The universe is incredibly old, astoundingly vast and populated by trillions of planets -- so where are all the aliens? Astronomer Stephen Webb has an explanation: we're alone in the universe. In a mind-expanding talk, he spells out the remarkable barriers a planet would need to clear in order to host an extraterrestrial civilization -- and makes a case for the beauty of our potential cosmic loneliness. "The silence of the universe is shouting, 'We're the creatures who got lucky,'" Webb says.

Stephen Webb: Where are all the aliens?

Where are all the aliens?

Our search for extraterrestrial life continues, but the deeper mystery lies in why the vast, ancient universe remains silent.

Could Earth's perfect conditions be unique in the cosmos? The 'Rare Earth' hypothesis suggests we might be the universe's sole storytellers.

Exploring cosmic silence: Despite trillions of planets, Earth may be uniquely suited for life, hinting at rare cosmic conditions.