Update at 2147 GMT on Wednesday: The Pentagon says the contrail was caused by a plane. "We have no evidence to suggest that this was anything other than a contrail caused by an aircraft," Pentagon spokesman Colonel David Lapan said, according to ABC News.

Update at 1640 GMT on Wednesday: The Federal Aviation Administration says no fast-moving unidentified objects were seen in the area on radar and no commercial companies had applied to launch rockets from the region.

What appeared to be a rocket blasting into the skies off the southern California coast on Monday was probably just an approaching plane, a Harvard astronomer who tracks space launches says.

Around 5 pm local time on Monday, a KCBS news helicopter shot a video showing what appeared to be a missile launch above the Pacific Ocean. A Pentagon spokesman said that so far there is no explanation for the observation, according to the Associated Press. "Nobody within the Department of Defense that we've reached out to has been able to explain what this contrail is, where it came from," Pentagon spokesman Colonel Dave Lapan said. "So far, we've come up empty with any explanation."

So what was it? Jonathan McDowell, an astronomer who tracks space launches at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says it was probably an optical illusion caused by a plane.

"If it's coming over the horizon, straight at you, then it rises quickly above the horizon," he told New Scientist. "You can't tell because it's so far away that it's getting closer to you – you'd think it was just going vertically up," he says.

The fact that it occurred at twilight would have emphasised the contrail, he adds. "It's critical that it's at sunset – it's a low sun angle. It really illuminates the contrail and makes it look very dense and bright." See other jet contrails that look like missiles.