A watery exoplanet of unexpected composition -- and perhaps origin

"Our solar system's planets follow a very tight trend: the heavier the planet is, the less heavy elements, like water, there are in the atmosphere", he said.

In orbit around a star 437 light years away, is a planet known as HAT-P-26b that is roughly the size of Neptune, but much closer to its host star. Now thanks to a joint effort between NASA and a team of scientists from various universities such as the University of Maryland and University of Exeter, a new study is showing the most detailed analysis of a warm Neptune to date. Researchers believe that, compared to the gas giants in our own solar neighborhood, this planet either developed later in the history of its solar system, closer to its star - or both.

The discovery of such a primordial atmosphere on this Neptune-sized planet has implications for how scientists think about the birth and development of planetary systems.

"Astronomers have just begun to investigate the atmospheres of these distant Neptune-mass planets, and nearly right away, we found an example that goes against the trend in our solar system", said Hannah Wakeford, a postdoctoral researcher at NASAs Goddard Space Flight Centre in the US.

It's a memorable moment for the scientists involved. The astronomers also detected water and evidence of exotic clouds in the planet's air. The atmosphere has been described as ancient, helping shed light on the nature of planets this size and how they may form.

But in the case of HAT-P-26b, we've got a world that's not far out from its host star - and is in fact virtually clinging to it - and yet its atmosphere is nearly entirely composed of hydrogen and helium, much like Jupiter and Saturn. "This exoplanet, HAT-P-26b, we were expecting it to look like Neptune with great amounts of water but it actually looks more like Jupiter".

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It turned out that the composition of HAT-P-26b is much closer to Jupiter than Uranus or Neptune.

When the planet passes in front of its star, light is filtered through the planet's atmosphere before it reaches the telescope on Earth.

A recent study carried out with the help of NASA's Hubble and Spitzer telescopes have discovered a distant Neptune-sized planet - HAT-P-26b - which comprises of a primitive atmosphere comprising entirely of hydrogen and helium. This pointed towards how rich the planet is in elements which are heavier than hydrogen and helium.

HAT-P-26b, however, with a metallicity of just about 4.8 times the metallicity of our sun, runs closer to Jupiter rather than Neptune. "We expected the smaller worlds to be formed further out, where they would accumulate clumps of icy debris and richer heavy elements during the formation in the [protoplanetary] disk".

"Not too long ago, it was exciting just to find an exoplanet", said Drake Deming, a professor of astronomy at UMD and a co-author of the study.

  • Regina Walsh