Universiteit Leiden

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Denis Bajram

Halo of gas shows how exoplanets are slowly losing their atmosphere

Twee teams of astronomers, including from the universities of Amsterdam and Leiden, have discovered how hot gas giants are surrounded by large halos of thin helium gas, an indication that they are slowly losing their atmosphere. The helium, that was recently seen for the first time by the Hubble space telescope has now been mapped precisely for two planets using a method that was developed in the Netherlands.The two results will be published in Science on 7 December independently of one another.

Two gas giants

The two gas giants in question orbit very closely around their mother star, which means that they are very hot. One of these is WASP-69b, a planet the size of Jupiter but with a temperature of over 1000 º Celsius. The second planet is HAT-P-11b, comparable to Neptune in size. The planets are around 100 to 150 light years from the Earth. 

Forgotten

‘Gas giants are made up largely of helium, but this gas is very difficult to observe,' says Javier Alonso Floriano from Leiden University and co-researcher in one of the studies. 'Twenty years aso it was already predicted that the helium gas might be visible in infrared light, but little effort was made to search for it, and then it was forgotten. That is until earlier this year, when the Hubble spae telescope first saw a dip at exactly the right infrared colour, which pointed to the possibility of the presence of helium.’

Measuring from the ground

‘What our two research groups discovered independently of one another is that this helium is easier to observe using ground-based telescopes,' says Lorenzo Pino from the University of Amsterdam. ‘We can measure the size and speed of the gas, and how it surrounds the planets like a thin halo. That's something you can't mesure with Hubble.'

Artistist's impression of the helium halo (Denis Bajram)

The Dutch method

This technique uses high-resolution spectrosopy that can measure the colour of the light of the planet up to one in one hundred thousand. Both teams used a brand-new Spanish-German spectrograph, CARMENES, in Andalusia in Spain, near to Granada. The method itself was largely developed by Dutch astronomers. While a planet seen from the Earth passes in front of its mother star, a small amount of starlight penetrates the planet's atmosphere, which, depending on the type of gas, is absorbed in very specific colours. This can be measured using the speed of the planet, which creates a kind of Doppler effect. 

Atmosphere dissipates

‘What is so great is that we have now finally found a good way of measuring the thin gas around such a hot planet,' commented Aurélien Wyttenbach from Leiden University and the University of Geneva. 'This gas shows how, and at what speed, the hot giants are losing their atmosphere. In very early history this was also an important process for the Earth, but there's no longer anywhere we can study it, except for these kinds of extreme planets. Measuring a lot more planets will make it possible for us to test our ideas more extensively.' 

Source: astronomie.nl

Publications

  • 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
  • Ground-based detection of an extended helium atmosphere in the Saturn- mass exoplanet WASP-69b. Lisa Nortmann, Enric Pallé, Michael Salz, Jorge Sanz-Forcada , Evangelos Nagel, F. Javier Alonso-Floriano et al.

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