300 light years from Earth in the constellation Musca, the gas giant TYC 8998-760-1 b, along with a companion planet, orbits an infant K-class star about 17 million years old. We’re probably looking at a brown dwarf here rather than a gas giant like Jupiter, for TYC 8998-760-1 b is about 14 times Jupiter’s mass, nudging into brown dwarf territory, and it appears to be roughly three times as large, unusual for brown dwarfs. The planet’s separation from its host star is pegged at 160 AU.
An inflated atmosphere due to processes still unknown? We don’t know, but both this and the companion planet have been directly imaged. Now TYC 8998-760-1 b resurfaces through work with the European Southern Observatory’s Very Large Telescope, as reported in the latest issue of Nature. Led by first author Yapeng Zhang (Leiden University, The Netherlands), the team of astronomers detected carbon isotopes in the object’s atmosphere, showing higher than expected carbon-13 content.
Here is the image, first released in 2020, of the TYC 8998-760-1 system, showing the two young planets. One of the co-authors of the Zhang paper, Alexander Bohn (also at Leiden), worked on the earlier imaging as well. The caption is the original one from ESO accompanying the image and does not describe the later work by Zhang’s team.
Image: This image, captured by the SPHERE instrument on ESO’s Very Large Telescope, shows the star TYC 8998-760-1 accompanied by two giant exoplanets, TYC 8998-760-1b and TYC 8998-760-1c. The two planets are visible as two bright dots in the centre (TYC 8998-760-1b) and bottom right (TYC 8998-760-1c) of the frame, noted by arrows. Other bright dots, which are background stars, are visible in the image as well. By taking different images at different times, the team were able to distinguish the planets from the background stars. The image was captured by blocking the light from the young, Sun-like star (top-left of centre) using a coronagraph, which allows for the fainter planets to be detected. The bright and dark rings we see on the star’s image are optical artefacts. Credit: ESO/Bohn et al.
Different forms of the same atom, isotopes vary in the number of neutrons housed in the nucleus, so that while carbon-12 has six neutrons to go along with its six protons, carbon-13 has seven neutrons and carbon-14 has eight. The distinctions are useful because while chemical properties remain largely the same, isotopes can be distinguished as they react to different conditions and are formed in different ways.
Zhang’s team drew on the distinction between carbon-13 and carbon-12 as marked by the way each absorbs radiation at slightly different colors, using ESO’s Spectrograph for Integral Field Observations in the Near Infrared (SINFONI), mounted on the Unit 3 instrument of the VLT. Expecting about one in 70 carbon atoms to be carbon-13, they found twice the number. The planet’s formation seems to be implicated, according to co-author Paul Mollière (Max Planck Institute for Astronomy, Heidelberg):
“The planet is more than one hundred and fifty times further away from its parent star than our Earth is from our Sun. At such a great distance, ices have possibly formed with more carbon-13, causing the higher fraction of this isotope in the planet’s atmosphere today.”
Image: This is Figure 3 from the paper. Caption: The two planets inside the CO snowline denote Jupiter and Neptune at their current locations, whereas TYC 8998 b is formed far outside this regime, where most carbon is expected to have been locked up in CO ice and formed the main reservoir of carbon in the planet. We postulate that this far outside the CO snowline, the ice was 13CO-rich or 13C-rich through carbon fractionation, resulting in the observed 13CO-rich atmosphere of the planet. A similar mechanism has been invoked to explain the trend in D/H within the Solar System. Future isotopologue measurements in exoplanet atmospheres can provide unique constraints on where, when and how planets are formed. Credit: Zhang et al.
The study of isotope abundance ratios has proven significant in studying not only interstellar chemistry and star formation but also the evolution of the Solar System. While its uses in analyzing exoplanet atmospheres are in their infancy, the hope is that future work on a range of exoplanets will offer clues to their formation.
The paper is Zhang et al., “The 13CO-rich atmosphere of a young accreting super-Jupiter,” Nature 595 (14 July 2021), 370-372. Abstract.