A space engineering professor said aliens should be the "absolute last thing" one would consider regarding the unidentified flying objects the US military spotted and shot down over the past week.
When asked during a Pentagon briefing, defence officials didn't rule out the possibility that the origin of the so far unexplained objects is extraterrestrial.
An "unidentified object" shot down by a US fighter jet over Lake Huron on Sunday was the fourth encounter in eight days and the latest military strike in the extraordinary chain of events over US airspace.
But a professor in space engineering has said there is no evidence to support such speculation, and that there many more human activities that the objects can be associated with.
Professor Keith Ryden, head of Surrey Space Centre, told The Mirror: "I haven't seen any evidence that would indicate that, Quite often, you have objects flying at high altitudes, such as balloons collecting data for meteorological purposes.
"And there are also UAVs (unmanned aerial vehicles) conducting all sorts of experimental activities, solar powered even.
"So there are a lot of other things that happen at those altitudes, which would be a quite plausible human activity rather than that.
"It would need a lot more evidence for anybody to conclude that there was anything extraterrestrial at the moment. It would be the absolute last thing to consider for this type of thing."
When approached by The Mirror, NASA as well as the European Space Agency declined to comment on the speculation.
The US Air Force general overseeing North American airspace said on Sunday that he would not rule out aliens or any other explanation yet, deferring to US intelligence experts.
Asked whether he had ruled out an extraterrestrial origin for three airborne objects shot down by US warplanes in as many days, General Glen VanHerck said: "I'll let the intel community and the counterintelligence community figure that out. I haven't ruled out anything."
He added: "At this point, we continue to assess every threat or potential threat, unknown, that approaches North America with an attempt to identify it."
US and Canadian authorities had restricted some airspace over the lake earlier Sunday as planes were scrambled to intercept and try to identify the latest object.
According to a senior administration official, the object was octagonal, with strings hanging off, but had no discernable payload, and was reportedly flying low at about 20,000 feet.
In late January, a white balloon appeared over the US and hovered above the nation for days before fighter jets downed it off the coast of Myrtle Beach, South Carolina.
That event played out over live stream, with many Americans having been captivated by the drama playing out in the skies as fighter jets scramble to shoot down objects.
The Pentagon officials said they are still trying to determine what exactly the objects were and drew a strong distinction between the three shot down over this weekend and the balloon from China, which they accused was part of a broad surveillance operation.
