de Kleer, Katherine
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De Kleer explains Pluto's reclassification as a straightforward consequence of the International Astronomical Union's three-part planet definition (orbits the sun, is gravitationally rounded, and has "cleared its orbital path") — Pluto fails the third criterion because it orbits within the debris-filled Kuiper Belt, a fate it shares historically with Ceres (once considered a planet, then reclassified as an asteroid, now a "dwarf planet"). She personally finds the categorization scientifically useful but not meaningful to her own research interest, noting she often refers informally to large moons as "planets" since many (Io, Europa, Titan) have volcanoes, atmospheres, and active geology comparable to or exceeding some actual planets.
Much of the conversation centers on Io, Jupiter's innermost large moon and de Kleer's specific research focus — the most volcanically active body in the solar system, with hundreds of active volcanoes, plumes reaching hundreds of kilometers high, and lava more fluid than most on Earth (flowing in sheets due to low viscosity, forming towering "curtains" of lava, best approximated by Hawaiian eruptions but more extreme). She explains Io's heat source is tidal heating: because Io, Europa, and Ganymede orbit in an integer-ratio ("resonant") relationship, their gravitational tugs compound coherently rather than canceling out, stretching Io's shape slightly with each close pass to Jupiter and generating enough mantle friction to melt rock. She frames Io as a natural laboratory for a broader goal of planetary science — building a general theory of planetary processes not biased toward Earth's specific conditions — noting Io's style of highly fluid, high-volume volcanism resembles the very early volcanic history thought to have occurred on essentially all terrestrial planets and moons, offering an unusual present-day window into early solar-system conditions. She contrasts Io's "heat-pipe" volcanism (magma simply punching through a rigid crust) with Earth's plate tectonics, and notes Earth's plate tectonics — recycling surface material and refreshing chemical nutrients — appears to be genuinely unique among solar system bodies and may itself be a subtle biosignature visible to a hypothetical outside observer tracking volcano locations over time.
On habitability elsewhere, de Kleer explains that the leading candidate environments are subsurface oceans in icy moons rather than any solid surface (surfaces are considered too radiation-blasted and dry), with Europa considered especially promising because — unlike Ganymede or Callisto, whose oceans are sandwiched between two ice layers — its ocean sits in direct contact with rock, permitting hydrothermal chemistry potentially analogous to Earth's deep-ocean hydrothermal vent ecosystems. She distinguishes Europa (likely requires drilling through kilometers of ice to sample) from Enceladus (Saturn's moon, which vents ocean material directly into space via south-polar geysers, making a flyby sample-collection mission more feasible in principle) — though she notes some models suggest Enceladus's ocean may cyclically freeze and thaw entirely, which would limit the time available for life to have evolved there. She also discusses Titan as a distinct possibility for atmospheric "prebiotic chemistry" (methane broken apart by radiation and reforming into complex organic molecules in a dense, cold atmosphere), while cautioning that liquid water is only "canonically" assumed necessary for life due to human biological bias, not proven necessary in principle.
On the 2020 phosphine-on-Venus announcement, de Kleer is openly skeptical, stating she does not believe the phosphine detection itself was real, noting the signal was weak, based on only one spectral line, required heavy data processing to extract, and did not meet the rigor she'd expect for such a claim — and separately notes that even a confirmed phosphine detection would only weakly imply biology, since the reasoning ("we ruled out every abiotic explanation we could think of, therefore life") is a fragile inferential chain. On the Drake Equation and the odds of alien life, she notes that only the first few terms (star counts, planets per star, potentially habitable planets) are becoming empirically tractable, while the terms governing how likely life is to arise and survive remain essentially unconstrained guesses; she personally would "put money on there being aliens" but not on humanity obtaining definitive evidence of them in her lifetime.
Discussing 'Oumuamua, the first confirmed interstellar object detected passing through the solar system, de Kleer walks through its two unusual properties (an elongated, non-spherical shape unlike typical solar system bodies, and unexplained acceleration as it left the solar system) and explains that both are reasonably well accounted for by non-alien hypotheses — either a nitrogen-ice fragment from a Pluto-like body in another star system, or hydrogen ice from a "failed" star-forming molecular cloud fragment — with outgassing/sublimation as sunlight hit it explaining both the acceleration and its elongated, soap-bar-like shape from asymmetric surface erosion. Invoking the Sagan standard that extraordinary claims require extraordinary evidence, she argues the scientifically responsible position is skepticism toward an alien-artifact explanation given available, more mundane explanations, while acknowledging — in a broader reflection on the tension between rigor and speculation — that the same scientific caution that protects against false positives can also make it harder to seriously entertain genuinely novel possibilities when only one or two anomalous data points exist.
On asteroids, de Kleer distinguishes primordial, largely unaltered asteroids from "differentiated" fragments — remnants of larger planetesimals that had already separated into core, mantle, and crust before being shattered by collisions — describing the latter as a rare, indirect window into planetary interior composition (including Earth's own core and mantle) that can't otherwise be directly sampled. She notes ongoing efforts to catalog and track near-Earth asteroids for impact risk (discussing Apophis, a close 2029 pass now assessed as non-threatening) and the theoretical, largely untested prospects for deflecting a threatening asteroid, complicated by many asteroids being loosely bound "rubble piles" rather than solid bodies. She closes on career advice, emphasizing that long-term scientific satisfaction depends as much on genuinely enjoying the day-to-day technical methods of a field as on finding its big questions interesting, and recommends several literary works (Nabokov's Pale Fire, Rilke's Duino Elegies, Olga Grushin's The Dream Life of Sukhanov) that share a "slow reveal" structure she finds intellectually compelling.
Excerpts
On why Io captivated her research career:
“Io is the most volcanically active object in the solar system... it puts out more volume of magma per volcano than volcanoes on Earth today. But I think the reason that it's most interesting to me is as a laboratory for understanding planetary processes... broadly, planetary science is trying to broaden that framework so you have a narrative for how each planet became different from every other planet.”
On why Europa is considered more promising for life than Ganymede or Callisto:
“Europa doesn't have this ice shell at the bottom of the ocean... the water and rock are in direct interaction, and so that means you can basically dissolve a lot of material out of the rock — you potentially have this hydrothermal activity that's injecting energy and nutrients for life to survive.”
On her skepticism toward the 2020 Venus phosphine claim:
“I don't personally believe that phosphine was detected in the first place... it was a very tenuous detection, and only one line of the species was detected, and a lot of really complicated data analysis methods had to be applied to even make that weak detection.”