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Two teams of astronomers, amongst whom Lorenzo Pino from the University of Amsterdam, have discovered how hot gas giant planets are shrouded in large halos of tenuous helium gas, suggesting that they are slowly losing their atmospheres. The helium, that was only recently discovered in data from the Hubble Space Telescope, has now been meticulously mapped using a method that was earlier developed in the Netherlands. The results from the two groups have separately been published in Science.

Artists impression of the helium halo around exoplanet HAT-P-11b. Image: Denis Bajram

The studies entail two gas giants that rotate around their mother stars very quickly, causing them to be very hot. One, WASP-69b is a planet the size of Jupiter but with a temperature of over a thousand degrees Celsius. The second planet is HAT-P-11, comparable in size to Neptune. The planets are between 100 and 150 light years away from earth.

Gas giants

‘Gas giants consist of mostly helium, but that’s a very difficult element to observe,’ says Javier Alonso Floriano of the University of Leiden and co-author of one of the studies. ‘However, that the helium gas should be visible in infrared light was predicted twenty years ago already, but back then nobody looked for it after which this theory sank into oblivion. Until earlier this year, when the Hubble space telescope first saw a tiny dip in precisely the expected color of infrared that pointed to the existence of helium.

‘What these two research groups have now independently discovered is that this helium is much easier to observe from the ground,’ says Lorenzo Pino of the University of Amsterdam. ‘we can measure the extent and speed of the gas, and how it sits around the planets like a tenuous halo. Hubble can’t see these things.’

The technique uses high resolution spectroscopy that is able to split the planet’s composite light in hundreds of thousands of colors. Both teams used a brand new Spanish-German spectrograph, CARMENES, in Andalusia (Spain).  Dutch astronomers significantly contributed to the development of the method. While a planet passes in front of its mother star as seen from Earth, a tiny bit of starlight passes through the planet’s atmosphere, being absorbed differently depending on the types of gas in it. This is measured by using the doppler effect caused by the speed of the planet’s movement.


‘The beauty of this is that we have now finally found a good method to measure the tenuous gas round such a hot exoplanet,’ says Aurélien Wyttenbach of the University of Leiden and the University of Geneva. ‘This gas shows how, and with which speed, the hot gas giants are losing their atmospheres. We know how this process worked in the very early history of Earth, but these days we no longer have any place where we can research this, except for these types of extreme planets. By measuring many more of these planets, we can extensively test our theories about this.’

Publication details

Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune exoplanet. R. Allart, V. Bourrier, C. Lovis, D. Ehrenreich, J.J. Spake, A. Wyttenbach, L., Pino, F. Pepe, D.K. Sing