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A world just over 300 light years away allowed for the very first detection of isotopes in the atmosphere of an exoplanet.
In the haze around a gaseous exoplanet named TYC 8998-760-1b, astronomers have detected a form of carbon known as carbon-13. This discovery suggests that the exoplanet formed far from its mother star, in the cold parts of its system beyond a specific snow line.
According to the researchers, the discovery gives us a new way to examine the poorly understood process of planetary formation.
“It’s really quite special that we can measure this in an exoplanet atmosphere, at such a great distance,” said astronomer Yapeng Zhang of Leiden University in the Netherlands.
The TYC 8998-760-1 b, discovered in 2019, was already pretty special. It belongs to an extremely rare group of exoplanets – the ones we have been able to directly imitate.
Stars are very, very bright and planets very dark in comparison, so we usually identify them by detecting the effect they have on their host star, either by gravity or by painstakingly dimming the star’s light when she walks past.
These techniques work best for planets close to their stars, but TYC 8998-760-1 b orbits its star at a fairly large distance – around 160 astronomical units. Pluto, for context, orbits the Sun at a distance of 40 astronomical units.
The exoplanet is also a chonk, clocked at about 14 times the mass and twice the size of Jupiter, which means it’s relatively bright with reflected starlight. So a team of researchers led by Zhang took a closer look to see if the light reflected from the star could tell them anything.
Specifically, they used an instrument called the Spectrograph for Full Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope at the European Southern Observatory in Chile. This instrument observes a spectrum of light; the team was looking for absorption characteristics.
These are dark lines in a spectrum that occur when certain wavelengths of light are absorbed by certain elements. The researchers found that the wavelengths absorbed by TYC 8998-760-1b are compatible with carbon-13, probably mainly related to carbon monoxide.
Isotopes are quite interesting. These are all forms of the same element that have the same number of protons and electrons, but different number of neutrons.
Carbon-12, the most common stable carbon isotope, contains six. Carbon-13 has six protons and six electrons, but seven neutrons. This is important because their formation pathways are different and they behave differently depending on their environmental conditions.
On TYC 8998-760-1 b, the researchers expected a certain abundance of carbon. The amount of carbon-13 they found in the exoplanet’s atmosphere was double that expected abundance. The team thinks this may tell us something about the conditions in which TYC 8998-760-1b was formed.
“The planet is more than 150 times farther from its mother star than our Earth is from our Sun,” said astrophysicist Paul Mollière of the Max Planck Institute for Astronomy in Germany.
“At such a great distance, ice may have formed with more carbon-13, causing the highest fraction of this isotope in the planet’s atmosphere today.”
This region would be beyond the carbon monoxide snow line – the distance from the star beyond which carbon monoxide condenses and freezes gas into ice (different gases have different snow lines ).
All the exoplanets forming this far from the star’s heat would incorporate these carbon monoxide ices. Since the known planets in the solar system are closer than this distance from the Sun, they would not form with as much carbon monoxide as TYC 8998-760-1b, the researchers argued.
We have a similar phenomenon here in the solar system, where Neptune and Uranus are richer in deuterium, an isotope of hydrogen with one proton and one neutron (normal hydrogen has only one proton), than Jupiter. This is attributed to the formation of the planet beyond the water snow line.
Detecting isotopes in the atmosphere will not yet be possible for many exoplanets, but as our telescopes continue to improve, it could provide a new way to study the formation of exoplanets, the researchers said.
“It is expected that in the future, isotopes will further help to understand exactly how, where and when planets are formed,” said astronomer Ignas Snellen of the University of Leiden. “This result is just the start.”
The research was published in Nature.
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