Aliens could plausibly sterilise earth from light years away using an electron beam, according to a great Kurzgesagt episode: ▶ this video An electron beam seems to be the stealthiest of the three extermination options, the other two being a star laser and antimatter "rods from God."
in “GoPro Ride Through an Electron Beam Irradiator at Full Beam Power [video]” · View on HN