Back to the books
Benedict, a theologian by training, is likely to switch from universal pastor back to scholar.
"My sense is that he will lay low out of deference to the new pope, that he will stay out of the way and under the radar," Hilgartner said. He expects the pope to behave mostly like a retired scholar, doing lots of reading and maybe a little writing.
Benedict was rumored to be working on his fourth encyclical before he announced he would resign, Hilgartner said. Encyclicals are papal letters to the church, often on pressing matters that carry the weight of the office the pope with them.
"He had written the encyclical on hope, the encyclical on love, and another one on social justice and charity," Hilgartner said, adding that the rumored fourth may be on faith. As a retired pope, Benedict's final encyclical would not carry the weight of the office.
That is something Benedict had not imposed on his previous scholarly works while in office.
"He was careful not to bless his own writings with the papacy," said Pia de Solenni, a moral theologian from Seattle.
When he published books as the pope his byline was "Joseph Ratzinger -- Pope Benedict XVI," de Solenni noted.
"I think he was willing to engage with others." She said his books are "a sharing of ideas and he's putting his ideas out on paper. To me it's an incredible mark of his humility."
One thing for sure: he won't be writing any more tweets. The Vatican said the official Twitter handle @pontifex will be retired along with Benedict.
Life beyond the walls of the Vatican
Benedict said he no longer had the strength to go on. After he announced his retirement, the Vatican said he had begun thinking about leaving the office after a strenuous papal visit barnstorming across Mexico and Cuba.
When he leaves the office he will give up his Fisherman's Ring, which takes its name from St. Peter's occupation. It will be destroyed along with "the lead seal of the pontificate," Vatican spokesman Federico Lombardi said.
He will also be giving up his personal security detail, the 100 to 120 members of the Swiss Guard who are responsible for round-the-clock protection of the pope.
"He received security like any other head of state," former Swiss guard member Andreas Widmer said.
While best known for their Renaissance-era dress uniforms -- brightly striped puffy-sleeved shirts and pants -- along with their ceremonial battle axes, they are a formidable modern security detail, according to Widmer, who now runs the entrepreneurship program at the School of Business and Economics at the Catholic University of America.
Widmer had a kinship with Benedict in the late 1980s while he was a young German-speaking member of the guard and Benedict, whose native tongue is German, was a top cardinal serving John Paul II.
He described Benedict as an "unbelievable introvert." He said Benedict was always friendly with people at the Vatican one on one, even beggars on the streets, but large crowds sapped his energy.
The task of protecting two popes would have meant doubling the Swiss Guard force, a group unaffiliated with other Swiss security forces, as the guard predates the Swiss state.
But Widmer suspects that would not have been an issue anyway. His hunch is that Benedict will retire and remain cloistered.
"My guess is Benedict is not going to leave the Vatican," Widmer said. "It's not like he's going to make these huge moves. My guess is anything he's going to write and say will only come out after he dies."
A turbulent time
Before he became pope at the age of 78 Benedict had talked at length about retiring.
Speculation has swirled over what finally pushed him to step aside -- Vatileaks, the sexual abuse crisis, or the growing tide of secularism.
The "Vatileaks" scandal began with his butler leaking documents showing disarray and mismanagement and led to an internal review that was reported to contain details of gay sex scandals (reports which the Vatican calls baseless) and money woes came to a close this week when three cardinals reported their finds back to the pontiff.