Epsilon Indi Ab's atmosphere points to solids formed in deep cold

The mature giant planet Epsilon Indi Ab has a deuterium-to-hydrogen ratio similar to Uranus and Neptune, but about twice the levels found in Jupiter, Saturn, and the early Sun's surrounding gas. Its atmospheric chemistry also suggests the planet gathered solid material formed in an exceptionally cold environment.

Star system: Epsilon Indi A | Distance from the Sun: 11.9 light years

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Epsilon Indi Ab's atmosphere points to solids formed in deep cold

Star system: Epsilon Indi A | Distance from the Sun: 11.9 light years

Epsilon Indi Ab, a mature giant planet, has a deuterium-rich atmosphere. Deuterium is a heavier form of hydrogen, and the proportion of deuterium to ordinary hydrogen can help trace where a planet's building materials formed. The planet's measured ratio resembles those of Uranus and Neptune, and is about twice the ratios in Jupiter, Saturn, and the gas from which the Solar System formed.

The James Webb Space Telescope detected deuterated methane in the planet's atmosphere. This is methane in which some hydrogen atoms are replaced by deuterium. The signal reached 48 sigma, a statistical measure indicating a very strong detection. From this signal, the atmospheric deuterium-to-hydrogen ratio was measured as (4.4 ± 0.7) × 10⁻⁵.

Carbon, oxygen, nitrogen, and sulfur are all enriched in Epsilon Indi Ab to similar levels, comparable to those in Jupiter. The measured deuterium ratio also constrains the solid material the planet gathered: those solids must have had a deuterium-to-hydrogen ratio of at least about 1 percent. That is at least 30 times the typical ratio in Solar System comets. Such high ratios are found in the interstellar comet 3I/ATLAS and in organic molecules in star-forming regions. The results indicate that Epsilon Indi Ab accreted pristine solids formed in a very cold environment.

Key findings
Deuterated methane was detected in Epsilon Indi Ab with a 48-sigma signal.
The planet's atmospheric deuterium-to-hydrogen ratio is (4.4 ± 0.7) × 10⁻⁵.
Its ratio is similar to Uranus and Neptune, and about twice those of Jupiter, Saturn, and the protosolar nebula.
Carbon, oxygen, nitrogen, and sulfur show similar enrichment levels, comparable to Jupiter.
The planet's building solids had a deuterium-to-hydrogen ratio of at least about 1 percent, pointing to formation in a very cold environment.

Scientific study

The deuterium-rich atmosphere of a mature super-Jupiter