A trick of the light reveals out-of-this-world water vapor

In this week’s edition of Nature, researchers in the US present the strongest evidence yet of a water-vapor signature in the atmosphere of the so-called ‘hot-Jupiter’ planet, HD 189733b. The finding settles doubts that arose from previous studies with ambiguous or contradictory results.

Hot-Jupiters are planets outside our Solar System that orbit their parent stars closely and have masses similar to, or greater than, that of Jupiter. Of the 300 currently known extrasolar planets, HD 189733b is the most easily observable. It has been heavily scrutinized by both by ground- and space-based observatories and yet, until now, the presence of water — considered essential for supporting life — had been uncertain as it failed to show in previous observations.

Carl Grillmair from the Spitzer Science Center in California and his colleagues conclusively detected the signature of water absorption by using the refined ‘secondary eclipse method’ to study the infrared spectrum of HD 189733b. This method can be used to better understand other planetary atmospheres in extreme environments, and has been used in previous studies, but Grillmair and colleagues have pushed it past its previous application.

The secondary eclipse method measures the infrared and visible spectra of the planet through a geometric trick: the light spectra are measured before and after the planet passes behind its parent star during a secondary eclipse. Then, the planet’s contribution is deduced by subtraction from that of the parent star when it was recorded in isolation.

From the averaged observations of ten secondary eclipses of HD 189733b made with the Spitzer Space Telescope’s Infrared Spectrograph (IRS) between mid-June and mid-December 2007, Grillmair and his colleagues were rewarded with finding the characteristic signature of the water molecule.

Commenting in a related News and Views article, Drake Deming of the NASA Goddard Space Flight Center notes that the technique developed by the researchers “might be used to prove a nearby habitable world.”