AUSTRALIA: Did Galileo discover Neptune?

Astronomy may need to credit Galileo Galilei with the discovery of the planet Neptune a full 234 years before the recognised discovery date of 1846, an Australian physicist suggests. Galileo certainly observed Neptune nearly 400 years ago - he recorded it in his notebooks - but apparently thought it was a star, not a planet.

Now Professor David N Jamieson, head of the school of physics at the University of Melbourne says there is evidence Galileo did indeed recognise Neptune as a planet. If so, the planet's discovery would be added to a list of breakthroughs including the discovery of Jupiter's moons, the rings of Saturn, and the phases of Venus.

In a paper published in Australian Physics and reproduced on this site, Jamieson explains that Galileo observed Neptune in December 1612 and on 28 January 1613. The scientist's notes suggest that during the latter observation he saw Neptune move when it passed close to a star, but he did not follow the matter up.

But Jamieson says the notebooks have an unlabelled ink mark on a 6 January observation. The mark could have been made by Galileo after the 28 January observation to indicate the position of Neptune.

"If so, this would suggest he did indeed form the hypothesis that he had seen a new planet which had moved right across the field of view during his observations of Jupiter over the month of January 1613."

Jamieson says the date on which the ink mark was made could be identified using trace element analysis of the ink. The analysis would identify differences in the composition of ink used on 6 January and 28 January, indicating when the mark was made.

But Jamieson says there is one other intriguing possibility. Galileo sent cryptic anagrams to his correspondents as a means of establishing the primacy of his discoveries.

"It is therefore possible that there remains, undiscovered in the Galileo literature, an anagram put there by Galileo to establish the date of his discovery of Neptune," he says. "If Galileo had used his observations to propose the discovery of a new planet, it would have been the first time a planet had been discovered by humanity since deep antiquity, and would be without precedent in recorded history."

* This year is the 400th anniversary of Galileo's first discoveries using an astronomical telescope and has been designated the International Year of Astronomy by the United Nations.