Five Hundred Light Years

Camila let Polly stand on the back of her chair during the first observation of the night. The chair was carbon fibre. The control room was lit only by red lamps to preserve night vision.

The target was a star called HD 189733. It is a Sun-like star sixty-three light years from Earth. It has at least one planet, a Jupiter-sized gas giant orbiting very close in. Tonight's observation was a transit. The planet was going to pass in front of its star, and Paranal was going to measure the light that passed through the planet's atmosphere on the way to here.

This is called transmission spectroscopy. The light from a star is a known thing. The light that has been filtered through an alien atmosphere has gaps in it where particular molecules have absorbed particular wavelengths. The gaps are the signatures of the molecules.

They had already detected sodium and water vapour in this planet's atmosphere. Tonight, they were looking for carbon monoxide.

"Why does it matter?" Polly asked, quietly.

Camila did not look up. "The chemistry of an atmosphere tells you about the star, the planet, and, sometimes, whether life could be there. HD 189733b is a gas giant. There is no life on it. The atmosphere is fifteen hundred degrees Celsius and rains liquid silicate. But understanding its atmosphere helps us understand smaller, cooler planets where life is possible. We are not yet good enough to look at Earth-sized planets directly. We are practicing on the big easy targets."

A small green line on her screen began to dip. The transit had started.

"There," Camila said. "The planet is in front of the star. The light is dropping by one and three quarters percent."

Polly perched still. The light hitting the telescope had left the star sixty-three years ago. Sixty-three years ago, humans had not yet landed on the Moon.