White Dwarf Iron Depletion vs. Stellar Age — The Unrun Quiet Expansion Test

Polluted white dwarfs are planetary autopsies. When a white dwarf accretes material from tidally disrupted asteroids and planetesimals, its photosphere encodes the elemental composition of those bodies — iron, silicon, magnesium, calcium — with extraordinary precision. The MNRAS 2026 paper (arXiv:2507.16777) by Rogers, Bonsor et al. now combines photospheric abundances with circumstellar disk mineralogy (from Spitzer/IRS spectra) to constrain bulk composition of exoplanetary fragments. Eight systems are characterized. One — GD56 — is accreting core-rich material with iron core mass fraction 0.59. Another — WD1150-153 — matches Bulk Earth.

The Quiet Expansion Filter (Ivliev 2026, arXiv:2606.13914) predicts that a post-threshold civilization engaged in distributed resource extraction would preferentially mine metallic (M-type) asteroids — iron-nickel, platinum-group metals — leaving remnant belt populations anomalously depleted in metallic iron relative to the stellar metallicity baseline. This depletion would appear in WD disk spectra as anomalously LOW iron-to-silicate ratios.

No study has run a scatter plot of iron-to-silicate ratio vs. total stellar age (main sequence lifetime + WD cooling age) with the Quiet Expansion prediction as the explicit prior. The dataset exists. The test has not been done.

The Specific Prediction

Under Quiet Expansion, post-threshold probes operate over galactic timescales — mining asteroid belts of stars that are now significantly older or already dead. The prediction is directional:

Iron-depleted WD systems (Fe/Si below chondritic baseline for that stellar metallicity) should cluster among the oldest total stellar ages — systems where probes had billions of years to selectively extract metallic fraction
Iron-enriched systems (GD56-type, core mass fraction >> 0.5) represent natural differentiation events — a differentiated body shedding its iron core via tidal disruption — and should show no age correlation

The clean statistical signature: iron depletion correlates with stellar age but NOT with spectral type, WD mass, or cooling temperature (which would indicate systematic measurement artifacts). If the correlation appears only for iron-poor outliers at high total age, it survives as an anomaly.

State of the Dataset (2026)

MNRAS 2026 (arXiv:2507.16777): 8 WD systems with both circumstellar disk emission AND photospheric metals — the intersection that allows simultaneous mineralogy (from disk emission) and bulk composition (from photospheric absorption). Key findings:

All 8 show 10 μm silicate emission features (olivine + pyroxene mixture)
Fe/Ca abundance ratio varies by ~2 orders of magnitude across the sample
GD56: core mass fraction 0.59 ± 0.09 — accreting differentiated planetary core fragment; iron-enriched
WD1150-153: Bulk Earth composition — accreting undifferentiated or mantle-dominated material
Amorphous vs. crystalline ratio tracks thermal history: high crystalline fraction = past thermal processing (differentiation, impacts, stellar irradiation)

arXiv:2512.14254 (December 2025): "A Possible Indication of Metallic Iron in White Dwarf Dusty Disks from their 'Dirtiness'" — metallic iron enhances near-infrared opacity in disk emission spectra. The best-fit metal-to-silicate mixing ratio for G29-38 is ~unity. This is the photometric signature that would be suppressed in iron-depleted systems: "clean" (low dirtiness) disk spectra at old stellar age.

Broader WD pollution sample: When combining photospheric data from the wider literature, ~100+ polluted WDs have measured photospheric abundance ratios including Fe/Si. The subset with circumstellar disk emission is ~25–30. The subset with BOTH disk emission AND well-constrained total stellar age (WD cooling age + main sequence lifetime from mass-metallicity relation) is likely ~15–20. This is marginal for statistical power but sufficient for a first anomaly search.

Why This Test Hasn't Been Run

Prior: No published study has used Quiet Expansion as an alternative hypothesis. The default prior is natural stochasticity — differentiation events, volatile loss, composition diversity from planet formation. Outliers are attributed to "accreting a core fragment" rather than "preferential extraction over Gyr timescales."
Age data: WD total stellar ages require combining WD cooling models (from mass and temperature) with main-sequence lifetime estimates (from mass-metallicity-age relations for the progenitor). This combination is standard but requires care for each system.
Iron-depletion vs. iron-enrichment: The GD56 result (high iron core fraction) is the opposite of Quiet Expansion's prediction. It represents natural differentiation. Quiet Expansion predicts the OTHER tail — anomalously low iron — which requires explaining why a system has LESS iron than its stellar metallicity predicts.

What the Test Looks Like

Data: Fe/Si or Fe/(Mg+Si) photospheric ratios from published WD abundances, combined with total stellar age.

Test: Scatter plot of log(Fe/Si) vs. total stellar age. Annotate by WD mass and spectral type (DA vs. DB) to control for measurement systematics. Look for:

Iron-depleted outliers (log(Fe/Si) > 1σ below chondritic baseline for that [Fe/H]) clustering at high total age
No corresponding clustering pattern for non-iron elements (Si, Ca, Mg) — this would rule out systematic bias toward old stars

Alternative hypotheses to rule out:

WD diffusion timescales: heavier elements (Fe) sink faster in DA WDs, depleting Fe relative to lighter elements over time. This would produce a spurious correlation of low Fe/Si with WD cooling age — NOT total stellar age. The test should use TOTAL age, and control for WD cooling age separately.
Differentiation: high iron = accreted core fragment; low iron = accreted mantle fragment. Both are natural and produce scatter. The Quiet Expansion signal requires systematic excess of mantle-like (low-iron) composition correlating with stellar age — not just scatter.
Selection bias: WDs with circumstellar disks may not be representative of the full WD population. But this would produce a bias in both directions, not preferentially iron-depleted at old ages.

The JWST Path to Significance

With ~25–30 systems currently characterized, a ~2σ anomaly is detectable but inconclusive. JWST MIRI Cycle 4–5 observations targeting WDs with known age and circumstellar emission could expand the sample to ~80–100 systems. At that scale:

A 3σ correlation of iron depletion with stellar age (if real) becomes detectable
Control tests (Si vs. age, Mg vs. age) can confirm the signal is iron-specific
Systems from the Galactic thick disk (old, metal-poor) vs. thin disk (young, metal-rich) can be compared to break the degeneracy between stellar age and metallicity

Confidence level: theoretical. The prediction is physically coherent; the test is technically feasible; no result yet.

Key Facts

GD56: core mass fraction 0.59 (0.37 by mole) — most iron-enriched system in the Rogers 2026 sample
WD1150-153: Bulk Earth composition — consistent with chondritic starting point
Fe/Ca ratio varies by ~2 orders of magnitude across the WD pollution sample
arXiv:2512.14254: metallic iron detectable as "dirtiness" (near-IR opacity enhancement) in current WD disk systems
MNRAS 2026 (MNRAS 542:293) published March 2026
Sample at current JWST characterization: ~25–30 systems; ~80–100 needed for 3σ

Cross-Realm Connections

concept arabah fermi analog: The Arabah copper polity is the Bronze Age proof of concept — distributed industrial resource extraction leaves no institutional trace, only anomalous geochemistry. The WD iron-depletion test is the astronomical version of looking for the Arabah in the asteroid record.
concept quiet expansion debris disks: This test is a sub-question within the broader Quiet Expansion detection program. The WD stellar age correlation is the first test requiring only archival data; the MIRI mineralogy test requires new observations.
concept quiet expansion asteroid signatures: The inner belt M-type test (NEOWISE) and the WD iron depletion test are complementary: one looks for depletion in surviving asteroid populations; the other looks for the depletion signature in accreted material that has already been processed by the WD system.
concept fermi paradox: If the test returns a positive result (iron-poor outliers correlating with stellar age), it would be the first anomalous astronomical datum requiring Quiet Expansion as a competing hypothesis — not proving it, but establishing it as non-negligible.
concept fidelity survival model: Civilizations that fail their own durability inversion — centralizing knowledge, abandoning distributed structures — may end up as iron-depleted WD debris disks. The civilizations that survived left no institutional trace (no Dyson sphere, no SETI beacon) precisely because they distributed rather than centralized.

Abhishek