Astronomers study a hot Jupiter in unprecedented detail thanks to SPIRou!

An international team of astronomers has measured the most precise composition of the hot Jupiter Tau Boötis b’s atmosphere, providing us with a better understanding of giant planets. Using the SPIRou spectropolarimeter at the Canada-France-Hawaii Telescope in Hawaii, a team led by Stefan Pelletier, a PhD student at Université de Montréal's Institute for Research on Exoplanets (iREx), studied the atmosphere of the gas giant exoplanet Tau Boötis b, a scorching hot world that takes a mere three days to orbit its host star.

Their detailed analysis, presented in a paper published today in the Astronomical Journal, shows that the atmosphere of the gaseous planet contains carbon monoxide, as expected, but surprisingly did not identify water, a molecule that was anticipated to be prevalent and should be easily detectable with SPIRou.

Tau Boötis b is a planet that is 6.24 times more massive than Jupiter and 8 times closer to its parent star than Mercury to the Sun. Its host star, Tau Boötis, located 51 light years from Earth is 40% more massive than the sun and is one of the brightest known planet-bearing stars in the sky.

Discovered in 1996, Tau Boötis b was one of the first exoplanets ever detected thanks to the radial velocity method. The radial velocity method studies the slight back-and-forth motion of a star generated by the gravitational tug of its planet. The planet’s atmospheric composition has been studied a handful of times before, but never with an instrument as powerful as SPIRou.

“SPIRou’s high resolution and infrared wavelength range open a new window into the atmosphere of planets likeTau Boötis b,” says Dr. Luc Arnold, CFHT resident astronomer and SPIRou instrument scientist. “These are the kinds of observations that the instrument was designed for and we look forward to seeing what SPIRou uncovers next.”

Studying hot Jupiters to better understand Jupiter and Saturn
“Hot Jupiters like Tau Boötis b offer an unprecedented opportunity to probe giant planet formation”, said co-author Björn Benneke, astrophysics professor and Pelletier’s PhD supervisor at Université de Montréal. “The composition of the planet gives clues as to where and how this giant planet formed.”

The key to revealing the formation location and mechanism of giant planets is imprinted in their atmospheric composition. The extreme temperature of hot Jupiters allows most molecules in their atmospheres to be in gaseous form and detectable with current instruments, enabling astronomers to precisely measure the content of their atmospheres.

“In our Solar System, Jupiter and Saturn are much colder,” continues Benneke. “Some molecules such as water are frozen and hidden deep in their atmospheres. Thus, we have a very poor knowledge of their abundance. Studying exoplanets provides a better way to understand our own giant planets. For example, the low amount of water on Tau boötis b could mean that our own Jupiter is drier than we had previously thought.”

SPIRou: a unique instrument

Tau Boötis b is one of the first planets studied with SPIRou, which started observations at CFHT in 2018. SPIRou is an infrared spectropolarimeter which takes the light from a single object and breaks the light into its component infrared colors; colors our eyes are unable to detect. The observations allow astronomers to study the object’s characteristics-- temperature, motion, and in the case of Tau Boötis b, the composition of the planet’s atmosphere.

“This spectropolarimeter can analyze the planet’s thermal light — the light emitted by the planet itself — in an unprecedentedly large range of colours, and with a resolution that allows for the identification of many molecules at once: water, carbon monoxide, methane, etc.” explains iREx researcher Neil Cook, a co-author that is an expert on the SPIRou instrument.

The team spent 20 hours observing the exoplanet with SPIRou between April 2019 and June 2020. This exquisite dataset allowed the researchers to make a detailed analysis of the molecular content of the hot Jupiter's atmosphere.

“We measured the abundance of all major molecules that contain either carbon or oxygen,” explains Pelletier. “Since they are the two most abundant elements in the universe, after hydrogen and helium, that gives us a very complete picture of the content of the atmosphere.”

Tau Boötis b, like most planets, does not pass in front of its star as it orbits around it, from Earth’s point of view. Previously, the study of exoplanet atmospheres has mostly been limited to these "transiting" planets - those that cause periodic dips in the brightness of their star when they pass between us and the star, blocking some of the light.

“It is the first time we got such precise measurements on the atmospheric composition of a non-transiting exoplanet. This work opens the dloor to studying in detail the atmospheres of a large number of exoplanets, even those that do not transit their star,” explains PhD student Caroline Piaulet, also a co-author of the study.

Searching for water

Assuming a similar composition as in the Solar System, models show that water vapour should be present in large quantities in the atmosphere of an exoplanet similar to Tau Boötis b. It should thus have been easy to detect with an instrument such as SPIRou.

“We expected a strong detection of water, with maybe a little carbon monoxide,” explains Pelletier. “We were, however, surprised to find the opposite, carbon monoxide, but no water.”

The team worked hard to make sure the results could not be attributed to problems with the instrument or the analysis of the data.

“Once we’ve convinced ourselves the content of water was indeed much lower than expected on Tau Boötis b, we were able to start searching for formation mechanisms that could explain this,” says Pelletier.

A composition similar to Jupiter

The analysis of Pelletier and colleagues allowed them to conclude that Tau Boötis b’s atmospheric composition has roughly five times as much carbon as that found in the Sun, quantities similar to that measured for Jupiter.

This may be a hint that hot Jupiters could form much further from their host star, at distances that are similar to the giant planets in our Solar System, and simply experienced a different evolution, which included a migration towards the star.

“According to what we found for Tau Böotis b, it would seem that, at least composition-wise, hot Jupiters may not be so different from our own Solar System giant planets after all,” concludes Pelletier.

About this study
“Where is the Water? Jupiter-like C/H ratio but strong H2O depletion found on Tau Boötis b using SPIRou,” by Stefan Pelletier et al., was published DATE in the Astronomical Journal.

In addition to Stefan Pelletier, Björn Benneke, Neil Cook and Caroline Piaulet, the team includes Institute for research on exoplanets (iREx) members Antoine Darveau-Bernier, Anne Boucher, Louis-Philippe Coulombe, Étienne Artigau, David Lafrenière, Simon Delisle, Romain Allart, René Doyon, Charles Cadieux and Thomas Vandal, all based at Université de Montréal, and seven other co-authors from France, the United States, Portugal and Brazil.

Funding was provided by the the Technologies for Exo-Planetary Science (TEPS) CREATE program, the Fonds de recherche du Québec - Nature et technologies (FRQNT), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Trottier Family Foundation and the French National Research Agency (ANR).

Additional links
Scientific article at arXiv.org
Université de Montréal press release (English)
Université de Montréal press release (French)

About SPIRou

SPIRou was designed, funded and constructed thanks to a worldwide consortium of institutes, namely IRAP/OMP/UPS, IPAG/OG/UJF, LAM/OHP/Pythéas/UAM, IAP/OP, CNRS/INSU in France, NRC-H and Université de Montréal in Canada, ASIAA in Taiwan, OG in Switzerland, LNA in Brazil, CAUP in Portugal and, of course, CFHT.