A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter

New JWST/MIRI observations of the cold super-Jupiter Eps Ind Ab confirm the presence of ammonia while revealing a suppressed feature and fainter-than-expected near-infrared emission, which the authors attribute to the presence of thick water-ice clouds in its atmosphere.

The Gist

Imagine you are looking for a lost sibling in a very dark room. You have a powerful flashlight (the James Webb Space Telescope, or JWST), but your sibling is hiding behind a giant, glowing lamp (their parent star). To see them, you need to block out the lamp's glare. That's exactly what astronomers did to find and study Eps Ind Ab, a massive "super-Jupiter" planet orbiting a nearby star.

This paper is like a detective's second visit to the crime scene. The team went back with a sharper lens to solve a mystery about what this cold, giant planet is made of.

Here is the story of what they found, explained simply:

1. The Mystery of the "Missing" Smell

Planets have atmospheres, just like Earth does. When a planet gets very cold (around -200°F or -130°C, which is cold for a planet!), a gas called ammonia usually starts to show up in its atmosphere. Think of ammonia like a distinct smell in a room; if you have the right nose (or in this case, the right telescope filters), you can tell it's there.

The team looked at Eps Ind Ab using two specific "colors" of light:

• Filter A (10.6 microns): This light gets blocked easily by ammonia. If ammonia is present, the planet looks dim here.
• Filter B (11.3 microns): This light passes right through ammonia. The planet looks bright here.

The Discovery: The planet was indeed much brighter in Filter B than in Filter A. This confirmed the "smell" of ammonia! The planet has it.

2. The Twist: The Smell is Too Faint

Here is where it gets weird. The team expected the ammonia "smell" to be very strong, like a room full of cleaning products. Instead, it was more like a faint whiff. The difference in brightness between the two filters wasn't as huge as the computer models predicted for a planet of this temperature.

It's as if you walked into a room expecting a strong perfume, but you only smelled a hint of it. Why?

3. The Culprit: Thick Clouds

The scientists considered a few suspects:

• Suspect A: The planet is made of different stuff. Maybe it has very little metal or nitrogen. But this didn't quite fit the other clues.
• Suspect B (The Winner): Thick Clouds. The team believes the planet is covered in a thick, fluffy blanket of water-ice clouds.

The Analogy: Imagine trying to see a campfire through a thick fog. The fire (the planet's heat and the ammonia gas) is still there, but the fog (the water-ice clouds) blocks some of the light and mutes the colors. These clouds are so thick that they hide the deep "ammonia smell" and also block the planet's near-infrared light, making it look much dimmer than expected in other parts of the spectrum.

This is a big deal because it suggests that cold giant planets might be wrapped in icy blankets, changing how we think about their weather and formation.

4. Confirming the Identity

Before this study, there was a tiny bit of doubt: Is this point of light actually a planet, or just a distant background star?

To prove it's a planet, you have to show it moves along with its parent star. Eps Ind A is a very fast-moving star (like a car speeding down a highway). If the point of light was a background star, it would stay still while the planet moved.

• The Result: The team took a new picture two years later. The point of light had moved exactly the same amount and in the same direction as the star. Case closed: It is definitely a planet sharing a family bond with its star.

5. Weighing the Planet

Using all the new data, the team updated their calculations for the planet's weight and orbit.

• Weight: It's about 7.6 times the mass of Jupiter. (So, a very heavy super-Jupiter).
• Orbit: It travels in a slightly oval path, taking about 110 years to go around its star once.

The Big Picture

This paper tells us that the universe of cold planets is full of surprises.

• The Pattern: Eps Ind Ab isn't alone. Another cold object, a "failed star" called WISE 0855, also has this faint ammonia signal.
• The Lesson: It seems that the coldest giant worlds in the galaxy might all be wearing thick, watery coats (clouds) that hide their true chemical signatures.

In summary: Astronomers used the James Webb Space Telescope to take a second look at a cold, giant planet. They confirmed it has ammonia, but the signal was weaker than expected. They solved the mystery by realizing the planet is likely covered in thick, water-ice clouds that are acting like a foggy window, hiding the planet's true nature. This discovery helps us understand how these distant, icy worlds are formed and what their atmospheres are really like.

Technical Summary

Here is a detailed technical summary of the paper "A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter."

1. Problem Statement

The James Webb Space Telescope (JWST) has opened a new window for directly imaging cold ($\sim$200–300 K), solar-age giant exoplanets. However, early observations of these objects reveal tensions with standard atmospheric models:

• Near-IR Faintness: Cold giant planets are consistently fainter than predicted by cloud-free, solar-metallicity models in the 3–5 $\mu$m range.
• Ammonia Detection: While ammonia ($NH_3$) absorption is expected in cold atmospheres, the depth of this feature in the mid-infrared (MIR) is uncertain.
• Specific Case: Eps Ind Ab, a $\sim$275 K super-Jupiter, was previously detected by JWST/MIRI at 10.6 $\mu$m and 15.5 $\mu$m, but its near-IR flux was undetected by ground-based telescopes (VLT/NaCo), contradicting standard models. The physical mechanism suppressing the near-IR flux and the depth of the ammonia feature remained unclear.

2. Methodology

The authors conducted a multi-faceted analysis combining new observations, archival data, and atmospheric modeling:

• New Observations:
  • Instrument: JWST/MIRI Coronagraphic Imager.
  • Filter: F1140C (11.3 $\mu$m), chosen to sit outside the strong 10–11 $\mu$m ammonia absorption band.
  • Epoch: May 10, 2025 (Program GO #5037), approximately two years after the initial detection.
  • Technique: Reference Differential Imaging (RDI) using DI Tuc as a Point Spread Function (PSF) reference star to subtract stellar light.
• Data Reduction:
  • Processed using spaceKLIP (based on jwst pipeline and pyKLIP).
  • Utilized raw uncal.fits files to apply custom pipeline adjustments (e.g., increased jump rejection thresholds).
  • Employed three independent photometric extraction methods (spaceKLIP injection, STPSF simulation, and target acquisition contrast) to derive robust fluxes and uncertainties.
• Orbital Analysis:
  • Re-fitted the orbit using orvara, incorporating:
    • New direct imaging astrometry (2025).
    • Previous JWST (2023) and VISIR/NEAR (2019) astrometry.
    • Hipparcos-Gaia acceleration (Proper Motion Anomaly).
    • Radial Velocity (RV) data from CES, UVES, and HARPS (corrected for perspective acceleration).
• Atmospheric Modeling:
  • Grid Models: Used the Sonora Flame Skimmer grid (clear and cloudy atmospheres) to test metallicity, C/O ratios, and vertical mixing ($K_{zz}$).
  • Cloud Models: Generated custom PICASO models with Virga cloud microphysics to simulate self-consistent water-ice clouds.
  • Comparisons: Compared observed colors (F1065C-F1140C, L'-F1140C) against models and a sample of other cold brown dwarfs and exoplanets (e.g., GJ 504 b, WISE 0855).

3. Key Results

A. Atmospheric Composition & Clouds

• Ammonia Detection: The planet is significantly brighter at 11.3 $\mu$m (F1140C) than at 10.6 $\mu$m (F1065C), with a color of 0.88 $\pm$ 0.08 mag. This confirms the presence of an ammonia absorption feature ($11\sigma$ detection).
• Shallow Ammonia Feature: The ammonia feature is shallower than predicted by cloud-free, solar-metallicity models.
  • Hypothesis 1 (Low Metallicity): Ruled out. Low-metallicity models predict brighter near-IR flux, contradicting archival non-detections.
  • Hypothesis 2 (Nitrogen Depletion): Models with 85–95% nitrogen depletion fit the mid-IR colors but struggle to fully explain the near-IR faintness without additional enrichment of C/O and metallicity.
  • Preferred Hypothesis (Thick Water-Ice Clouds): Models with thick water-ice clouds (optical depth $\tau \approx 416$) successfully reproduce the observed mid-IR colors (bluer F1065C-F1140C) and suppress the near-IR (3–5 $\mu$m) flux to match archival non-detections.
• Composition: Even with clouds, the best-fit models require elevated metallicity ($\sim$3$\times$ solar) and high C/O ratio (2.5$\times$ solar), posing a challenge for standard planet formation theories.

B. Orbital Dynamics

• Common Proper Motion: The re-detection of the companion in 2025 confirms common proper motion with the host star, definitively proving it is a bound planet and not a background source.
• Updated Parameters:
  • Mass: $7.6 \pm 0.7 M_{Jup}$ (higher than previous estimates).
  • Eccentricity: $0.24^{+0.11}_{-0.08}$ (lower than previous estimates).
  • Semi-major Axis: $20.9^{+5.8}_{-3.3}$ AU.
  • Inclination: $102.3 \pm 1.7^\circ$.
• The changes are primarily driven by corrected perspective acceleration in RV data and updated binning, rather than the new astrometric point alone.

C. Comparative Analysis

• Eps Ind Ab and the cold brown dwarf WISE 0855 share remarkably similar mid-IR colors and shallow ammonia features, suggesting a universal trend for very cold ($<300$ K) substellar objects where ammonia features are suppressed.
• This sample of cold planets (including 14 Her c and TWA 7 b) consistently appears fainter than expected in the 3–5 $\mu$m range, supporting the "thick cloud" hypothesis.

4. Significance and Conclusions

• First Direct Imaging of Ammonia in a Cold Giant: This work provides robust photometric evidence for ammonia in Eps Ind Ab, the coldest directly imaged exoplanet with such a detection.
• Cloud Physics: The results strongly suggest that thick water-ice clouds are present in the atmospheres of cold giant planets. These clouds suppress both the near-IR emission and the depth of the ammonia absorption feature, resolving the tension between observations and cloud-free models.
• Formation Challenges: The inferred high metallicity and C/O ratio in a massive planet ($\sim$7.6 $M_{Jup}$) challenge current core-accretion and gravitational instability formation models, which typically predict metallicities closer to the host star.
• Future Directions: The paper highlights the need for spectroscopic follow-up (planned for GO #8438 and #8714) to definitively confirm cloud species and measure precise metallicity/C/O ratios. It also calls for a broader survey of cold exoplanets to determine if shallow ammonia features and thick clouds are a universal property of the coldest substellar atmospheres.