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Astronomers using the near-infrared spectrograph (NIRSPEC) on the Keck II telescope at the WM Keck Observatory have confirmed the existence of water and a lack of methane in the water. atmosphere of HR 8799c, an extrasolar planet about 7 times larger than Jupiter that revolves around its host host every 200 years. While other astronomers had already performed similar measurements of the HR 8799c, the new data demonstrates the power of the combination of high resolution spectroscopy and a technique known as adaptive optics .
Artistic view of the HR 8799 planetary system. Image credit: Observatory W. M. Keck / Adam Makarenko / C. Alvarez.
Taking pictures of exoplanets is a daunting task. The light of the host stars far surpasses the planets, making them difficult to see.
Up to now, more than a dozen exoplanets have been directly imaged, including HR 8799c and three of its planetary companions.
In fact, the HR 8799 – located about 129 light-years away from the constellation Pegasus – is the only multiple-planetary system to have been photographed.
Once the image of a planet is obtained, astronomers can use instruments, called spectrometers, to separate the light from the planet, like a prism transforming sunlight into a rainbow, thus revealing the fingerprints digital chemicals. Until now, this strategy has been used to know the atmosphere of several giant exoplanets.
"At the moment, with Keck, we can learn more about the physics and dynamics of these giant alien planets, which look nothing like our own planets in the solar system," said Dr. Ji Wang, astronomer at Ohio State University.
The planetary system HR 8799 is the first planetary system beyond ours that astronomers have imitated directly. Captured in 2008 using the near-infrared adaptive optics of the Keck Observatory, the photo revealed three planets orbiting a young star named HR 8799 (center). In 2010, astronomers announced they detected a fourth planet in the system – HR 8799e. Image credit: CNI-HIA / C. Marois / Observatory W. M. Keck.
In this new study, Dr. Wang and his colleagues have combined the NIRSPEC instrument with the powerful adaptive optics of the Keck Observatory, a method for creating sharper images using a guide star in the sky to measure and correct turbulence of the earth's atmosphere.
"This is the first time this technique has been demonstrated on planets imaged directly using the L-band, a type of infrared light with a length of about 3.5 micrometers and a region of the spectrum with many detailed chemical fingerprints, "astronomers said.
"The L band has been largely neglected before because the sky is brighter at this wavelength. If you were an alien with eyes fixed on the L-band, you would see an extremely bright sky. It is difficult to see the exoplanets through this veil. "
The addition of adaptive optics has made the L band more accessible for the planet HR 8799c study.
In the study, the team performed the most accurate measurements ever done on the planet's atmospheric constituents, confirming that it had water and lacked methane, as previously thought.
"We are now more certain of the lack of methane on this planet. This may be due to mixing in the atmosphere of the planet. Methane, which we expected to be on the surface, could be diluted if the convection process reveals deeper layers of the planet that do not contain methane, "said Dr. Dimitri Mawet, scientist at Caltech and NASA Jet Propulsion Laboratory.
The results were published in the November 20, 2018 issue of Astronomical Journal.
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Ji Wang et al. 2018. Detection of water in the atmosphere of HR 8799 c with high dispersion spectroscopy in L-band using adaptive optics. A J 156, 272; doi: 10.3847 / 1538-3881 / aae47b
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