The findings will appear in Astrophysical Journal Letters.

This year, astronomers including Dr. Swain’s group reported finding water vapor and methane swirling around HD x189733b. And in Thursday’s issue of the journal Nature, a different group of astronomers, led by Carl J. Grillmair of the California Institute of Technology, now report that they, too, have detected water around the same planet, using a technique more precise than that used in earlier research.

Image This artist's concept shows a cloudy Jupiter-like planet that orbits very close to its fiery hot star. Credit... NASA/JPL-Caltech/T. Pyle

As seen from Earth, HD 189733b passes directly in front of and behind its parent star as it orbits. Taking advantage of those eclipses, Dr. Swain’s group used the Hubble Space Telescope to compare the near-infrared light from the star alone (when the planet was hidden behind it) with the combined light from both.

The difference between the two spectrums revealed the light emitted from the planet, and the mix of colors in the planet’s light contained the telltale signs of carbon dioxide at concentrations of between one part per million and one part per 10 million, compared with Earth at about 385 parts per million.

Even that much carbon dioxide was a bit of a surprise, because the simplest chemistry equations predicted that carbon would prefer to form carbon monoxide or methane molecules. One possibility is that the intense ultraviolet radiation from the star, just three million miles away, is spurring chemical reactions to produce the observed carbon dioxide.

“The theorists will have no problem explaining it,” said L. Drake Deming, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and a member of Dr. Swain’s team.