When a planet passes in front of its star, some starlight filters through its atmosphere. Gases in the atmosphere block certain colors and let others through, like the colored glass of a stained-glass window. By seeing which colors are dimmed, you can figure out what the air of a distant world is made of.
Astronomers pointed the William Herschel Telescope at HAT-P-44b, a puffy hot Saturn. Over 246 millionths of the star's brightness, they spotted the signature of sodium: this metal in the upper atmosphere absorbs yellow-orange light. The signal is robust — four times stronger than random fluctuations.
And there's more. The spectrum revealed hints of water vapor and light scattering by hydrogen molecules. This means the atmosphere isn't empty: it's mostly hydrogen with a bit of water. Heavy elements (anything heavier than hydrogen and helium) are about seven times more abundant than in the Sun. The temperature at the altitude where the spectrum forms is around 800 K, like a kiln for firing ceramics.
HAT-P-44b turns out to be almost a twin of the well-studied WASP-39b. Both are hot, puffy Saturns, both with sodium, water, and a hint of clouds. But HAT-P-44b is a bit cooler and richer in heavy elements. Such similarities help astronomers test atmospheric models on sister planets.
The sodium in the planet's atmosphere absorbs the same yellow-orange light as in street sodium lamps, which glow with a warm amber color.