2023-06-17 00:32:44
An international team led by Stefan Pelletier, Ph.D. student at the Trottier Institute for Research on Exoplanets of the University of Montreal announced today that they have carried out 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 is published in the journal Nature.
“It’s really rare that an exoplanet hundreds of light years away can tell us something that would otherwise be impossible to know about our own solar system,” Pelletier said. “That is the case with this study.”
A big, warm, strange world
WASP-76b is a strange world. It reaches extreme temperatures because it is very close to its parent star, a massive star located 634 light-years away in the constellation Pisces: about 12 times closer than Mercury is to the Sun. With a mass similar to that of Jupiter, but almost six times greater in 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. In particular, in a study also published in Nature in March 2020, a team found an iron signature and hypothesized that there may be iron rain on the planet.
Aware of these studies, Pelletier became motivated to obtain new independent observations of WASP-76b using the high-resolution optical spectrograph MAROON-X on the Gemini-North 8-meter telescope in Hawai’i, part of from the Gemini International Observatory, operated by NSF’s NOIRLab.
“We recognized that the powerful new MAROON-X spectrograph would allow us to study the chemical composition of WASP-76 b with an unprecedented level of detail for a giant planet,” says Björn Benneke, professor of astronomy at UdeM , co-author of the study and Doctoral Research Director of Stefan Pelletier.
A composition similar to that of the Sun
Within the Sun, the abundances of almost all the elements of the periodic table are known with great precision. In the giant planets of our solar system, however, this is only true for a handful of elements, whose compositions remain poorly constrained. And this has hampered the understanding of the mechanisms governing the formation of these planets.
Because it is so close to its star, WASP-76 has a temperature well over 2000°C. At these degrees, many elements that would normally form rocks here on Earth (such as magnesium and iron) are vaporized and present 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 abundance of many elements measured by Pelletier and his team in the atmosphere of the exoplanet – such as manganese, chromium, magnesium, vanadium, barium and calcium – corresponds very closely to that of its star. host as well as that of 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. It is, however, different from the composition of rocky planets like Earth, which form in more complex ways.
The results of this new study indicate that the giant planets could retain an overall composition that mirrors that of the protoplanetary disk from which they formed.
Impoverishment of other very interesting elements
However, other elements were depleted in the planet relative to the star – a result that Pelletier found particularly interesting.
“Those elements that seem 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 matched our predictions, such as 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 may be extremely sensitive to temperature. Depending on the condensation temperature of an element, it will be in gaseous form and present in the upper part of the atmosphere, or will condense in liquid form where it will sink to deeper layers. When in gaseous form, it plays an important role in the absorption of light and can be observed in the observations of astronomers. Once condensed, it cannot be detected by astronomers and becomes 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. “A bit like two pots of water, one at -1°C which is frozen and the other at +1°C which is liquid. For example, calcium is observed on WASP-76 b, but it 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 that it has been unambiguously detected on an exoplanet, and is of great interest to astronomers because they know that it can have a significant impact on hot giant planets.
“This molecule plays a role similar to ozone in the Earth’s atmosphere: it is extremely effective in warming the upper atmosphere,” explained Pelletier. “This causes temperatures to increase with altitude, instead of decreasing as typically seen on colder planets.”
One element, nickel, is clearly more abundant in the exoplanet’s atmosphere than astronomers expected. Many hypotheses could explain this; one is that WASP-76 b may have accumulated material from a planet similar to Mercury. In our solar system, the small rocky planet has become enriched with metals like nickel due to the way it was formed.
Pelletier’s team also found that the iron uptake asymmetry 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 therefore likely a global process such as a temperature difference or clouds present on one side of the planet but not the other, rather than d be the result of condensation in liquid form as previously suggested.
Confirm and leverage lessons learned
Pelletier and his team are very keen to learn more about this exoplanet and other ultra-hot giant planets, in part to confirm their hypothesis about the very different atmospheres that might prevail on planets that are slightly different in temperature.
They also hope other researchers will take what they’ve learned from this giant exoplanet and apply it to better understand our own solar system planets and how they came to be.
“Generations of researchers have used the measured abundances of Jupiter, Saturn, Uranus and Neptune for hydrogen and helium to compare gas planet formation theories,” Benneke said. “Similarly, 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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