Radiometric dating of meteorites and lunar samples constrains planetary accretion timescales to typically 1–10 million years. Isotopic anomalies reveal the sequence and timing of accretion events, fractionation between inner and outer solar system material, and the presence of short-lived radioactive isotopes that influenced early planetary heating.
You already know that radiometric dating uses the predictable decay of parent isotopes into daughter products to measure absolute ages, and that meteorites and lunar samples preserve material from the earliest solar system. Planetary accretion chronology applies these tools to answer a deceptively simple question: how fast did the planets come together, and in what order? The answer turns out to be surprisingly precise — and surprisingly fast.
The key technique is short-lived radionuclide chronometry. Isotopes like aluminum-26 (half-life ~717,000 years) and hafnium-182 (half-life ~8.9 million years) were present when the solar system formed but have long since decayed to undetectable levels. Their former presence is recorded as excesses of their daughter products (magnesium-26 and tungsten-182, respectively) locked into minerals that crystallized early. By measuring how much daughter excess a sample contains relative to a reference, you can determine when that mineral formed relative to the oldest solar system solids — calcium-aluminum-rich inclusions (CAIs), which date to 4.567 billion years ago and serve as "time zero."
Using these clocks, the chronology becomes remarkably clear. CAIs formed first. Chondrules (the rounded grains in chondrite meteorites) formed within the next 1–3 million years. Iron meteorite parent bodies — small planetesimals that melted and differentiated — accreted and separated their metal cores within just 1–2 million years of CAI formation, meaning planet-building began almost immediately. Mars-sized embryos likely assembled within 5–10 million years. Earth's final assembly, including the Moon-forming giant impact, is constrained by hafnium-tungsten systematics to roughly 30–100 million years after solar system formation — slow by comparison, but still geologically instantaneous.
The chronological picture also reveals that short-lived radioactive isotopes were not just clocks but engines. Aluminum-26 was abundant enough in early planetesimals to melt their interiors through radioactive heating, driving differentiation (separation of metal cores from silicate mantles) even in bodies only tens of kilometers across. This means the timing of accretion directly controlled a body's thermal fate: accrete early while aluminum-26 is still live, and you melt and differentiate; accrete a few million years later, and you remain a cold, undifferentiated rubble pile. The meteorite record preserves both outcomes, giving us a physical archive of how accretion timing shaped planetary structure from the very beginning.