A potential new tracer of exoplanet formation

First measurement of isotopes in the atmosphere of an exoplanet

July 14, 2021

An international team of astronomers, including scientists from the Max Planck Institute for Astronomy, have become the first in the world to detect isotopes in the atmosphere of an exoplanet. It concerns different forms of carbon in the gaseous giant planet TYC 8998-760-1 b at a distance of 300 light-years in the constellation Musca (Fly). The weak signal was measured with ESO’s Very Large Telescope in Chile and seems to indicate that the planet is relatively rich in carbon-13. The astronomers hypothesize that this is because the planet formed at a great distance from its parent star. The research will appear in the scientific journal Nature.

Cartoon about the discovery of carbon-13 in the atmosphere of an exoplanet. In reality, the astronomers were sitting behind their desks analysing the spectra of the exoplanet TYC-8998 b made by ESO’s Very Large Telescope in Chile.

© Image: Daniëlle Futselaar (Artsource)

Cartoon about the discovery of carbon-13 in the atmosphere of an exoplanet. In reality, the astronomers were sitting behind their desks analysing the spectra of the exoplanet TYC-8998 b made by ESO’s Very Large Telescope in Chile.

© Image: Daniëlle Futselaar (Artsource)

Isotopes are different forms of the same atom but with a varying number of neutrons in the nucleus. For example, carbon with six protons typically has six neutrons (carbon-12), but occasionally seven (carbon-13) or eight (carbon-14). This property does not change much the chemical properties of carbon. Still, isotopes form in different ways and often react slightly differently to the prevailing conditions. Isotopes, therefore, provide applications in a wide range of research fields: from detecting cardiovascular disease or cancer to studying climate change and determining the age of fossils and rocks.

Astronomers from several countries, among them Paul Mollière from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, discovered an unusual ratio between those isotopes in the atmosphere of the young giant planet TYC 8998-760-1 b. Carbon is present primarily in the form of CO (carbon monoxide) gas. The planet itself exhibits a mass of about 14 Jupiter masses and has almost twice the size of Jupiter. Therefore, astronomers classify it as a super-Jupiter.

The group of scientists, led by first author Yapeng Zhang, a PhD student at Leiden Observatory, The Netherlands, successfully distinguished carbon-13 from carbon-12 because it absorbs radiation at slightly different colours. “It is really quite special that we can measure this in an exoplanet atmosphere, at such a large distance,” says Zhang. The astronomers had expected to detect about one in 70 carbon atoms to be carbon-13, but it seems to be twice as much for this planet. The idea is that the higher abundance of carbon-13 is somehow related to the formation of the exoplanet.

Illustration of the birth environments of planets in a proto-planetary disk that formed around a young star. The two planets inside the CO snowline denote Jupiter and Neptune at their current locations, while TYC 8998 b is formed far outside this regime. At such a distance from the parent star, most carbon is expected to have been locked up in CO-ice and built the planet’s main carbon reservoir. Consequently, the ice was rich in carbon-13 resulting in the observed isotope ratio in the planet’s atmosphere.

Image: Yapeng Zhang (Leiden Observatory)/MPIA graphics department

Illustration of the birth environments of planets in a proto-planetary disk that formed around a young star. The two planets inside the CO snowline denote Jupiter and Neptune at their current locations, while TYC 8998 b is formed far outside this regime. At such a distance from the parent star, most carbon is expected to have been locked up in CO-ice and built the planet’s main carbon reservoir. Consequently, the ice was rich in carbon-13 resulting in the observed isotope ratio in the planet’s atmosphere.

Image: Yapeng Zhang (Leiden Observatory)/MPIA graphics department

Mollière explains: “The planet is more than one hundred and fifty times farther 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.” Suppose the enrichment in carbon-13 is connected to the freeze-out of CO in the planet-forming protoplanetary disks. In that case, this could mean that Solar System planets did not collect much carbon-13-rich ice. A reason may be that in the Solar System, the distance beyond which CO begins to freeze out of the gas phase, known as the CO snowline, lies beyond Neptune’s orbit. Therefore, CO ices have likely rarely been incorporated into the Solar System planets, leading to a higher isotope ratio. Mollière wrote the data analysis software and contributed to interpreting the results.

The exoplanet itself, TYC 8998-760-1 b, was discovered only two years ago by Leiden PhD student Alexander Bohn, co-author of the article. He adds: “It’s awesome that this discovery has been made close to ‘my’ planet. It will probably be the first of many.”

Ignas Snellen, professor in Leiden and the driving force behind this subject for many years, is above all proud. “The expectation is that in the future, isotopes will further help to understand exactly how, where and when planets form. This result is just the beginning.”

Background information

Besides Paul Mollière fom MPIA, the following scientists contributed to the results featured in the paper: Yapeng Zhang, Ignas A. G. Snellen, Alexander J. Bohn, Matthew A. Kenworthy, Frans Snik (all Leiden Observatory), Christian Ginski (Anton Pannekoek Institute for Astronomy, University of Amsterdam), H. Jens Hoeijmakers (Observatoire de Genève and Lund Observatory ), Eric E. Mamajek (Jet Propulsion Laboratory, California Institute of Technology and University of Rochester), Tiffany Meshkat (IPAC, California Institute of Technology), Maddalena Reggiani (KU Leuven)