It seems like something always affects Prince Harry and Meghan Markle's plans.

Five months after Prince Harry and Meghan Markle officially left the monarchy, the royal prince is expected to have already adapted in his environment. Since his wife is an American citizen, it should be easier for him to embrace his new world away from the royal family.

However, the royal prince is about to turn into a resident alien, as he has not done anything about a possible U.S. citizenship.

According to Express, the cutoff to be considered as a resident is 183 days. The process is also known as the substantial presence test.

To pass this, a person should be physically present in the U.S. "for 31 days during the current year and 183 in the last three years."

In Prince Harry's case, it has been 159 days since the Duke and Duchess of Sussex left Canada and relocated in California. It means that he is nearing the cutoff point, and he could be liable to pay taxes once it happens.

Meghan could sponsor Prince Harry. However, immigration and tax expert David Lesperance told the news outlet that Prince Harry is unlikely to be in the U.S. on a green card since this would make him a U.S. citizen for tax purposes.

However, this process can be refuted since Prince Harry is also a resident in another country.

With that said, if the Duke of Sussex were to stay in the U.S. past that threshold, what Lesperance fears would definitely happen.

If he refuses to have a visa instead of a green card, then he could choose to be a U.S. taxpayer this year and a U.K. taxpayer next year.

The set-up is called the O-visa, and Lesperance believes that this could give Prince Harry the maximum flexibility he wants.

"In another world, [Harry] could say, 'we're having another baby, I want to be home changing diapers, our activity for shooting a movie will be in the U.S. so this year I will be a U.S. person for tax purposes, next year I may not be'," he explained. "Whereas, if you have a green card, you are a U.S. person for tax purposes until you give up that green card."

It is highly likely that the royal prince would avail a visa called the O-1 non-immigrant visa. This allows individuals with remarkable achievement in their field to stay in a country without shouldering taxes.

The tax expert added that Prince Harry could use his background as the founder and organizer of the Invictus Games to qualify for this visa.

Although he has a lot of choices to choose from, an insider revealed earlier this year that Prince Harry is not considering getting a green card.

An insider told E! News that Prince Harry has a valid reason why he refuses to apply or does not consider U.S. citizenship.

"Harry is not applying for a Green Card or dual citizenship anytime soon, which will come as a surprise to many because that is what most people assumed he'd do on moving to the U.S.," the insider said.

READ MORE: Princess Diana's Death: 3 Times the Royal Family Disappointed, Hurt the Princess of Wales