A tattoo on the wrist of a wealthy merchant in a 17th-century group portrait is the only one ever found in depictions from that era and may well be the oldest in Dutch painting, experts from the Amsterdam Museum have said.

The painting, the Oppercommissarissen der Walen, attributed to Wallerant Vaillant, shows a group of prominent citizens in charge of the city’s harbour, quays and cranes. Among them was rich merchant Wesssel Smits, who has now been found to sport a barely visible tattoo of a comet, complete with tail, something that had gone unnoticed through the ages.

According to curator Judith van Gent, Smits was born in 1618 or 1619, when a bright comet lit up the sky.

Tailed comets were seen as harbingers of pestilence and war, but, Gent said, rather than being superstitious, Smits was proud of having been born under the sign of the comet. “He came from a family of intellectuals. His father was part of a group of scientists and Jacob Cats, who wrote a pamphlet about the comet, was a relative,” Gent told broadcaster NOS.

The painting came to the museum in the 19th century but it wasn’t until a thorough inspection following a show at the H’art museum in Amsterdam that the tattoo was found.

Not only did Smits defy superstition, he also had a tattoo when his Protestant faith would have prohibited “tattoo marks”, as mentioned in Leviticus 18:28.

According to tattoo artist Henk Schiffmacher, there are German and French 17th-century portraits which show people with tattoos to show they had been on a pilgrimage but they were Catholics.

“It is unclear what Smits, who commissioned the portrait and is portrayed centre stage, wanted to convey. He is surrounded by three fellow protestants who apparently did not object. There is a story there but we may never find out what it is,” he said.

The blue ink has faded and that means Smits had had for years, suggesting a sea voyage to Asia, perhaps for trade purposes. Not enough is known about his life to be sure, Gent said.

The painting is now in the Willet-Holthuysen museum on Amsterdam’s Herengracht for the next few months. Schiffmacher, meanwhile, is offering a free comet tattoo to five visitors to the museum who can best explain why they want one.