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 kept dark, lit only by red lamps to preserve night vision. Camila's face was mostly in shadow, lit by the soft red of the screens.

The target was a star called HD 189733. It is a Sun-like star sixty-three light years from Earth, in the constellation Vulpecula. It has at least one planet, HD 189733b, which is 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 was called transmission spectroscopy. It was, Camila said, one of the main ways astronomers worked out what exoplanet atmospheres were made of. 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 were the signatures of the molecules.

They had already, on previous nights, detected sodium and water vapour in HD 189733b's atmosphere. Tonight, they were looking for carbon monoxide.

Polly tilted her red head. "Why does it matter?" she asked, quietly.

Camila did not look up. "Because the chemistry of an atmosphere tells you about the star, the planet's temperature, the planet's formation history, 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 what is in its atmosphere helps us understand the atmospheres of smaller, cooler planets where life is possible. We are not yet good enough at this to look at Earth-sized planets directly. We are practicing on the big easy targets."

A small green line on her main screen began to dip. The transit had started. Polly watched the line drop.

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

Polly perched still. One and three quarters percent of a star sixty-three light years away. The light hitting the telescope at that moment had left the star sixty-three years ago. Sixty-three years ago, when this light was beginning its trip toward Polly, humans had not yet landed on the Moon. The first communications satellite had not been launched. Tomas the ranger had not been born.

The transit would last two hours and twelve minutes. During that time, Camila would take spectra continuously, comparing the light from the star alone with the light from the star plus the atmosphere. She would not know for another two months, after the data had been processed, whether they had detected carbon monoxide.

"It is slow," Camila said. "We do the observation tonight. The data goes back to Garching, in Germany, where they process it. Then a graduate student spends maybe a year writing the paper. By the time we publish, this night will feel like a memory."

Polly thought about that. The light had taken sixty-three years to get here. The result would take another two years to be human knowledge. Time was strange in this room.

The green line stayed flat and low.