Was our solar system once home to an advanced civilization other than our own — perhaps one that predated humanity by hundreds of millions of years before being wiped out by an asteroid impact or some other cataclysm?

There’s no evidence for such a pre-human indigenous technological species, though people have been speculating about one since ancient times. But a respected space scientist points out in a provocative new paper that if the existence of home-grown intelligent space aliens has never been established, it’s never been ruled out either.

And if a race of smart and perhaps spacefaring aliens did make their home in our solar system, traces of their lost civilization might still be out there somewhere in the system just waiting for us to find them.

Related: Are Human Beings the Real Ancient Aliens?

At least that’s what the scientist, Penn State astrophysicist Dr. Jason T. Wright, argues in the paper, “Prior Indigenous Technological Species.” The paper was posted on the online research archive ArXiv on April 24, 2017.

What sorts of traces does Wright have in mind? He’s not saying we might dig up aliens’ fossilized bones. Rather, he’s talking about “technosignatures.” The term encompasses a range of possible artifacts, including archaeological ruins and old mining operations as well as synthetic chemicals or nuclear isotopes that could have been created only by technological processes.

Wright argues that if a prior indigenous technological species did exist in our solar system, it might have arisen on Earth or a “pre-greenhouse” Venus, or on Mars when it still had flowing water.

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But we’re unlikely to find any artifacts from this species on Earth, he says, because surface erosion and the shifting tectonic plates on our planet would probably have erased them long ago (or changed them so dramatically that we might not recognize them as products of technology).

Similarly, any artifacts left on Venus would probably have been destroyed long ago by the planet's harsh atmosphere or the violent “resurfacing” the planet underwent hundreds of millions of years ago.

If any technosignatures were left on Mars, however, Wright believes they might still exist. But given the red planet’s thick dust, he writes that “it is unlikely that artifacts might be obvious from space imagery, or even from the sort of shallow probing performed by the various Martian rovers.”

In other words, just because the orbiters and rovers we've sent to Mars haven't turned up any technosignatures doesn’t mean they’re not there. They might simply be lying deep under the surface.

Related: Why Finding Signs of Life May Be Harder Than We Thought

Other places to look for technosignatures include our moon (again, probably under the surface) and the rocky moons and asteroids of the outer solar system. And as Wright told NBC News MACH in an email, there might even be “large structures free-floating in space” — some sort of space stations left over from this hypothetical civilization.

What do other experts make of Wright’s ideas?

Dr. Paul Horowitz, a research professor of physics and electrical engineering at Harvard University and a noted expert on the search for extraterrestrial intelligence (SETI), praised the paper as “very nice” but cast doubt on the notion that our solar system might have been home to the kind of species Wright discusses.

“If I had to guess, I’d say it’s highly unlikely, but not impossible, that an ancient civilization existed — probably even more unlikely we’ll ever find evidence,” Horowitz told MACH in an email.

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In a separate email to MACH, Dr. Neil deGrasse Tyson called Wright’s ideas “intriguing,” but added with a measure of understatement that efforts to confirm the existence of another home-grown technological species would “require substantial upticks in our space exploration budgets.”

Wright himself isn’t actually calling for a massive new effort to look for evidence of the hypothetical species. But he does hope his paper has an impact on the way scientists think about the possibility of ancient intelligent life in our neck of the cosmic woods.

“I’d like people who think about ancient Earth — archaeologists and paleontologists — to consider how we can rule out a prior indigenous technological species,” he said. “Before what [geologic] time are we sure that there was no complex life, no technology, on the surface of the Earth? I feel that planetary scientists can tell us how old the surfaces of Venus and Mars are, and how long artifacts should last.”

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