Guests of honor William and Kate – in the dress she wore in L.A. on the last night of their 2011 tour and beautiful Kiki McDonough special edition morganite earrings – were the last to arrive, greeted by Jason Knauf, chief executive of the Royal Foundation and Hannah Jones, CEO of the Earthshot Prize. Guests were asked not to purchase new frocks or suits for the evening.
Presenter Emma Thompson said she came in the outfit she wore when she last saw Prince William — her suit and the badge that signifies her damehood.
Samir Hussein/WireImage Prince William and Kate Middleton
"On our little notes they said 'Please do not buy anything new for this.' Can you imagine? The relief! So I got my dame suit that I wore when I went to the palace," she shared.
Thompson also showed off a ring that her husband Greg Wise "bought me for 30 bucks in New York" and her vegan sneakers. She spoke about the evening as an opportunity to help counter the hopelessness of issues like food waste, saying, "One of the solutions is going to be here and that makes me happy."
Joe Maher/Getty Prince William and Kate Middleton
From a pool of 15 finalists, five winners will soon be announced, one each from five categories the minds behind Earthshot have deemed critical focus areas to save the planet: Protect and Restore Nature, Clean Our Air, Revive Our Oceans, Build a Waste-Free World, and Fix Our Climate.
Each of these five winners will receive $1.3 million to advance their work, and the Duke of Cambridge will host a ceremony each year through 2030 to award five more recipients the resources to pursue innovative solutions and scale up their ideas to repair the planet.
"This is an absolutely incredible community," finalist Sam Teicher, who is working to restore coral reefs, told PEOPLE before Sunday's event. "The other finalists are so inspiring. We're already thinking about the synergies that exist between us all and the different partners within the Earthshot community."
Teicher continued, "There are a lot of people talking about Earthshot and these solutions and the other solutions out there. ... [T]here's already good progress being made because of Prince William."
Clara Amfo and Dermot O'Leary are hosting the event, which will be streamed from London's Alexandra Palace with performances by Ed Sheeran, KSI and Yemi Alade, Shawn Mendes, and Coldplay (on a set powered by 60 cyclists!).
Legendary naturalist (and Cambridge kid favorite!) Sir David Attenborough will speak about the Earthshot Prize and the importance of finding solutions to environmental challenges. Emma Watson, David Oyelowo, Mo Salah, and Thompson are among the presenters celebrating change-makers who have a passion for global environmental issues throughout the evening.
The inspiration for Earthshot goes back to when William was a child studying U.S. President John F. Kennedy's wildly ambitious 1960s Moonshot initiative, which sought to harness innovation to send a man to the moon within 10 years.
"Humans have an extraordinary capacity to set goals and strive to achieve them," William shared in a TED Talk in July.
AARON CHOWN/POOL/AFP via Getty Prince Louis, Princess Charlotte and Prince George
But the reason was also personal for the royal, according to Royal Foundation chief executive Jason Knauf, who revealed, "The challenge the Duke set himself was, 'What is the maximum positive personal contribution I can make in the next 10 years in the fight against climate change? What am I going to do in the next decade that means I can look my children in the eye and say that I did my bit?' Every aspect of the Prize bears the stamp of his contribution."
And on Thursday, William said he hoped he wouldn't pass down the "burden" of worrying about the environment to his three children: 8-year-old Prince George, 6-year-old Princess Charlotte, and 3-year-old Prince Louis.
For more information about William's campaign to repair the planet and spur innovation, you can watch the five-part series The Earthshot Prize: Repairing Our Planet on Discovery's streaming platforms or pick up Earthshot: How to Save Our Planet wherever books are sold.