An international team of scientists, using the ground-based Gemini
Observatory telescope in Chile, is the first to directly measure the amount
of both water and carbon monoxide in the atmosphere of a planet in another
solar system roughly 340 light-years away.
The team is led by Assistant Professor Michael Line of Arizona State
University's School of Earth and Space Exploration, and the results have
been recently published in the journal Nature.
There are thousands of known planets outside of our own solar system (called
exoplanets). Scientists use both space telescopes and ground-based
telescopes to examine how these exoplanets form and how they are different
from the planets in our own solar system.
For this study, Line and his team focused on planet "WASP-77Ab," a type of
exoplanet called a "hot Jupiter" because they are like our solar system's
Jupiter, but with a temperature upwards of 2,000 degrees Fahrenheit.
They then focused on measuring the composition of its atmosphere to
determine what elements are present, compared with the star it orbits.
"Because of their sizes and temperatures, hot Jupiters are excellent
laboratories for measuring atmospheric gases and testing our
planet-formation theories," Line said.
While we cannot yet send spacecraft to planets beyond our solar system,
scientists can study the light from exoplanets with telescopes. The
telescopes they use to observe this light can be either in space, like the
Hubble Space Telescope, or from the ground, like the Gemini Observatory
telescopes.
Line and his team had been extensively involved in measuring the atmospheric
compositions of exoplanets using Hubble, but obtaining these measurements
was challenging. Not only is there steep competition for telescope time,
Hubble's instruments only measure water (or oxygen) and the team needed to
also gather measurements of carbon monoxide (or carbon) as well.
This is where the team turned to the Gemini South telescope.
"We needed to try something different to address our questions," Line said.
"And our analysis of the capabilities of Gemini South indicated that we
could obtain ultra-precise atmospheric measurements."
Gemini South is an 8.1-meter diameter telescope located on a mountain in the
Chilean Andes called Cerro Pachón, where very dry air and negligible cloud
cover make it a prime telescope location. It is operated by the National
Science Foundation's NOIRLab (National Optical-Infrared Astronomy Research
Laboratory).
Using the Gemini South telescope, with an instrument called the Immersion
GRating INfrared Spectrometer (IGRINS), the team observed the thermal glow
of the exoplanet as it orbited its host star. From this instrument, they
gathered information on the presence and relative amounts of different gases
in its atmosphere.
Like weather and climate satellites that are used to measure the amount of
water vapor and carbon dioxide in Earth's atmosphere, scientists can use
spectrometers and telescopes, like IGRINS on Gemini South, to measure the
amounts of different gases on other planets.
"Trying to figure out the composition of planetary atmospheres is like
trying to solve a crime with fingerprints," Line said. "A smudged
fingerprint doesn't really narrow it down too much, but a very nice, clean
fingerprint provides a unique identifier to who committed the crime."
Where the Hubble Space Telescope provided the team with maybe one or two
fuzzy fingerprints, IGRINS on Gemini South provided the team with a full set
of perfectly clear fingerprints.
And with clear measurements of both water and carbon monoxide in the
atmosphere of WASP-77Ab, the team was then able to estimate the relative
amounts of oxygen and carbon in the exoplanet's atmosphere.
"These amounts were in line with our expectations and are about the same as
the host star's," Line said.
Obtaining ultra-precise gas abundances in exoplanet atmospheres is not only
an important technical achievement, especially with a ground-based
telescope, it may also help scientists look for life on other planets.
"This work represents a pathfinder demonstration for how we will ultimately
measure biosignature gases like oxygen and methane in potentially habitable
worlds in the not-too-distant future," Line said.
What Line and the team expect to do next is repeat this analysis for many
more planets and build up a "sample" of atmospheric measurements on at least
15 more planets.
"We are now at the point where we can obtain comparable gas abundance
precisions to those planets in our own solar system. Measuring the
abundances of carbon and oxygen (and other elements) in the atmospheres of a
larger sample of exoplanets provides much needed context for understanding
the origins and evolution of our own gas giants like Jupiter and Saturn,"
Line said.
They also look forward to what future telescopes will be able to offer.
"If we can do this with today's technology, think about what we will be able
to do with the up-and-coming telescopes like the Giant Magellan Telescope,"
Line said. "It is a real possibility that we can use this same method by the
end of this decade to sniff out potential signatures of life, which also
contain carbon and oxygen, on rocky Earth-like planets beyond our own solar
system."
Reference:
Michael R. Line et al, A solar C/O and sub-solar metallicity in a hot
Jupiter atmosphere, Nature (2021).
DOI: 10.1038/s41586-021-03912-6
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