Isotope ratios
Elemental abundances alone leave room for many formation stories. Isotopes tighten the narrative because different nuclei are produced and fractionated in different places: helium burning versus CNO cycling in stars, ion-exchange and selective photodissociation in discs, and ice–gas partitioning around snow lines (Öberg et al. 2011; Milam et al. 2005 for the local ISM baseline).
In exoplanets, that idea moved from speculation to observation when Zhang et al. (2021) measured $^{12}\mathrm{C}/^{13}\mathrm{C}\approx31$ in TYC 8998-760-1 b — substantially more $^{13}$C-rich than the local ISM ($\sim68$) or the Sun ($\sim91$) — and argued for ice accretion beyond the CO snow line. Snellen (2025) highlights minor C and O isotopes as one of the most interesting new HRS frontiers precisely because of that formation potential. JWST has since shown that oxygen isotopes are accessible too (e.g. Gandhi et al. on VHS 1256 b). At the same time, updated measurements for some early targets have drifted back toward ISM-like values, so the field is still calibrating how much fractionation to expect — and when a “solar-like” ratio is the informative result.
Building a comparative sample
My contribution is to grow that sample across companions, isolated brown dwarfs, and stars, with homogeneous retrieval methods.
| Object | $^{12}\mathrm{C}/^{13}\mathrm{C}$ | Facility | Reference |
|---|---|---|---|
| β Pic b | $58^{+18}_{-15}$ | CRIRES+ | A&A 2026 |
| GQ Lup B | $53^{+7}_{-6}$ | CRIRES+ | SupJup IV |
| GQ Lup A (host) | $51^{+10}_{-8}$ | CRIRES+ | SupJup IV |
| TWA 28 (CRIRES+) | $81^{+28}_{-19}$ | CRIRES+ | SupJup II |
| J0856 | $79^{+20}_{-14}$ | CRIRES+ | SupJup II |
| TWA 27A (JWST) | $79^{+14}_{-11}$ | NIRSpec | A&A 2025 |
| TWA 28 (JWST) | $75^{+2}_{-2}$ | NIRSpec | A&A 2025 |
| HD 19467 B | $154^{+19}_{-17}$ | NIRSpec | A&A 2026 |
Young SupJup targets typically land near the local ISM, consistent with fragmentation or disc collapse rather than extreme ice enrichment. β Pic b, despite sitting interior to the CO snow line, is likewise ISM-like. GQ Lup A and B matching each other is especially telling: without a stellar anchor, a planetary ratio is harder to interpret. JWST adds oxygen: $^{16}\mathrm{O}/^{18}\mathrm{O}\approx645$ and $681$ for TWA 27A and TWA 28. The older companion HD 19467 B sits higher in $^{12}\mathrm{C}/^{13}\mathrm{C}$, a reminder that age and birth environment both matter.
Stars as the other half of the story
Planets inherit (and possibly fractionate) material that was already processed by Galactic chemical evolution. Measuring rare isotopes in M dwarfs — building on earlier stellar work such as Crossfield et al. (2019) and the SPIRou parameter advances of Cristofari et al. (2022) — provides that baseline. Our Nature Astronomy study of 32 nearby M dwarfs finds decreasing $^{12}\mathrm{C}/^{13}\mathrm{C}$ with metallicity, matching models with ongoing $^{13}$C enrichment from novae. Planetary isotope ratios only make sense against that backdrop.