(7:53 p.m. EDT) — The biggest cruise ship in the world is getting another sister.

Royal Caribbean Cruises Ltd. has revealed plans to build a fifth Oasis-class ship, less than a week after Royal Caribbean’s 5,479-passenger Harmony of the Seas set sail on its inaugural cruise. The ship is scheduled for delivery 2021.

Additionally, the company said it would double Celebrity Cruises’ new Edge Class with two ships slated for 2021 and 2022. The first two ships, which are scheduled to debut 2018 and 2020, represent a new class that touts “small-ship itineraries with large-ship amenities.” Edge-class ships will be smaller than the line’s Solstice Class but larger than the Millennial Class.

Both line’s presidents shared their thoughts on the expansion, in a statement.

Speaking to Harmony of the Seas’ recent delivery, Royal Caribbean President and CEO Michael Bayley said the warm reception of the vessel “is evidence that growing this class of ship will continue to drive exceptional performance for the brand.”

Celebrity Cruises President Lisa Lutoff-Perlo said there had been “a tremendous amount of interest around the new standard of style Edge-class will introduce.”

Royal Caribbean expects a fourth Oasis-class ship to launch spring 2018. Like Harmony of the Seas, it will carry 5,479 passengers (slightly more than Oasis and Allure) and feature a mix of Oasis-class and Quantum-class features.

All three new ships will undergo construction at Saint-Nazaire’s STX shipyard, where Royal Caribbean’s fourth Oasis-class ship and Celebrity’s two Edge-class ships are being built. Deployment, ship names and more details of what passengers can expect onboard have not yet been released.

The company also has a fourth Quantum-class ship on the horizon, expected to launch April 2019.

See a full list of new ships on order from now through 2020.

–By Gina Kramer, Associate Editor