Astronomers using the Near-Infrared Spectrograph (NIRSPEC) on the Keck II telescope at the W. M. Keck Observatory have confirmed the existence of water — as well as a lack of methane — in the atmosphere of HR 8799c, an extrasolar planet approximately 7 times the mass of Jupiter that orbits its host star every 200 years. While other astronomers had previously made similar measurements of HR 8799c, the new data demonstrate the power of combining high-resolution spectroscopy with a technique known as adaptive optics.
An artist’s impression of the HR 8799 planetary system. Image credit: W. M. Keck Observatory / Adam Makarenko / C. Alvarez.
Taking pictures of exoplanets is a formidable task. Light from the host stars far outshines the planets, making them difficult to see.
More than a dozen exoplanets have been directly imaged so far, including HR 8799c and three of its planetary companions.
In fact, HR 8799 — which is located approximately 129 light-years away in the constellation Pegasus — is the only multiple-planet system to have its picture taken.
Once an image of a planet is obtained, astronomers can use instruments, called spectrometers, to break apart the planet’s light, like a prism turning sunlight into a rainbow, thereby revealing the fingerprints of chemicals. So far, this strategy has been used to learn about the atmospheres of several giant exoplanets.
“Right now, with Keck, we can learn about the physics and dynamics of these giant exotic planets, which are nothing like our own solar system planets,” said Dr. Ji Wang, an astronomer at Ohio State University.
The HR 8799 planetary system is the first planetary system beyond our own that astronomers directly imaged. Captured in 2008 using Keck Observatory’s near-infrared adaptive optics, the picture 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: NRC-HIA / C. Marois / W. M. Keck Observatory.
In the new study, Dr. Wang and colleagues coupled the NIRSPEC instrument with Keck Observatory’s powerful adaptive optics, a method for creating crisper pictures using a guide star in the sky as a means to measure and correct the blurring turbulence of Earth’s atmosphere.
“This is the first time the technique has been demonstrated on directly imaged planets using what’s known as the L-band, a type of infrared light with a wavelength of around 3.5 micrometers, and a region of the spectrum with many detailed chemical fingerprints,” the astronomers said.
“The L-band has gone largely overlooked before because the sky is brighter at this wavelength. If you were an alien with eyes tuned to the L-band, you’d see an extremely bright sky. It’s hard to see exoplanets through this veil.”
The addition of adaptive optics made the L-band more accessible for the study of the planet HR 8799c.
In the study, the team made the most precise measurements yet of the atmospheric constituents of the planet, confirming it has water and lacks methane as previously thought.
“We are now more certain about the lack of methane in this planet. This may be due to mixing in the planet’s atmosphere. The methane, which we would expect to be there on the surface, could be diluted if the process of convection is bringing up deeper layers of the planet that don’t have methane,” said Dr. Dimitri Mawet, a scientist at Caltech and NASA’s Jet Propulsion Laboratory.
The results were published in the November 20, 2018 issue of the Astronomical Journal.
Ji Wang et al. 2018. Detecting Water in the Atmosphere of HR 8799 c with L-band High-dispersion Spectroscopy Aided by Adaptive Optics. AJ 156, 272; doi: 10.3847/1538-3881/aae47b