Unexpected carbon-13 found in exoplanet’s atmosphere
TYC 8998-760-1 b (center dot) is one of two planets discovered orbiting a star (top left) located about 300 light-years away in the constellation Musca.
ESO/Bohn et al.
There are about 89 times as many carbon-12 atoms in our solar system as there are atoms of the element’s second most abundant isotope, 13C, whose nucleus contains one extra neutron. In the interstellar medium, or the gas between stars, the ratio is lower, at around 68. Scientists expect young stellar systems—that is, those on the order of millions of years old—to have a 12C/13C ratio close to that of the interstellar medium because it is the background in which the system formed. Researchers believe older systems, like ours, have a higher 12C/13C ratio because they formed 4.5 billion years ago, when less 13C had been produced in stars.
But in a recent paper, Yapeng Zhang and Ignas Snellen of Leiden University in the Netherlands and colleagues find that a planet in a young system has a 12C/13C ratio of just 31 among carbon monoxide molecules. The result could reveal the conditions of the planet’s formation, especially in regard to the CO snow line that separates gas and ice phases in interplanetary space.
The planet, known as TYC 8998-760-1 b, and its star are more than 300 light-years away from Earth. The star is about as massive as the Sun but is only 17 million years old. TYC 8998-760-1 b is 14 times as massive as Jupiter and orbits its star at 160 AU. (Pluto’s orbital radius is 40 AU.) Due to the system’s youth, the planet continues to accrete circumplanetary material.
The researchers observed the planet on two nights in June 2019 using a spectrograph on the Very Large Telescope in Chile. Looking specifically at wavelengths between 2.10 and 2.45 μm, they estimated isotope abundances in the planet’s atmosphere using CO as a marker. The carbon isotopes vibrate at different frequencies, so the researchers were able to distinguish the spectra of 12CO from 13CO by analyzing the molecules’ absorption of infrared radiation from the planet. The analysis revealed the 12CO/13CO ratio as 31.
Researchers suggest that a snow line marks the distance from a star at which it becomes cold enough for carbon monoxide to freeze into ice. The ice may contain more carbon-13 than the gas.
Y. Zhang et al., Nature 595, 370 (2021)
The researchers suggest the planet’s distance from its star likely contributed to the excess of 13C. CO is found in space as either a gas or an ice, depending on its temperature, and the planet is so far out that much of its material likely came from ices. In cool gas, exothermic reactions favor replacing 12C atoms in CO molecules with the heavier isotope, which, along with other isotope fractionation processes, can increase the amount of 13CO. Then, if the gas freezes, the resulting ice has a lower 12C/13C ratio. The researchers say that the freezing out of heavy atoms could have allowed TYC 8998-760-1 b to accrete extra 13C.
That process would explain the constant ratio in our solar system. According to the Nice model, a widely accepted scenario for the solar system’s evolution, all the planets formed inside the Sun’s CO snow line. TYC 8998-760-1 b, on the other hand, likely formed farther out than its star’s snow line, which is estimated to be at 20 AU. It is difficult to discern how, where, and when planets formed, so looking at isotope ratios could provide another clue. Researchers expect to see similarly low 12C/13C ratios on other planets that formed far from their stars. (Y. Zhang et al., Nature 595, 370, 2021 .)