Springboards of exoplanets

Astronomers have collected some of the best data ever obtained on the composition of a planet known as HR 8799c, a giant young gas planet representing about 7 times the mass of Jupiter that revolves around its star every 200 years .

The team used state-of-the-art instrumentation at the W.M. Keck observatory in Maunakea, Hawaii, to confirm the existence of water in the planet's atmosphere, as well as the lack of methane.

While other researchers have already done similar measurements on this planet, these new, more robust data demonstrate the power of high-resolution spectroscopy combined with a technique known as adaptive optics, which corrects # 39; blur effect of the Earth's atmosphere.

"This type of technology is exactly what we want to use in the future to look for signs of life on a planet similar to the Earth.We are not there yet, but we are moving forward," Dimitri said. Mawet, Associate Professor of Astronomy. at Caltech and a researcher at JPL, which Caltech manages for NASA.

Mawet is co-author of a new article on the results published today in the Astronomical Journal. The main author is Ji Wang, a former postdoctoral researcher at Caltech and now an adjunct professor at Ohio State University.

Taking pictures of planets that revolve around other stars – 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 is the only multi-planets system to have been photographed. Discovered using adaptive optics on the Keck II telescope, the direct images of the HR8799 are the very first of a planetary system orbiting a star other than our sun.

Once an image is obtained, astronomers can use instruments, called spectrometers, to separate light from the planet, in the manner of a prism transforming sunlight into a rainbow, thus revealing the fingerprints of chemicals. Until now, this strategy has been used to know the atmosphere of several giant exoplanets.

The next step is to do the same thing for smaller planets closer to their stars (the closer a planet is to its star and the smaller it is, the harder it is to see).

The ultimate goal is to search for chemicals in the atmosphere of Earth-like planets that revolve around the "habitable zone" of the star, including biological signs that could indicate life, such as water, oxygen and methane.

Mawet's group hopes to do just that with an instrument on the next 30-meter telescope, a giant telescope planned for the late 2020s by several national and international partners, including Caltech.

But for now, scientists are perfecting their technique using the Keck Observatory and, in doing so, are learning more about the composition and dynamics of giant planets.

"For the moment, with Keck, we can already learn about the physics and dynamics of these giant exotic planets, which look nothing like our own planets in the solar system," Wang said.

In the new study, researchers used a Keck II telescope instrument called NIRSPEC (near infrared cryogenic scale spectrograph), a high-resolution spectrometer operating in infrared light.

They associated the instrument with the powerful adaptive optics of the Keck Observatory, a method for creating sharper images with the help of a guide star in the sky in order to measure and to correct the fuzzy turbulence of the Earth's atmosphere.

This is the first time that this technique is demonstrated on planets imaged directly using the L-band, a type of infrared light of about one wavelength. 3.5 micrometers and a region of the spectrum with many detailed chemical fingerprints.

"The L band has been largely neglected before because the sky is brighter at this wavelength," Mawet said. "If you were an alien with eyes set on the L-band, you would see an extremely bright sky – it's hard to see exoplanets through that veil."

The researchers say that the addition of adaptive optics has made the L band more accessible for the planet HR 8799c study. In their study, they carried out the most accurate measurements ever done on the atmospheric constituents of the planet, confirming that it had water and lacked methane as previously thought.

"We are now more certain of the lack of methane on this planet," said Wang. "This may be due to a mixture in the atmosphere of the planet.The methane, which we would expect to find on the surface, could be diluted if the convection process causes layers to appear. deeper of the planet without methane. "

The L band is also useful for measuring the carbon / oxygen ratio of a planet – a plotter indicating where and how a planet is formed. Planets form from swirling disks of matter around the stars, particularly from a mixture of hydrogen, oxygen and carbon – rich molecules, such as water. , carbon monoxide and methane.

These molecules freeze out disks forming the planet at different distances from the star, to boundaries called snow lines. By measuring the carbon / oxygen ratio of a planet, astronomers can learn more about its origins.

The Mawet team is now preparing to use its newest instrument at the Keck Observatory, called Keck Planet Imager and Characterizer (KPIC). He will also use high resolution spectroscopy assisted by adaptive optics, but can also see planets paler than HR 8799c and closer to their stars.

"KPIC is a stepping stone for our future thirty-meter telescope instrument," said Mawet. "For the moment, we are learning a lot about the myriad ways the planets of our universe are formed."

Explore further:
Direct observations of a planet orbiting a star at 63 light-years

More information:
Ji Wang et al., Water Detection in the HR 8799c Atmosphere with High Adaptive Optics L-Band Dispersion Spectroscopy, The astronomical journal (2018). DOI: 10.3847 / 1538-3881 / aae47b