Using the upgraded GRAVITY+ instrument on ESO’s Very Large Telescope Interferometer (VLTI), astronomers have measured the ratio of carbon isotopes in the atmosphere of the young exoplanet Beta Pictoris b, offering new insights into how giant planets form.
This artist’s impression shows how the gas giant exoplanet inside the disc of Beta Pictoris may look. Image credit: L. Calçada / ESO.
Frist discovered in November 2008 by astronomers using ESO’s Very Large Telescope, Beta Pictoris b is a gas giant similar to Jupiter but is 9 to 13 times more massive.
The exoplanet orbits its host star, Beta Pictoris, at a distance of 8 times the Earth-Sun distance.
The star also hosts at least two more planets and a circumstellar disk of gas and dust that could, in time, evolve into a torus of icy bodies much like Solar System’s Kuiper Belt.
In a new study, Max-Planck-Institut für Astronomie Ph.D. student Antonia von Stauffenberg and colleagues aimed to investigate Beta Pictoris b’s origin and potential atmospheric variability.
They observed the planet with the recently upgraded GRAVITY+ instrument.
“We applied a method proposed a few years ago to identify a planet’s birthplace inside its planet-forming disk,” they explained.
“By measuring the relative abundance ratio between two different versions of carbon (C) locked inside carbon monoxide (CO) gas in Beta Pictoris b’s atmosphere, it should be possible to infer whether the planet formed outside or inside a region in the disk where carbon monoxide was present as ice.”
“Considering the irradiation by the host star heating the disk from its center, this would directly translate to the distance from the star at which the planet formed.”
“The radius at which the temperature is low enough to turn gas into ice is commonly referred to as the snowline.”
“Isotopes exhibit the same number of positively charged protons in the nucleus of an atom, but differ in the number of neutral neutrons, like in the two carbon isotopes: carbon-12 (12C) and carbon-13 (13C).”
“As a consequence, they have slightly different masses but exhibit similar chemical properties.”
“In space, carbon is often found in association with oxygen, forming 12CO and 13CO molecules.”
“Interestingly, in an earlier attempt to assess the diagnostic ratio between 12CO and the somewhat heavier 13CO, our colleagues utilized the original GRAVITY instrument before its upgrade, yielding a comparatively low ratio.”
“We already suspected that GRAVITY may have been inadequate to properly resolve the key signals in this dataset and advised caution in interpreting the results.”
“Still, following the rationale of the scenario mentioned above, this face value suggests that Beta Pictoris b might have grown in the outer disk beyond the snowline by accumulating CO ice rather than CO gas.”
“However, at a range of about 10 AU from the host star, Beta Pictoris b currently circles the disk clearly between the star and the snowline, where CO should have been present predominantly as a gas.”
“Assuming the result was correct, this finding would indicate Beta Pictoris b may have migrated through the disk.”
The astronomers also found subtle hints that the observed levels of flux coming from the planet vary over time.
“Despite its low significance, the dominating variations seem to be linked to the planet’s rotation period of approximately 8.7 hours,” they said.
“If true, this may hint at clouds or chemical processes in Beta Pictoris b’s atmosphere. However, more sensitive observations are required to confirm the result.”
“In the proposed scheme to recover a gas giant’s birthplace, the new, more precise 12CO/13CO abundance ratio clearly shifts Beta Pictoris b into the warmer, inner range of the natal planet-forming disk, consistent with the planet’s current location.”
“In addition, the ratio broadly matches values commonly found in the Solar System and the interstellar medium, which pervades the space between the stars in the Milky Way.”
“The overwhelming majority of about a dozen young giant gas planets probed for the CO ratio show similar values.”
“This consistency may actually be bad news, because the carbon isotope abundance ratio doesn’t seem to be that diagnostic after all, when used as a probe to identify a planet’s distance from its host star.”
“The most likely explanation is that any potential variance during planet formation is too small to be caught by the proposed method.”
“This means that the 12CO/13CO ratio currently fails to be sufficiently decisive to tell us anything specific about individual planet-forming environments.”
“Therefore, it is very likely we are missing some crucial physics that govern CO ice chemistry in planet-forming disks.”
“Thus, the 12CO/13CO ratio may not tell us much about the differences between the milder gaseous environments and the cold, CO-ice-laden realm after all.”
“For now, it seems the wide-orbit giant gas planets refuse to reveal their origins.”
“New tools that can distinguish between planet formation scenarios are needed, and GRAVITY+ may play a vital role in finding and evaluating them.”
The findings were published today in the journal Astronomy & Astrophysics.
_____
A. von Stauffenberg et al. 2026. 13CO and potential variability in β Pictoris b with GRAVITY+. A&A 711, L2; doi: 10.1051/0004-6361/202660275
