An international team led by Stefan Pelletier, a PhD student at the University of Montreal’s Trottier Institute for Exoplanet Research, announced today that they have completed a detailed study of the extremely hot giant exoplanet WASP-76 b.
Using the Gemini North telescope’s MAROON-X instrument, the team was able to identify and measure the abundance of 11 chemical elements in the planet’s atmosphere.
These include rock-forming elements whose abundance is not even known for the giant planets of the solar system such as Jupiter or Saturn. The team’s study was published in the journal Nature.
“It’s really rare that an exoplanet hundreds of light-years away can tell us something about our own solar system that we couldn’t otherwise know,” Pelletier said. “That’s the case with this study.”
A big, warm and strange world
WASP-76b is a strange world. It reaches extreme temperatures because it is so close to its parent star, a massive star 634 light-years away in the constellation Pisces: about 12 times closer to the Sun than Mercury. With a mass similar to that of Jupiter but almost six times the volume, it is quite ‘bloated’.
Since its discovery by the Wide Angle Search for Planets (WASP) program in 2013, numerous teams have studied it and identified various elements in its atmosphere. Notably, in a March 2020 study also published in Nature, a team found an iron signature and hypothesized that there may be iron rain on the planet.
Aware of these studies, Pelletier was motivated to obtain new independent observations of WASP-76b using the MAROON-X high-resolution optical spectrograph at the Gemini-North 8-meter telescope in Hawaii, part of the Gemini International Observatory and operated by NSF’s NOIRLab .
“We realized that the powerful new MAROON-X spectrograph would allow us to study the chemical composition of WASP-76 b with a level of detail unprecedented for a giant planet,” says Björn Benneke, Professor of Astronomy at UdeM and co-author by the study and doctoral research leader by Stefan Pelletier.
A composition similar to that of the sun
Within the Sun, the abundances of almost every element on the periodic table are known with great precision. On the giant planets in our solar system, however, this only applies to a handful of elements whose composition is still largely unknown. And this has complicated understanding of the mechanisms that control the formation of these planets.
Because of its proximity to its star, WASP-76 has a temperature well over 2000°C. At these levels, many elements that would normally form rocks here on Earth (like magnesium and iron) vaporize and exist in gaseous form in the upper atmosphere. The study of this particular planet provides unprecedented insight into the presence and abundance of rocky elements on giant planets, as these elements lie deeper in the atmosphere and are undetectable on cooler giant planets like Jupiter.
The abundances of many elements in the exoplanet’s atmosphere – such as manganese, chromium, magnesium, vanadium, barium and calcium – measured by Pelletier and his team correspond very closely to those of its star and our own Sun.
These abundances are not random: they are the direct product of the Big Bang, followed by billions of years of stellar nucleosynthesis, so scientists measure roughly the same composition in all stars. However, it differs from the composition of rocky planets like Earth, which form in a more complex way.
The results of this new study suggest that the giant planets may retain an overall composition consistent with that of the protoplanetary disk from which they formed.
Depletion of other very interesting elements
However, other elements on the planet were depleted relative to the star – a result Pelletier found particularly interesting.
“The elements that appear to be missing from WASP-76b’s atmosphere are precisely those that require higher temperatures to vaporize, such as titanium and aluminum,” he said. “Meanwhile, those that fit our predictions, like manganese, vanadium, or calcium, all vaporize at slightly lower temperatures.”
The discovery team’s interpretation is that the observed composition of the upper atmospheres of giant planets could be extremely temperature sensitive. Depending on the condensation temperature of an element, it exists in gaseous form in the upper part of the atmosphere or condenses in liquid form where it sinks to lower levels. In gaseous form, it plays an important role in absorbing light and can be observed in observations by astronomers. Once condensed, it cannot be detected by astronomers and completely disappears from their observations.
“If this discovery is confirmed, it would mean that two giant exoplanets that have slightly different temperatures could have very different atmospheres,” Pelletier said. “A bit like two pots of water, one at -1°C that is frozen and the other at +1°C that is liquid. For example, calcium is observed on WASP-76 b, but may not be on a slightly colder planet.”
First detection of vanadium oxide
Another interesting discovery by Pelletier’s team is the detection of a molecule called vanadium oxide. It is the first time it has been positively detected on an exoplanet and is of great interest to astronomers, who know it can have significant effects on hot giant planets.
“This molecule plays a similar role to ozone in Earth’s atmosphere: it’s extremely effective at warming the upper atmosphere,” explained Pelletier. “This causes temperatures to increase with altitude instead of decreasing as is typically seen on colder planets.”
One element, nickel, is much more abundant in the exoplanet’s atmosphere than astronomers expected. Many hypotheses could explain this; One is that WASP-76b may have accumulated material from a planet similar to Mercury. In our solar system, the small rocky planet is enriched with metals such as nickel due to its formation.
Pelletier’s team also found that the asymmetry in iron uptake between the eastern and western hemispheres of WASP-76b, reported in previous studies, is also present for many other elements. This means that the underlying phenomenon causing this is likely a global process, such as B. a temperature difference or clouds present on one side of the planet but not on the other, and not the result of condensation in liquid form as previously suggested.
Confirm and use the insights gained
Pelletier and his team are keen to learn more about this exoplanet and other ultra-hot giant planets, also to confirm their hypothesis about the very different atmospheres that might exist on planets with slightly different temperatures.
They also hope other researchers will take what they have learned from this giant exoplanet and apply it to better understand the planets in our own solar system and how they formed.
“Generations of researchers have used the measured abundances of hydrogen and helium for Jupiter, Saturn, Uranus and Neptune to compare theories of gas planet formation,” Benneke said. “Measurements of heavier elements such as calcium or magnesium on WASP-76 b will also help to better understand the formation of gas planets.”