A new paper published in The Astronomical Journal presents an analysis of the atmosphere of one of the first exoplanets ever discovered – Tau Boötis b. This planet was detected using the radial velocity method, which measures the gravitational influence of a planet on its star by studying the wobble of the star. 

Tau Boötis b is, like COCONUTS-2b, about six times more massive than Jupiter. It’s also eight times closer to its star than Mercury is to the Sun. That star is only about 51 light-years from us, 40% more massive than the Sun, and one of the brightest stars known to have planets. Using the SPIRou instrument at the Canada-France-Hawaii Telescope, scientists gathered spectrographic data in infrared wavelengths over the course of 20 hours altogether, measuring the abundances of any molecule that contained carbon or oxygen. The results were surprising because we make assumptions about other planetary systems based on what we know of our own solar system, and that includes the prevalence of water vapor. 

Tau Boötis b should have had a lot of water vapor and a little carbon dioxide, and it just didn’t match the expectations. In fact, it had a lot of carbon dioxide and no water, which gave the team a new mystery to solve — how had this composition happened? Quite possibly, what we consider to be hot Jupiters could actually form farther from their stars at distances similar to our own gas giants, and then migrate in. This means we’ve found yet another challenge to our understanding of planetary formation and evolution. In other words, it’s a Thursday.

More Information

CFHT press release

University of Montreal press release

“Where Is the Water? Jupiter-like C/H Ratio but Strong H2O Depletion Found on τ Boötis b Using SPIRou,” Stefan Pelletier et al., 2021 July 28, The Astronomical Journal