Stellar Recipes for Planets express
Two stars, AF Lep A and ε Indi A, have become the focus of close attention. Using the GHOST spectrograph on the Gemini telescope, astronomers split their light into colors and, from absorption lines, determined how much carbon, oxygen, sulfur, and thirteen other elements are in the stars' atmospheres. It's like telling what fuel is made of by the color of its flame.
The comparison showed that in both stars, the C/O, C/S, and O/S ratios are nearly solar — deviations are less than 1.5 sigma, meaning within the margin of error. But the planets orbiting these stars have higher carbon, oxygen, and sulfur content than the stars themselves. It's as if the planets "over-salted" the soup: they took more ingredients from the common pot than remained in the broth.
Such enrichment is a strong argument for the core accretion scenario. First, solid particles clump together in the protoplanetary disk, forming a dense core; then this core, like a vacuum cleaner, attracts gas and small pebbles from the disk. As a result, the planet ends up with more heavy elements than the star it formed around. This isn't the only possible path for planet formation, but for AF Lep b and ε Indi Ab, it looks the most likely.
Original abstract
as written by the authors arXiv:2608.15912v1
JWST has enabled the measurement of carbon, oxygen, and sulfur abundances in the atmospheres of directly imaged planets. Interpretation of these abundances from a planet formation standpoint requires the corresponding abundances for the host star. In this work, we present detailed characterizations of the cold imaged planet hosts AF Lep A and $$\epsilon$$ Indi A using high-resolution Gemini/GHOST spectra. We derive the atmospheric parameters $$T_{\rm eff}$$ and $$\log{g}$$ using two different approaches, revealing differences in $$T_{\rm eff}$$ up to $$\sim260\,$$K. The derived parameters are subsequently incorporated in measurement of 16 elemental abundances (C, O, Na, Mg, Si, S, K, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y) and several abundance ratios. Utilizing both the spectral fit and the equivalent width methods, we find solar C/O, C/S and O/S ratios ($$<1.5\sigma$$) for AF Lep A and $$\epsilon$$ Indi A. We compare our measured abundances and their ratios with those of the planets AF Lep b and $$\epsilon$$ Indi Ab, with the elevated abundances for the planets relative to their host stars strongly indicating formation by core-accretion pathways.
The difference in the surface temperature of AF Lep A, obtained by two different methods, reaches 260 kelvins: that's like the difference between a hot summer day and a freezing winter night.
In the Strugatsky brothers' novella "The Kid," humans explore a planet that seems too "perfect" in composition, as if someone deliberately assembled it from the right elements. Real observations show that planets can indeed differ noticeably in chemistry from their stars, and this is a natural result of their birth.