When plants take in too much energy, they don’t get fat—they lighten up. They absorb more sunlight than they need to power photosynthesis, and they get rid of the excess solar energy by emitting it as a very faint glow. The light is far too dim for us to notice under normal circumstances, but it can be measured with a spectrometer. Called solar-induced fluorescence (SIF), it’s the most accurate signal of photosynthesis that can be observed from space.