The head of the government's UFO office says alien 'motherships' could be flying through our solar system and sending probes to Earth in a new science paper.

The draft paper, published last week has learned, has also sparked a row with the UFO chief's predecessor over the physics of strange craft seen in our skies.

The six-page article, titled 'Physical Constraints on Unidentified Aerial Phenomena', was co-written by Dr. Sean Kirkpatrick, the director of the Pentagon's UFO unit called the All-domain Anomaly Resolution Office (AARO), and former Harvard Astronomy Department Chair, Professor Avi Loeb.

Loeb is also the founder of the Galileo Project, which aims to use high-resolution telescopes and artificial intelligence algorithms to spot and photograph UFOs around Earth.

He gained national attention for his theory that an interstellar comet that passed through our solar system in 2017, dubbed 'Oumuamua, could in fact be an alien probe, and has written books on the possibility of extraterrestrial life.

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