Université de Montréal astronomers have used the Gemini-North Telescope in Hawai’i to study the chemical composition of an ultra-hot giant exoplanet in unprecedented detail, allowing for a much better understanding of these peculiar planets and providing insights into the composition of giant planets.
An international team led by Stefan Pelletier, a Ph.D. student at the Trottier Institute for Research on Exoplanets (iREx) at the Université de Montréal (UdeM) announces today the detailed study of the extremely hot giant exoplanet WASP-76 b in the journal Nature. Thanks to the MAROON-X instrument on the Gemini-North Telescope, the team was able to identify and measure the abundance of 11 chemical elements in the atmosphere of the planet, including rock-forming elements whose abundances are not even known for the giant planets like Jupiter or Saturn in the Solar System.
“Truly rare are the times when an exoplanet hundreds of light years away can teach us something that would otherwise likely be impossible to know about our own Solar System,” said Stefan Pelletier. “This is the case with this study.”
Getting another view of a peculiar planet
WASP-76 b is a strange world. It reaches extreme temperatures because it is very close to its parent star, a massive star 634 light-years away in the constellation of Pisces: approximately 12 times closer than Mercury is to the Sun. With a mass similar to that of Jupiter, but almost six times bigger by volume, it is quite “puffy”. Since its discovery by the Wide Angle Search for Planets (WASP) program in 2013, many teams have studied it and identified various elements in its atmosphere. Notably, in a study also published in Nature in March 2020, a team found an iron signature and hypothesised that there could be iron rain on the planet.
Stefan Pelletier, aware of these studies, became motivated to obtain new, independent observations of WASP-76 b using the MAROON-X high-resolution optical spectrograph on the Gemini-North 8-metre Telescope in Hawai’i, part of the International Gemini Observatory, operated by NSF’s NOIRLab.
“We recognized that the powerful new MAROON-X spectrograph would enable us to study the chemical composition of WASP-76 b with a level of detail unprecedented for any giant planet,” says Prof. Björn Benneke, co-author of the study and Stefan Pelletier’s PhD research supervisor at Université de Montréal.
The Gemini-North Telescope, seen here, was used by Stefan Pelletier and colleagues to assess the atmospheric composition of the ultra-hot exoplanet WASP-76 b. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/P. Horálek (Institute of Physics in Opava).
A composition similar to that of the Sun
For almost all elements in the periodic table, we know their abundances with great accuracy within the Sun. However, this is only true for a handful of elements in the giant planets in our Solar System, whose compositions remain poorly constrained. This has hampered our ability to understand the precise mechanisms governing the formation of these planets.
As it is so close to its star, WASP-76 b’s temperature reaches well above 2000°C. At these temperatures, many elements that would normally form rocks here on Earth (like magnesium and iron) are vaporised and present in gaseous form in the upper atmosphere. Studying this peculiar planet enables unprecedented insight into the presence and abundance of rock-forming elements in giant planets, since in colder giant planets like Jupiter, these elements are lower in the atmosphere and impossible to detect.
The abundance of many elements measured by Stefan Pelletier and his team in the exoplanet’s atmosphere, like manganese, chromium, magnesium, vanadium, barium, and calcium, match those of its host star as well as of our own Sun very closely. 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 are formed in a more complex manner. The results of this new study indicate that giant planets could maintain an overall composition that reflects that of the protoplanetary disc from which they formed.
However, other elements were depleted in the planet compared to the star – a result Stefan Pelletier found particularly interesting.
“ These elements that appear to be missing in WASP-76 b’s atmosphere are precisely those that require higher temperatures to vaporise, like titanium and aluminium, ” he explains. “ Meanwhile, the ones that matched our predictions, like manganese, vanadium, or calcium, all vaporise at slightly lower temperatures. ”
The discovery team’s interpretation is that the observed composition of the upper atmospheres of giant planets can be extremely sensitive to temperature. Depending on an element’s temperature of condensation, it will be in gas form and present in the upper part of the atmosphere, or condensed into liquid form where it will sink to deeper layers. When in gas form, it plays an important role in absorbing light and can be seen in astronomers’ observations. When condensed, it cannot be detected by astronomers and becomes completely absent from their observations.
“ If confirmed, this finding would mean that two giant exoplanets that have slightly different temperatures from one another could have very different atmospheres, “ says Stefan Pelletier. “ Kind of like two pots of water, one at -1°C that is frozen, and one that is at +1°C that 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 result Stefan Pelletier’s team had is the detection of one molecule: vanadium oxide. This is the first time it has been unambiguously detected on an exoplanet.
This molecule is of great interest to astronomers because they know it can have a big impact on hot giant planets. “ This molecule plays a similar role to ozone in Earth’s atmosphere: it is extremely efficient at heating up the upper atmosphere,” explains Stefan Pelletier. “This causes the temperatures to increase as a function of altitude, instead of decreasing as is typically seen on colder planets.”
One element, nickel, is clearly more abundant in the exoplanet’s atmosphere than what the astronomers were expecting. Many hypotheses could explain that. One of them is that WASP-76 b could have accreted material from a planet similar to Mercury. In our Solar System, the small rocky planet is enriched with metals like nickel because of how it was formed.
The team also found that the asymmetry in iron absorption between the east and west hemispheres of WASP-76 b reported in previous studies is similarly present for many other elements. This means the underlying phenomenon causing this is thus probably a global process such as a difference in temperature or clouds being present on one side of the planet but not the other, rather than being the result of condensation into liquid form as was previously suggested.
Following these exciting results, Stefan 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 vastly different atmospheres that could prevail on planets differing slightly in temperature.
They also hope other researchers will leverage what they learned from this giant exoplanet and apply it to better our understanding of our own Solar System planets and how they came to be.
“Generations of researchers have used Jupiter, Saturn, Uranus, and Neptune’s measured abundances for hydrogen and helium to benchmark formation theories of gaseous planets,” summarises professor Björn Benneke. “Likewise, the measurements of heavier elements such as calcium or magnesium on WASP-76 b will help further understanding the formation of gaseous planets.”
About this study
“Vanadium oxide and a sharp onset of cold-trapping on a giant exoplanet” by Stefan Pelletier et al., was published on June 14th 2023, in Nature. In addition to Stefan Pelletier and Björn Benneke, the team also includes Luc Bazinet and Olivia Lim, two graduate students at the Trottier Institute for Research on Exoplanets at the Université de Montréal, Mohamad Ali-Dib, a former Trottier Postdoctoral Fellow at iREx, now at NYU Abu Dhabi as well as 13 other co-authors from Canada, the United Arab Emirates, Sweden, France, United Kingdom, the United States, Italy, the Netherlands, and Germany.
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Trottier Institute for Research on Exoplanets
Université de Montréal, Montréal, Canada
Trottier Institute for Research on Exoplanets
Université de Montréal, Montréal, Canada