Vacharaesorn Vivacharawongse is the second of four estranged sons of King Vajiralongkorn. Until now they were widely believed to have been stripped of their royal status and to have no place in the line of succession to the throne.

But the succession became problematic after the king's eldest daughter Princess Bajrakitiyabha, the most plausible heir, collapsed last December. She is still in a coma and seems unlikely to recover.

For years there has been quiet talk in Thailand about the possibility of one of the estranged sons being rehabilitated and brought back to play a royal role.

Mr Vacharaesorn's unannounced visit could be the start of such a rehabilitation, although there has been no official statement from the palace.

He is one of five children born to King Vajiralongkorn's second wife Sujiranee, a former actress whom he divorced in 1996.

Mr Vacharaesorn and his three brothers, who were at school in UK at the time, were in effect banished from Thailand with their mother, and have lived in the US since then. His sister, Princess Sirivanavari was brought back to Thailand to live with her father.

In the past the four sons occasionally published letters appealing for reconciliation with their father, and to be allowed to return to Thailand, to no effect.

Mr Vacharaesorn has spent his first three days in Bangkok partly as a tourist, rediscovering the city, visiting temples, enjoying Thai food and riding in a tuk-tuk. He told journalists coming back was like a dream come true for him.

But has also had what appear to be official engagements, meeting Thailand's most senior religious figure, the Supreme Patriarch of the Buddhist Sangha who is appointed by his father, and visiting a childcare centre sponsored by the palace.

Many Thais will interpret this as showcasing his potential for royal duties.

Back in the US he has been the most visible of the four sons, very active in the Thai community there and openly supportive of the Thai monarchy.

He appears to be the one who is most suited to and interested in taking a higher profile role in the royal family.

Until her collapse last year Princess Bajrakitiyabha was seen as the most accomplished of the king's children who are living in Thailand. She is a lawyer who had worked with the UN and been Thailand's ambassador to Austria.

The King's youngest child, Prince Dipangkorn, who is 18, is believed to suffer from some learning difficulties, and it was widely assumed that Princess Bajrakitiyabha would play the role of regent, or even become queen, on the death of her father.

Thai succession law leaves it to the monarch to designate his successor, which King Vajiralongkorn has not yet done.

So could Mr Vacharaesorn, a successful and popular figure back in the US, fit the bill? It is far too soon to say.

More likely this will be seen as an exploratory first visit, with further trips in the future to see how he copes in the public spotlight, and with the rigid protocol and sometimes Byzantine politics of Thailand's royal court.

And it is impossible to know what King Vajiralongkorn's views are on this.

People in his inner circle have been known to go in and out of favour quite suddenly, sometimes disappearing from public view.

Whoever organised this return of a prodigal son will be doing so with understandable caution.