# GRAVITY+ measures 12CO/13CO of 91 in β Pictoris b, a planet of about 11 Jupiter masses

- Published: 2026-11-12 05:40 UTC
- Section: Sky & Space › Astronomy
- Tags: β Pictoris b, 12CO/13CO ratio, GRAVITY+, VLTI, giant planet formation, CO ice line, exoplanet atmosphere, directly imaged planet, petitRADTRANS, atmospheric variability
- URL: https://realtech.media/news/rt-2b5be2e3fe949f88ec9e0240_gravity_measures_12co/

**The carbon monoxide isotope ratio in the atmosphere of β Pictoris b is higher than an earlier measurement, which weakens 13CO as a tracer of where giant planets form.**

![GRAVITY+ measures 12CO/13CO of 91 in β Pictoris b, a planet of about 11 Jupiter masses](https://realtech.media/media/2026/11/d4875cb7c1845b4a.jpg)
*ESO/T. Preibisch, Wikimedia Commons, CC BY 4.0*

A team led by Antonia von Stauffenberg of the Max Planck Institute for Astronomy (MPIA) in Heidelberg measured the 12CO/13CO ratio in the atmosphere of the directly imaged planet β Pictoris b with the upgraded GRAVITY+ instrument. According to the paper in Astronomy and Astrophysics, the retrievals give a ratio of 91 (+24, −17) [1]. GRAVITY+ operates at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory in Chile.

## Why the carbon isotope ratio was used

The 12CO/13CO ratio was introduced as an indicator of where in the disk a planet formed. A value lower than that of the host star was suggested to mean that the planet accreted CO ice beyond the disk's CO ice line. Earlier observations of β Pictoris b with the original GRAVITY instrument gave a relatively low ratio, which appeared consistent with formation beyond the snowline and later inward migration. The researchers cautioned that instrumental limitations could have affected that result.

β Pictoris b is about 11 times the mass of Jupiter and lies in a 23-million-year-old planetary system. It orbits about 10 astronomical units from its star with a period of 23 years.

## New data and tentative variability

These are the first science observations with GRAVITY+ at a spectral resolution of R ≈ 4000. The planet's apparent brightness gives a signal-to-noise ratio of up to about 60 per wavelength point. The spectra were analysed with petitRADTRANS, and the retrievals robustly indicate 13CO [1]. The value of 91 is consistent with both a solar and an interstellar-medium-like ratio. It also agrees with independent measurements by González Picos et al. (2026) made with a different instrument.

The authors write that this corroborates recent interpretations that 13CO may be a less useful tracer of formation location in the disk than previously thought. Similar values have been measured in most of the roughly dozen young giant planets studied with this technique. According to von Stauffenberg, uncertainties in models and measurements still make it difficult to use 13CO as a formation tracer.

The observations span about 7 hours, which allowed a search for atmospheric variability. The authors report a tentative constraint on the variability amplitude of about 1.4 (+0.6, −0.7) % [1]. The brightness variations appear to follow a period of about 8.7 hours, close to the planet's rotation period. The evidence remains statistically weak, and more sensitive observations are needed to establish whether the changes are real.

## Sources

1. [Astronomy and Astrophysics](https://doi.org/10.1051/0004-6361/202660275)
2. [SciTechDaily](https://scitechdaily.com/a-planet-11-times-bigger-than-jupiter-is-challenging-what-we-thought-about-planet-formation)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-2b5be2e3fe949f88ec9e0240_gravity_measures_12co/