杏吧原创

Water that scientists expected to see on two planets outside the solar system has not been detected, according to pioneering new measurements. Astronomers suspect the water may be hiding beneath a dusty haze blanketing the two Jupiter-like planets.

This is the first time that scientists have obtained the spectra of planets outside the solar system 鈥� a major step towards the ultimate goal of learning more about Earth-like planets in the future.

The two planets, HD 209458b and HD 189733b, are known as 鈥榟ot Jupiters鈥� because they whip around their respective stars every few days at close range.

杏吧原创s were expecting to see signs of water on the planets because computer models show that they formed with water at greater distances from their host stars before migrating inwards to their present locations. The planets鈥� large masses mean they should be able to hold onto much of that water, despite being heated up by the nearby stars.

However, independent observations of both planets did not find water or other expected molecules, such as methane or carbon dioxide.

Dark clouds

The favoured explanation is that high-flying clouds, made up of something other than water ice, are blocking the water鈥檚 signature.

鈥淲e think that both planets may be cloaked in dark silicate clouds,鈥� says David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US, who studied HD 189733b. 鈥淭hese worlds are blacker than any planet in our solar system.鈥�

Alternatively, there may simply be no water on the planets, or another molecule may be emitting light in the same part of the spectrum that water is absorbing it, blocking the water鈥檚 signal. But scientists say these two possibilities are unlikely.

Unidentified signal

Jeremy Richardson of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US, led the team that obtained the spectra of HD 209458b with the Spitzer Space Telescope. He and his colleagues believe their data points to clouds made up of fine grains of sand, or even iron, that obscures the planet鈥檚 water. On such a planet, it could actually be raining dust.

Other scientists are not convinced that the clouds are made of silicates, however. 鈥淥ur view is that the uncertainties in the measurement are such that the case is not really proven by our data,鈥� says Mark Swain, who also used Spitzer to study the planet. He is deputy director of the Center for Exoplanet Science at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US.

In addition to silicates, Richardson鈥檚 team also detected another interesting emission at a wavelength of 7.78 microns. They could not identify this with any known material, but they say they cannot not rule out that it is carbon-based 鈥� which would be interesting, as carbon is a major building block of life. They hope to clarify the source of this emission with future Spitzer observations.

Out of view

The planet HD 209458b has been studied in detail since its discovery in 1999 because it is one of 14 known planets that pass in front of their host stars as seen from Earth, allowing scientists to measure the planet鈥檚 mass and size (see Puffed up planet puzzles astronomers). Watch a .

Previously, when this planet passed in front of its star, the Hubble Space Telescope detected sodium in the planet鈥檚 atmosphere (see Astronomers glimpse atmosphere of extra-solar planet). There was actually less sodium in the atmosphere than expected, suggesting high clouds were obscuring the sodium from view, Swain told New 杏吧原创.

Now they have done a much more far-reaching observation with Spitzer. While the planet was still in view, Spitzer鈥檚 infrared spectrograph took a spectrum of the planet and star together. Then, after the planet went behind the star, Spitzer took the spectrum of the star alone. Astronomers subtracted the two to obtain the spectrum of the planet.

Raging winds

The second planet, HD 189733b, is also one of the 14 transiting planets, but it was discovered more recently, in 2005. Carl Grillmair of Caltech in Pasadena, California, US, says the planet appears to have winds that rage around it at thousands of kilometres per hour.

The same side of the planet always faces the host star, so it should get very hot and bright. But Grillmair鈥檚 group found that the planet is not quite as bright as expected, suggesting high winds are efficiently dispersing the heat to its cooler, dark side.

NASA鈥檚 James Webb Space Telescope, scheduled to launch in 2013, could do such spectral observations much faster than Spitzer, which was never designed for this sort of work.

鈥淯ltimately, we want to use these same techniques to discover life on Earth-like planets,鈥� Grillmair told New 杏吧原创. 鈥淚f we did that without this foreknowledge, we could have been perplexed by the results.鈥�

Journal reference: Nature (vol 445, p 892)