An international team led by Stéphane Pelletier, a PhD student at the Trottier Institute for Exoplanet Research at the University of Montreal, announced that they have conducted a detailed study of the superheated giant exoplanet WASP-76 b.
Using the Gemini-North Telescope’s MAROON-X instrument, the team was able to determine and measure the abundances of 11 chemical elements in the planet’s atmosphere.
These include rock-forming elements whose abundance is not known even to the giant planets in the solar system such as Jupiter or Saturn. The team’s study is published in the journal nature.
“It’s really rare for an exoplanet hundreds of light-years away to tell us something that would have been impossible to know about our solar system,” Pelletier said. “That is the case with this study.”
A big, warm, weird world
WASP-76b is an alien world. It reaches extreme temperatures because it is so close to its parent star, a massive star located 634 light-years away in the constellation of Pisces: about 12 times closer than Mercury is to the Sun. With a mass similar to that of Jupiter, but about six times more massive, it is quite “bloated”.
Since its discovery by the Wide Angle Search for Planets Program (WASP) in 2013, many teams have studied it and identified various elements in its atmosphere. In particular, in a study also published in nature In March 2020, a team found an iron footprint and hypothesized that there might be iron rain on the planet.
Aware of these studies, Pelletier became excited to get new independent observations of WASP-76b using the MAROON-X high-resolution optical spectrometer on the Gemini-North 8-meter telescope in Hawaii, part of the Gemini International Observatory, which was operated by NSF’s NOIRLab. .
says Bjorn Beneke, Professor of Astronomy at UdeM, co-author of Research and Study Director Stefan Pelletier.
Composition similar to that of the Sun
Within the Sun, the abundances of nearly all of the elements of the periodic table are known with great accuracy. However, in the giant planets of our solar system, this applies only to a few elements, the components of which are still poorly constrained. This has hampered understanding of the mechanisms governing the formation of these planets.
Because it is so close to its star, WASP-76 has a temperature of more than 2,000 degrees Celsius. At these temperatures, many elements that would normally be rock here on Earth (such as magnesium and iron) are vaporized and exist in gaseous form in the upper atmosphere. Studying this particular planet allows unprecedented insight into the presence and abundance of rocky elements in giant planets, because in cooler giant planets like Jupiter, these elements are lower in the atmosphere and impossible to detect.
The abundances of many of the elements that Pelletier and his team measured in an exoplanet’s atmosphere — such as manganese, chromium, magnesium, vanadium, barium and calcium — match very closely with its star. host in addition to our sun.
This abundance is not random: it is a direct product of the Big Bang, followed by billions of years of stellar nuclear composition, so scientists measure roughly the same composition in all stars. However, it differs from the formation of rocky planets like Earth, which form in more complex ways.
The results of this new study indicate that giant planets may retain an overall composition that reflects the composition of the protoplanetary disk from which they formed.
The impoverishment of other very interesting items
However, other elements in the planet are depleted relative to the star—a result that Pelletier found particularly interesting.
“Those elements that appear to be missing from WASP-76b’s atmosphere are exactly those that require higher temperatures to vaporize, such as titanium and aluminum,” he said. “Meanwhile, those that match our expectations, such as manganese, vanadium or calcium, all vaporize at slightly lower temperatures.”
The discovery team’s explanation is that the observed composition of the giant planet’s upper atmosphere may be very sensitive to temperature. Depending on the condensation temperature of an element, it will be in gaseous form and present at the top of the atmosphere, or it will condense into liquid form as it will sink into deeper layers. When it is in gaseous form, it plays an important role in absorbing light and can be seen in astronomers’ observations. Once condensed, it is undetectable by astronomers and is completely absent from their observations.
“If confirmed, this discovery would mean that two giant exoplanets that have slightly different temperatures from each other could have very different atmospheres,” Pelletier said. “It’s a bit like two pots of water, one at -1°C that’s frozen and the other at +1°C that’s liquid. For example, calcium has been observed on WASP-76 b, but it might not be on a colder planet. a little. “
First detection of vanadium oxide
Another interesting discovery made by Pelletier’s team was the discovery of a molecule called vanadium oxide. It’s the first time it’s been unequivocally detected on an exoplanet, and it’s of great interest to astronomers because they know it can have a huge impact on hot giant planets.
“This molecule plays a similar role to ozone in Earth’s atmosphere: it is very effective at heating the upper atmosphere,” Pelletier explained. “This causes temperatures to increase with altitude, rather than decreasing as is typical on cooler planets.”
One element, nickel, is apparently more abundant in an exoplanet’s atmosphere than astronomers expected. Many hypotheses can explain this; One is that WASP-76 b may have accreted from a planet similar to Mercury. In our solar system, the little rocky planet has become rich in minerals such as nickel because of the way it formed.
Pelletier’s team also found that the asymmetry of iron uptake between the eastern and western hemispheres of WASP-76b that was reported in previous studies also exists for several other elements. This means that the underlying phenomenon causing this is likely to be a global process such as a temperature difference or clouds present on one side of the planet but not the other, rather than d being caused by condensation into liquid form as previously suggested.
Confirm and make use of the lessons learned
Pelletier and his team are very keen to learn more about this exoplanet and other super-hot giant planets, in part to confirm their hypothesis about very different atmospheres that might prevail on planets that are only slightly different in temperature.
They also hope that other researchers will take what they learned from this giant exoplanet and apply it to better understand the planets in our solar system and how they came to be.
“Generations of researchers have used the measured amounts from Jupiter, Saturn, Uranus and Neptune of hydrogen and helium to compare theories of formation of gaseous planets,” Beneke said. Likewise, measurements of heavier elements such as calcium or magnesium on WASP-76 b will help to better understand the formation of gaseous planets.
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