Missing water mystery of exoplanets solved

London: Astronomers have studied the atmospheres of ten hot, Jupiter-sized exoplanets in detail and solved a long-standing mystery – why some of these worlds seem to have less water than expected. Researchers used multiple observations from both the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope to study the planets, which are of various masses, sizes, and temperatures, across an unprecedented range of wavelengths.

“We found the planetary atmospheres to be much more diverse than we expected,” said David Sing of the University of Exeter in UK. All of the planets have a favourable orbit that brings them between their parent star and Earth. As the exoplanet passes in front of its host star, as seen from Earth, some of this starlight travels through the planet’s outer atmosphere.

“The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us,” said Hannah Wakeford, from the NASA Goddard Space Flight Centre. These fingerprints allowed the team to extract the signatures from various elements and molecules – including water – and to distinguish between cloudy and cloud-free exoplanets, a property that could explain the missing ater mystery.

The researcher’s models showed that, while apparently cloud-free exoplanets showed strong signs of water, the atmospheres of those hot Jupiters with faint water signals also contained clouds and haze – both of which are known to hide water from view.

To date, astronomers have discovered nearly 2,000 planets orbiting other stars. Some of these planets are known as hot Jupiters, hot, gaseous planets with characteristics similar to those of Jupiter.

They orbit very close to their stars, making their surface hot, and the planets tricky to study in detail without being overwhelmed by bright starlight. Due to this difficulty, Hubble has only explored a handful of hot Jupiters in the past, across a limited wavelength range. These initial studies have found several planets to hold less water than expected.

The researchers tackled the problem by making the largest ever study of hot Jupiters, exploring and comparing ten such planets in a bid to understand their atmospheres. Only three of these planetary atmospheres had previously been studied in detail; this new sample forms the largest ever spectroscopic catalogue of exoplanet atmospheres. The findings were published in the journal Nature.

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