CARMENES the “planet-hunter”: exploring evaporating atmospheres and water vapour beyond the Solar System

Instituto de Astrofísica de Andalucía (IAA-CSIC)
  • The spectrograph CARMENES studies the Cosmos measuring the temperature, composition and movement of extrasolar planets
  • New approaches to the study of planets from the ground are developed with the CARMENES project
  • Eleven German and Spanish institutions take part in the CARMENES project, including the Instituto de Astrofísica de Andalucía and the Instituto de Astrofísica de Canarias


Ever increasing numbers of detected extrasolar planets

According to the Extrasolar Planets Encyclopaedia, as of March the 1st, 2019, a total of 3,999 confirmed exoplanets were confirmed, including some previously dubious claims dating from the late eighties. The current count of confirmed exoplanets is now even higher. This numerous pool of planets detected outside the Solar System give ample grounds for the study of the characteristics of these planets.

Instruments such as CARMENES, a high-resolution spectrograph, can undertake their study. Spectrographs as this are instruments used for the separation of light into its sub-component wavelengths. This provides information about the temperature, chemical composition, or the motion of cosmic entities such as galaxies, stars, planets or comets, among others. CARMENES is a high-tech equipment co-developed by the Institute of Astrophysics of Andalusia together with ten other outstanding partners of Spain and Germany (full list at the bottom).

On 12 March 2019, the 4000-mark of detected extrasolar planets in our galaxy was surpassed.

The device, installed at the Calar Alto Observatory (CAHA), in Almería, is performing yet as a leading instrument: it has analysed the proportion of helium and water vapour in the atmospheres of several exoplanets. It has yet offered data with better resolution than those of the Hubble Space Telescope and opened new avenues in atmospheric studies.

“CARMENES is really 2 instruments in 1, observing simultaneously in the visible and in the infrared. This allows us, on the one hand, to make direct detections of planets avoiding false positives and, on the other, to undertake the study of planetary atmospheres The latter is possible thanks to the infrared channel (CARMENES-NIR), which was developed at the Institute of Astrophysics of Andalusia and which is a reference in its field worldwide”, says Pedro J. Amado, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) that has co-directed the development of CARMENES.


Evaporating atmospheres

Of the extrasolar planets found to date, many are classified as "hot jupiter" planets: giant gas planets that revolve around their star at a distance closer than that of Mercury to the Sun. The atmospheres of these planets suffer intense stellar radiation causing strong erosion, in some cases causing the full evaporation of their atmosphere.

“Studies of atmospheric escape in exoplanets have been made mostly by studying a spectral line of hydrogen, which requires observations from space”, as highlighted by Lisa Nortmann, a researcher at the Institute of Astrophysics of the Canary Islands (IAC) and who heads the study published in Science. However, adds Nortmann, “there is another tracer of atmospheric evaporation: helium, which we have observed from the ground with the CARMENES instrument in the planet WASP-69b, and which we interpret as a sign that its atmosphere is escaping and leaving a trace similar to the tail of a comet”.

Depiction of a "Hot Jupiter" exoplanet, HD 189733 b, showing a comet-like tail due to atmosphere evaporation caused by the heat.

Helium is a very light atom that is found in the outermost layers of the atmospheres where it is present. Unlike hydrogen, it presents a metastable state: an unstable but relatively long-lived state known as the helium triplet. This helium triplet state had been proposed as sensor of exoplanetary atmospheres.

“However, the triplet [state] was not observed until this year, with the Hubble Space Telescope", says Manuel López Puertas, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC). "But it did it at low spectral resolution, that is, without detail. Our observations from the ground present a higher resolution, which allows us to know, after modelling the data, if atmospheric escape occurs, its extension, the speed at which it expands or how much mass it is losing. Thus, we have shown that we have a new method for studying the atmospheres of extrasolar planets, and we hope that it will have a wide application in the future”.


Water vapour: tracer of clouds and aerosols

CARMENES has premiered also in the study of water vapour in the atmospheres beyond the Solar System, addressing another “hot Jupiter” planet called HD 189733b. Water had already been detected from space at several times, but its detection by CARMENES strengthens the arsenal of methods for the analysis of exoplanetary atmospheres from the ground. So, the use of support 4-meter telescopes becomes a complement to space telescopes or to 8-10 meter telescopes.

The detection of water vapour is complex. It requires a detection technique different of that used for helium. Helium presents a very strong spectral absorption line and is not affected by the Earth's atmosphere. Water, on the other hand, is a molecule with thousands of very weak spectral absorption lines instead of a single strong one. As an additional difficulty, the water vapour of the Earth atmosphere filters part of the relevant incident wavelengths of the light spectrum. This entails a notable difficulty for obtaining such signals with telescopes located on the ground.

Water vapour present in the Earth atmosphere interferes with attempts to measure from the ground its presence in exoplanets.

“As a novelty, compared with previous works, we have obtained our data in several and different bands (or spectral regions) from those used up to now, which allows us to use this technique not only to detect water but also to detect the presence of clouds and aerosols. This study opens the door to the study of molecular compounds, such as methane, water or carbon dioxide, in exoatmospheres”, using CARMENES, concludes Alejandro Sánchez López, researcher at the IAA-CSIC.


Carmenes, the planet hunter team

CARMENES is a unique instrument in the world, both in precision and stability. It works in vacuum conditions and with temperatures controlled to the 1/1000th of a degree. To build such a finely-tuned tool, the efforts of an international team had to be coordinated.

The participants in Spain include the Institute of Astrophysics of Andalusia (IAA-CSIC), which co-leads the project and has developed the infrared channel, as well as the Institut de Ciències de l'Espai (IEEC-CSIC), the Complutense University of Madrid (UCM), the Institute of Astrophysics of the Canary Islands (IAC) and the Center for Astrobiology (CAB, CSIC-INTA). Support was also obtained from the Max Planck Society (MPG), the Spanish Higher Council for Scientific Research (CSIC) and the members of the CARMENES consortium, with contributions from the Spanish Ministry of Economy and Finance (MINECO), the states of Baden-Württemberg and Lower Saxony, the German Foundation for Science (DFG), the Klaus Tschira Foundation (KTS), the Junta de Andalucía and the European Union through FEDER / ERF funds.



Image credits

Cactus and Milky Way picture is in the public domain and was downloaded from Pixabay.

Exoplanet HD 189733 b picture is in the public domain and was downloaded from Wikimedia Commons.

Earth atmosphere picture is in the public domain and was downloaded from the NASA Astrobiology Institute.

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