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Three and a half parsecs from the Sun, among the dim stars of the southern sky, hides a world that astronomers have been trying to see for thirty years. Epsilon Indi Ab is a gas giant, cold as a winter night: its temperature is about 275 kelvins, just above the freezing point of water. It lies so far from its star that a year there lasts longer than a human lifetime, and the star's light barely warms its upper layers.

Now this world is no longer just a dot on a graph. The James Webb Space Telescope turned its infrared eyes on the planet and captured images in the range from 4 to 25 micrometers, as if we had finally seen the face of a person we previously knew only by footsteps in the dark. The longest-wavelength frame, at 25 micrometers, set a record: no exoplanet had ever been photographed in such "thermal" light.

The new images allowed scientists to assemble the first-ever spectrum of thermal emission from a cold gas giant outside the Solar System — from 4 to 25 micrometers. A spectrum is light split into its colors, and it reveals the composition of the atmosphere. It turns out that Epsilon Indi Ab's air is rich in heavy elements — astronomers call this "metallicity," though it has little to do with metals in the everyday sense. Such enrichment was predicted by models: the more massive a planet, the more rocky material it gathered at birth.

There is also a hint of clouds. Water vapor in the planet's atmosphere should absorb some infrared light, making the planet appear dimmer at certain wavelengths. But the measurements showed that at 25 micrometers the planet is brighter than expected. The simplest explanation is that clouds of water-ice crystals hang in the upper atmosphere, reflecting heat back into space like a snowfield reflecting sunlight.

The system's age is estimated at 3.5 billion years — the planet has long cooled and no longer contracts like young hot giants. Comparing the measured luminosity with predictions from evolutionary models, astronomers found excellent agreement. This means our ideas about how gas giants age work even at the edge of what we've studied — with faint glow, low mass, and respectable age.

Epsilon Indi Ab is becoming a benchmark: now, when looking at other cold worlds, scientists will check them against this neighbor. And its atmosphere is the first of its kind to be split into colors so far into the infrared. Thirty years of observations have turned a dot into a world with weather, chemistry, and history.

Original abstract

as written by the authors arXiv:2603.08787

We present new JWST/NIRCam 4-5 $$\mu$$m (F410M, F430M) and JWST/MIRI 18-25 $$\mu$$m (F1800W, F2100W, F2550W) imaging detections of the nearby (3.6 pc) cold (275 K) gas giant exoplanet $$\epsilon$$ Ind Ab. The F2550W detection of $$\epsilon$$ Ind Ab constitutes the longest wavelength image of an exoplanet acquired to date. Combining three decades of radial velocity monitoring, Gaia-Hipparcos absolute astrometry, and relative astrometry from direct imaging (including the new NIRCam astrometry), we conduct a comprehensive re-analysis of $$\epsilon$$ Ind Ab's orbit and obtain a dynamical mass $$M_{\rm Ab} = 6.5^{+0.7}_{-0.6}\;M_{\rm Jup}$$. Using $$\epsilon$$ Ind Ab's NIRCam and MIRI photometry, we assemble the first 4-25 $$\mu$$m spectral energy distribution (SED) of a cold gas giant outside the Solar System. The NIRCam photometry supports a metal-enriched atmosphere for $$\epsilon$$ Ind Ab based on analysis with atmospheric model grids, consistent with predictions from the giant planet mass-metallicity relation. While the current data do not provide definitive evidence for or against the presence of water ice clouds, we tentatively find that the H$$_2$$O vapor absorption-dominated F2550W photometry is systematically brighter ($$>1\sigma$$, but $$<2\sigma$$) than predictions from cloud-free/rainout chemistry models and better explained by a cloudy model. We calculate a bolometric luminosity of $$\log L_{\rm bol}/L_\odot = -7.23 \pm 0.03$$ dex by directly integrating $$\epsilon$$ Ind Ab's SED. Combining this with the planet's dynamical mass and age ($$3.5 \pm 1.0$$ Gyr), we demonstrate excellent agreement with evolutionary model predictions in a new regime of low luminosities, low masses, and old ages. Our results establish $$\epsilon$$ Ind Ab as a benchmark system for planetary evolution studies and set the stage for the detailed atmospheric characterization of this temperate extrasolar world.