Young Giant Gas Planet Beta Pic b Refuses to Reveal its Origin

Results cast doubt on a seemingly reliable tool for inferring the origins of gas planets

To the point

• Beta Pictoris b, atmosphere, origin: Researchers used the upgraded GRAVITY instrument (GRAVITY+) to study the atmosphere of Beta Pictoris b, a young giant gas planet, to understand its origin and atmospheric variability.
• Improved data, new findings: The updated GRAVITY data show a higher ¹²CO/¹³CO ratio than previous results, aligning with other studies and suggesting a lack of variance in the abundance ratio in the disc during planet formation.
• Atmospheric variability, rotation: There are tentative signs of atmospheric changes linked to Beta Pic b’s rotation period, possibly indicating clouds or chemical processes, but further observations are needed.
• Diagnostic tool doubts, isotope ratio limits: The consistent isotope ratios across many young gas giants and the interstellar medium challenge the use of carbon isotope ratios as reliable indicators of planet formation location.

The young and evolving planetary system of the 23-million-year-old star Beta Pictoris (short: Beta Pic) is regarded as an iconic circumstellar dust disc, which hosts at least three giant gas planets. Discovered already in 2008 by direct imaging, Beta Pic b is the most massive of those planets, measuring approximately 11 Jupiter masses. It orbits its host star on a wide trajectory, taking about 23 years for one revolution.

Astronomers from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, the Observatoire de la Côte d’Azur (OCA), Nice, France and others observed Beta Pic b to investigate the planet’s origin and potential atmospheric variability with the recently upgraded GRAVITY+ instrument. It is mounted to the Very Large Telescope Interferometer (VLTI), operated by the European Southern Observatory (ESO) at the Paranal Site in Chile. MPIA’s PhD student Antonia von Stauffenberg is the main author of the study published as a letter in the journal Astronomy & Astrophysics.

“The GRAVITY+ interferometric instrument is highly stable [...], making it uniquely capable of high-fidelity characterization of directly imaged exoplanets”, says co-author and MPIA scientist Jonas Sauter. GRAVITY+ is an upgrade of the original GRAVITY instrument, equipped with improved adaptive optics.

What can we learn about Beta Pic b’s atmosphere and origin?

The team applied a method proposed a few years ago to identify a planet’s birthplace inside its planet-forming disc. By measuring the relative abundance ratio between two different versions of carbon (C) locked inside carbon monoxide (CO) gas in Beta Pic b’s atmosphere, it should be possible to infer whether the planet formed outside or inside a region in the disc where carbon monoxide was present as ice. Considering the irradiation by the host star heating the disc from its centre, 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.

The technical term for the different forms of an element, such as carbon, is an isotope. 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 ¹²C and 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.

Exciting tentative scenario

Interestingly, in an earlier attempt to assess the diagnostic ratio between ¹²CO and the somewhat heavier ¹³CO, MPIA scientist Matthieu Ravet utilised the original GRAVITY instrument before its upgrade, yielding a comparatively low ratio. The authors 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 Pic b might have grown in the outer disc beyond the snowline by accumulating CO ice rather than CO gas.

However, at a range of about 10 au (astronomical unit = the mean distance between the Sun and the Earth; 1 au = 149.6 million km) from the host star, Beta Pic b currently circles the disc 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 Pic b may have migrated through the disc.

New and better outcomes with GRAVITY+

Using GRAVITY+, von Stauffenberg and her collaborators now derived an updated and more precise ¹²CO/¹³CO abundance ratio in Beta Pic b’s atmosphere, which is significantly higher than the earlier value. While ¹²CO is clearly detected and its content is straightforward to determine, measuring ¹³CO requires a more sophisticated approach. Interestingly, the ratio is consistent with the value reported in the companion paper by González Picos et al. (2026), who employed a different instrument. This demonstrates the improved data quality GRAVITY+ delivers compared to its original design. The previous GRAVITY result was clearly affected by systematic uncertainties.

In addition, 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. If true, this may hint at clouds or chemical processes in Beta Pic b’s atmosphere. However, more sensitive observations are required to confirm the result.

Antonia von Stauffenberg says: “The ability to accurately constrain both isotopologues and potential rotational variability using ground-based observations of a bona fide planet such as Beta Pictoris b demonstrates the exceptional data quality achieved with the updated GRAVITY+ instrument.”

Doubting the significance of abundance ratios

In the proposed scheme to recover a gas giant’s birthplace, the new, more precise ¹²CO/¹³CO abundance ratio clearly shifts Beta Pic b into the warmer, inner range of the natal planet-forming disc, consistent with the planet’s current location. In addition, the ratio broadly matches values commonly found in the Solar System and the interstellar medium (ISM), 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 astronomers are missing some crucial physics that govern CO ice chemistry in planet-forming discs. 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.

Additional information

The results are based on data obtained as part of the GRAVTY+ Guarantee Time Programme (GTO) 114.27JS (PI: Laura Kreidberg). The GRAVITY+ consortium includes the following institutes: MPE, INSU/CNRS, University of Cologne, MPIA, CENTRA, University of Southampton, and the associated partners KU Leuven, University College Dublin, and Universidad Autónoma de México, in close collaboration with ESO and supported by the Max Planck Foundation.

MPIA astronomers involved in this study were Antonia von Stauffenberg, Jonas Sauter, Paul Mollière, Matthieu Ravet (also at Observatoire de la Côte d’Azur, Nice, France [OCA] and Université Grenoble Alpes, Grenoble, France), David Trevascus, Wolfgang Brandner, Gaël Chauvin (also at OCA), Laura Kreidberg, and Elisabeth Matthews.

Other researchers were:

A. Berdeu (Observatoire de Paris, Meudon, France), M. Bonnefoy (Institut de Planétologie et d’Astrophysique de Grenoble, France), G. Bourdarot (Max-Planck-Institut für extraterrestrische Physik, Garching bei München, Germany [MPE]), J.-B Le Bouquin (Université Grenoble Alpes: Saint-Martin-d’Hères, Auvergne-RhôneAlpes, France [UGA]), F. Eisenhauer (MPE), M. Houllé (UGA), F. Millour (OCA), J. Scigliuto (OCA), J. Wang (Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, USA), J. W. Xuan (Department of Astronomy, California Institute of Technology, Pasadena, USA [Caltech] and Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA), Y. Zhang (Caltech), and the GRAVITY+ Collaboration.

The Very Large Telescope Interferometer (VLTI), operated by the European Southern Observatory (ESO), is an infrared interferometer that combines light collected by the four Very Large Telescope (VLT) 8.2-metre Unit Telescopes (UTs) or the four movable 1.8-metre Auxiliary Telescopes (ATs). The VLTI is located at ESO’s Paranal Observatory in Chile.

GRAVITY+ is an upgrade to the VLTI and its original GRAVITY instrument. It enables imaging of fainter and more distant astronomical objects than previously possible, while also improving high-contrast precision on bright objects. Its consortium consists of 15 research institutes from seven European countries, including MPIA and ESO.

MN