglomeration, not a space ship.

But Loeb said his point was that objects like Oumuamua and Borisov could have been synthetic and that humans would be well served by developing techniques for determining if such visitors were constructed. He believes the possibility of extraterrestrial life is too important for humans to discount without investigation, especially considering how useful it would be in figuring out the origin of life.

“Intelligent life is more recent in the Earth’s history, but at the same time, given that it happened here, there is the possibility that it exists elsewhere,” Loeb said. “I don’t think we should pretend that we are the only ones — the smartest kid on the block — because very likely we aren’t the smartest kid on the block.”

The questions about what form alien beings might take are rooted in what is known as the Fermi paradox, named after Italian American physicist Enrico Fermi, who created the first nuclear reactor. He asked during a casual lunchtime conversation in 1950 why aliens have never been spotted, given the high probability of their existence.

SETI has been searching the skies for radio signals or some other sign of life beyond Earth for nearly four decades without a single peep.

Despite the failure, belief in the existence of extraterrestrial civilizations has only increased since Fermi’s time. That’s largely because powerful telescopes have recently detected numerous planets orbiting their stars at a habitable distance, known as the Goldilocks zone. Calculations indicate there are habitable planets around at least a quarter of the tens of billions of stars in the Milky Way Galaxy, possibly including the closest star, Proxima Centauri, 4.2 light-years from Earth.

Most astrophysicists believe life must have sprung up somewhere, some time, in the 13.5 billion years since the galaxy was formed. Given that our sun is 4.6 billion years old, Fraknoi said civilizations in other parts of the galaxy could have been using robotics, artificial intelligence and tapping the energy from their stars as many as 8 billion years before our solar system was created.

“In other words,” Fraknoi said, “there has been ample time for a civilization to become advanced enough to send alien microbes or micro-artifacts around the galaxy, including to our solar system.”

Astronomers have even concocted a sciency name, “directed panspermia,” to describe the act by an alien civilization of planting the seeds of life in another world.

Samantha Rolfe, a lecturer in astrobiology at Bayfordbury Observatory at the University of Hertfordshire in England, suggested recently that such organisms could be hidden inside what she called a microscopic “shadow biosphere” that is so different from ours that we don’t even recognize it as biological in origin.

“So why haven’t we found it? We have limited ways of studying the microscopic world as only a small percentage of microbes can be cultured in a lab,” she wrote in an article for the Conversation website. “We do now have the ability to sequence the DNA of unculturable strains of microbes, but this can only detect life as we know it — that contain DNA.”

Some have suggested that these alien life-forms could be small inactive spores floating in our solar system waiting for the right conditions to grow or as active monitors — transmitters — used by alien civilizations to determine whether Earthlings are a threat and might need to be eliminated.

Then again, a growing number of astronomers speculate that humanity itself might have originated somewhere else, possibly clinging to a chunk of rock ejected from a planet that was hit by a giant meteor.

“We know there are rocks on Earth that came from Mars, so you could imagine that microbes could have potentially survived the journey,” Loeb said. “So it’s possible we are all Martians. If you can do it from Mars, you can potentially bring life from other planets in other galaxies.”

Loeb recently published a paper calculating how asteroids could graze Earth’s atmosphere, scoop up microbes like the foamy cream off a latte, and potentially carry the seeds of life into outer space. Maybe, he and others suggest, this swapping of biological spores has happened since the beginning of time.

Either way, most experts believe an alien encounter is likely someday. The question, say those who think about such things, is whether humans will know it when they see it.

“Potentially, we could be part of an experiment where life was planted on Earth and someone is watching,” Loeb said. If that’s the case, “for sure they are disappointed. That would be my assessment by reading the morning newspaper.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite