Katherine de Kleer still calls moons planets

Lex Fridman opens with the question every planetary scientist hears from children: why is Pluto not a planet anymore, and should anyone be upset. Katherine de Kleer, a professor of planetary science and astronomy at Caltech, says the object looked unique when it was found. Later surveys filled the outer solar system with peers of similar size, some even larger, in the region now called the Kuiper belt. Keeping Pluto as a planet would have meant adding the rest of that belt, or at least every body above some cutoff, to the official inventory.

The International Astronomical Union did write down a definition. A planet has to orbit the Sun, a clause meant to exclude satellites and, de Kleer notes, a definition that was not forward-looking, because it technically strips every extrasolar planet of the name. It has to be massive enough for gravity to pull it into a sphere, a bar Pluto and many moons both clear. The third stipulation is the one that actually decides the case: the body must have cleared other material from its orbital path. Pluto has not. It is not big enough to knock or sweep its neighbors aside.

This is not the first demotion in the catalog. Ceres, the largest body in the asteroid belt, was listed as a planet when it was discovered, then recast as an asteroid once astronomers realized it was one of many, and is now called a dwarf planet. De Kleer studies solar-system objects and says her level of interest “has nothing to do with what it’s classified as.” The labels still matter as a shared framework for talking. They do not tell her what is worth looking at.

Her favorite objects in the solar system are all moons. When she speaks about them she often just calls them planets, because to her they are planet-like worlds: they have volcanoes, geology, and atmospheres. Fridman tries to bait her with movie tropes, moons as secret outliers and planets as the vanilla main event. She does have a preference, just not a romantic one about categories. Satellites are what she wants to explore next.

The exploration record, she argues, has been planet-heavy. There have been missions to Jupiter, Saturn, Mars, and Venus. The moons of the outer solar system are, in her view, the next frontier. Fridman asks whether the Kuiper belt is a garbage collector for inbound debris. She says the material out there is mostly leftover from the solar system’s own formation, not an interstellar dump, and that it is full of objects worth studying if anyone could get a close look.

Some of those distant bodies show unexpected ices, some are brighter than they should be, and people have even hypothesized subsurface oceans. If any of them sat in the inner solar system they would be studied intently. A flyby is currently all that is possible so far out. New Horizons proved how much a high-speed pass can do: Pluto went from a reddish dot to a world with real terrain, and the spacecraft later deflected to another Kuiper-belt object. Fridman still hates the thought of waving at a flyby that missed a biosphere. De Kleer does not pretend a photograph would settle that fear.

Planetary science, de Kleer says, sits on a border with astronomy, geology, climate science, chemistry, and even biology. Spacecraft are the tools the public knows: fly by, orbit, land, maybe drill, maybe listen for tectonic tremors. Her own path was the other one. Many colleagues grew up dreaming of NASA missions. She grew up looking at stars and playing with telescopes, and she still works primarily from Earth-based and orbital observatories.

The detail available from those telescopes surprised her when she moved into solar-system work. You can watch cloud features on Titan shear out in the atmosphere. You can watch Io’s lava flows expand from night to night. Spectroscopy reads composition. Hubble and, looking ahead in the conversation, James Webb sit in that same toolkit. On a good night she can separate at least a dozen of Io’s volcanoes by their infrared emission and track how the heat from each one changes.

A spacecraft dumps an extraordinary amount of information into a short window. Some questions need thirty or forty years. Titan’s orbital period is about thirty years; if you want its atmospheric seasons, you have to watch across that cycle. You will not park a mission that long. You can keep pointing telescopes. Spatial resolution from the ground is only a couple of hundred kilometers, which is why she still wants a spacecraft at Io, just not a human one. The moon is freezing away from the vents, the magma comes out above a thousand degrees, the air is sulfur dioxide at about a thousandth of Mars’s density, and Jupiter’s magnetosphere is a radiation trap filled with material stripped from Io itself.

Io is the most volcanically active object in the solar system. It has hundreds of active volcanoes, plumes that rise hundreds of kilometers, and more magma per volcano than anything erupting on Earth today. De Kleer cares about it as a laboratory for planetary processes. The field’s working picture of volcanism started on Earth. Move that picture to a tiny world with a tenuous atmosphere, a different heat source, and a deep seat inside another object’s magnetic field, and the old intuition fails. The job of planetary science, as she describes it, is to widen the story until every planet and moon has a place in it. She catches herself saying “planet” and reminds Fridman that she means the moons too.

The heat that matters is tidal. Earth’s interior still holds leftover heat of formation and heat from radioactive decay. Many outer-solar-system satellites are wrung by gravity. If several moons have orbital periods that are integer multiples of one another, they always meet at the same point in their orbits. Random encounters would cancel out; coherent ones add up and pump the orbits into ellipses. Io and Europa therefore spend part of each few-day circuit closer to Jupiter and part of it farther away. Nearer in, the moon is stretched by a couple of hundred meters; farther out, less so. That repeated deformation is enough friction, on Io, to melt mantle rock into magma.

Europa is in the same three-body resonance as Io and Ganymede, only farther from Jupiter, so the flexing is milder. The leftover heat may still drive hydrothermal activity at the seafloor, the kind of setting where interesting life evolved in Earth’s oceans. On Io the surface away from vents is far below freezing, black and red like Hawaiian lava but even more fluid, spreading in sheets because the magma is so hot and the viscosity so low. Water and carbon dioxide, the main volcanic gases on Earth, are missing; sulfur dioxide dominates. That outgassing chemistry is a different problem. The style of eruption, though, is not unique to today: every terrestrial planet and the Moon went through similarly effusive volcanism early on. Watching Io is, in that sense, a look at Earth’s youth. Asked whether an earthly volcano could end civilization, she guesses some other catastrophe will get there first.

Surfaces across most of the solar system are poor places to start life: the radiation is harsh and standing liquid is scarce. The canonical view, de Kleer is careful to say, is that liquids are required. That is why icy-moon oceans, and perhaps Titan’s atmosphere, sit on the short list of plausible settings. Ganymede and Callisto are thought to have subsurface oceans too, but with an ice shell under the water as well as above it. An ocean trapped between ice layers is close to pure water. There is little to dissolve and little to eat.

Europa is different, she says, if current models are right: the seafloor is rock, so water can leach minerals and hydrothermal vents can inject energy and nutrients. That rock–water contact is the habitability clue she keeps returning to. Fridman asks whether life could skip water and live in magma. She will not close the door. Scientists have only “life as we know it,” and a polar solvent that can dissolve molecules is the usual need. Liquid magma that stayed molten long enough would, in theory, tick the three classic astrobiology boxes of heat, liquid, and nutrients. Detecting anything in Europa’s ocean, she thinks, means drilling. Tectonic features might let ocean water reach the surface, and a spectrometer might catch the signature of radiation-killed cells, but that chain is a stack of ifs. Enceladus, a Saturnian moon with a similar ocean, offers a cruder shortcut: south-polar geysers that appear to throw ocean material into space, if a spacecraft can scoop it without destroying the evidence.

Between Enceladus and Titan she will not pick a winner so much as two different bets. Titan is the one place she can imagine life in an atmosphere. The air is denser than Earth’s, loaded with methane that radiation splits and reassembles into complex organics, prebiotic chemistry at freezing temperatures. It would still be a miserable visit: cold, unbreathable, and nowhere in the solar system is pleasant for humans. Enceladus has the plume you can fly through, and a problem of its own. Some models have the ocean freezing and thawing as the orbit wanders in and out of eccentricity. A sea that exists for a hundred million years may never get life started, and a freeze would be a mass extinction if it already had.

Asked how unique Earth is, de Kleer says science does not have an answer, including to a more local question: how many times life began here, once or a thousand times independently. It feels absurd to her that we are the only life in the universe, and it is still conceivable. The Drake equation puts planets in a central role. She first saw it as a college freshman and thought it was a named piece of common sense glued to a pile of unknowns. She now reads it as a language for the conversation they are having, a way to split the factors, not a machine for a number.

Exoplanet surveys are tightening only the early terms, she notes: stars, planets per star, a first scratch at which worlds might be habitable. The later factors, how often life starts and how often it lasts, remain huge blanks. She would put money on aliens existing. She would not put money on “definitive evidence” in her lifetime. Fridman dislikes that word. He worries a self-obsessed species would miss intelligence running on other timescales, in trees, rocks, ideas, or consciousness itself, and that we are more likely to meet somebody’s machines than their originators. De Kleer stays on the narrower point: looking for intelligent life at least gives us a better chance of recognizing it than hunting for an alien stop sign we would not know how to see.

Her favorite world, Earth aside, is still Io, the object that pulled her out of a planned astrophysics path in galaxy evolution and into planetary science. She likes moons because so much less is known and the questions are still large, then admits that may be unfair to planets: whether Mars ever had life is a huge question, and a fleet of robots has been sent to ask it. The worlds that test a theory, for her, are the extremes. Venus, with sulfuric-acid clouds, a crushing hot surface, and volcanism in the recent past and maybe today, is that kind of object. When Fridman raises the 2020 claim of phosphine in Venus’s air, she does not buy the detection. One spectral line, a tenuous signal, and a pile of complicated processing do not meet the historical standard. The jump from atmosphere to life was criticized for good reason. The hunger to find life, she adds, was obvious in how excited everyone got.

Fridman asks whether humans become a multi-planetary species, with Mars as the usual target. De Kleer thinks that if the species lasts long enough, and does not wipe itself out, colonization of other planets will happen. She does not expect it in her lifetime. Tourists who give years of their lives might go. A colony is another project. Temperature, unbreathable air, the need for a pressure suit, and radiation all look like problems technology can eventually cover. The hard step is a self-sustaining settlement: food, oxygen, and the rest produced from what is already on Mars, because everything you ship is finite.

Fridman has read that insects may be the easiest protein to farm there, a cockroach milkshake future that will not thrill people who hate bugs. He jokes about genetically engineering radiation-tolerant insects, then about an Austin allergy test that flagged only cockroaches, a handy excuse to miss the first wave. The joke does not change her answer about going. She would. She likes work in which the chance of death is not small, because meditating on death, especially when the task forces the thought, makes her value being alive. Most people do not. She does, at least as an idea of a hard trip taken only to see what a mind and body can do.

Fridman traces his own taste for that kind of journey to a Soviet childhood, the space race, and a scientist father, and wonders whether American culture still carries it. He thinks Elon Musk has pulled some of the rocket excitement back. De Kleer does not argue the cultural history. She has already drawn the engineering line she cares about: visiting with a full kit is imaginable, living off the land is the gap.

Fridman asks her to write the one-page report an alien scientist would file on Earth. She starts with volcanoes, then with a deeper professional frustration. Telescopes and spacecraft see a thin veneer. You can drill a meter and you are still on the skin; even Earth’s deepest holes are nothing next to the radius. A large part of the field is staring at that skin and inferring the interior. Given enough time, an orbital observer could notice that Earth’s volcanoes trace lines, the edges of plates, and that the Hawaiian chain records a plate sliding over a mantle plume. That is how you would deduce plate tectonics, which she says no other world in the solar system has.

Io’s volcanism is not the same machine. Earth has rigid plates riding on a partially molten layer. Io has what she calls heat-pipe volcanism: magma punches a hole through a cold, rigid crust and comes out. The pictures are spectacular. Low-viscosity lava leaves fissures in curtains that may rise a kilometer, a giant version of sheets sometimes seen in Iceland. With almost no atmosphere, plume particles follow ballistic paths and open into umbrellas instead of the tangled columns Earth builds by mixing with air. Sulfur dioxide may condense on ash and fall as snow, visible or not depending on the Sun angle. Sunlight is weaker there, not absent.

Plate tectonics is more than a geologic curiosity. A stagnant surface uses up free oxygen and reactants until life has nothing left to extract energy from. Recycling brings fresh interior material up and old surface material down, refreshing the raw inventory. An astrobiologist looking at Earth would also see the oxygen-rich air, the mild temperature, and the liquid water, and would flag the planet as a habitability candidate. Humans would not obviously be the headline. We are a tiny sliver of biomass. Aliens who counted individuals might notice ants, or the ocean, before people. Fridman guesses cities would stand out as hard corners against natural physics. De Kleer is less sure lights and power grids would parse as technology. He pushes the joke further: a city that lights up at night and sleeps by day might look like one organism with a circadian rhythm.

Every planet and moon, de Kleer says, has weather we barely understand, including the global circulation and why storms form at all. Clouds are one of the few windows. Circulation lifts a species from deeper, warmer air to a colder altitude, where it condenses. Where those clouds sit can tell you whether air rises at the equator and sinks at the poles or whether the atmosphere is stacked in several cells. Jupiter’s bands, vortices, and anti-vortices may be leaking information about deep interior flow. The condensates themselves are different chemistries: ammonia ice on Jupiter, methane ice on Uranus and Neptune, with colors that follow composition.

Uranus is her extreme case. It orbits on its side, so for years the Sun sits over the north pole, then over the equator, then over the south. Earth’s seasons are a mild tilt; Uranus is a decade-scale lamp swung from pole to pole. You would expect chaos. The atmosphere does not look as wild as that heating should imply. The fact is itself data: atmospheres can redistribute heat in ways solid surfaces cannot, a self-regulating buffer against extremes. Predicting how that buffer unrolls is still brutally hard. People cannot even nail Earth’s smaller variations.

Titan is one of the best-studied atmospheres in the solar system, which makes the next part sharper. Researchers built global circulation models and tried to forecast the coming season. The forecasts were wrong. De Kleer finds that exciting. A missed prediction means the theory was not sufficient, and you get to go back and change it until it fits. Fridman looks forward to the day other worlds have television weather, delivered with the same fake confidence.

The main asteroid belt is remnant debris between Mars and Jupiter, leftover from the solar system’s origin rather than a later import. That is why it is scientifically precious: it samples the raw mix that built Earth and the other planets, and how that mix was redistributed. De Kleer splits the population, roughly, in two. Some bodies accreted from the original disk and have stayed much the same, colliding and throwing off families whose orbits still show they were once one object. The group she finds more interesting is the wreckage of would-be planets.

Early planetesimals formed with enough energy and time to differentiate, heavy material sinking into cores, lighter rock rising into mantles and crusts. Giant collisions then tore them apart. Fragments of core, mantle, and crust are still drifting in the belt. Humans have never seen a planetary core or deep mantle in bulk; a little mantle comes up in volcanoes, and that is all. These shards are a chance to look at something like Earth’s interior after a few billion years of space weathering. There are too many objects to reassemble a lost planet, though orbits and mismatched compositions sometimes hint that two pieces started together.

Because the questions are statistical, she would rather measure thousands or hundreds of thousands of asteroids than fall in love with one. Brightness, a red or blue spectral slope, and silicate absorption features are still the crude bins in use, plus prior knowledge of what the solar system formed from. Surfaces in the space environment remain poorly understood. The belt looks crowded on posters and is mostly empty space, so flying through it is easier than it sounds. Landing is another matter. Some targets are solid; some are rubble piles, loose aggregates that may look like rock until you step. A few have been visited, sampled, and scheduled for return. A mission was then due to land on a suspected core fragment and find out what it actually is.

An impact is possible, de Kleer says, and it does not even require a last-minute collision to bend a trajectory. Not every near-Earth object is tracked. People are doing their best to catalog more of them, and missing one could be catastrophic. She brings up Apophis, a near-Earth asteroid once given a serious chance of hitting in 2029 and again in 2036. Follow-up work ruled out a strike. The flyby is still intimate: about a tenth of the way from Earth to the Moon, closer, apparently, than some geosynchronous communications satellites, and maybe even visible in a dark sky.

Fridman offers a piece of black humor. If something really is coming, he expects science and government to say it is a close call until the last minute, then admit a miscalculation. De Kleer does not swat the joke aside. Tracking is hard, uncertainties are large, and short observational arcs spread the error bars. Could a Bruce Willis crew plant a nuclear charge and shove the rock? In theory, if you knew five years out, depending on size and how far you needed to move it. She is not the deflection specialist, and she is clear that no one method covers every case. A bomb in a rubble pile might scatter material that then falls back together, Terminator 2 style, which would be a very expensive way to do nothing.

Fridman splits his robotic hopes. SpaceX landings make precise control look solvable; the DARPA Robotics Challenge, led at MIT by Russ Tedrake, produced viral videos of expensive humanoids falling on uneven ground, with noisy, intermittent human links that resemble remote operations on an asteroid. He wants the whole deflection job done by robots. Zooming out, he imagines a century or two of small probes photographing, landing, and streaming data, because the field is still starving for measurements. De Kleer sees exploration moving that way. NASA’s old model was one huge, over-tested mission that does everything and almost never fails. Cheap CubeSats can be built to do one thing. You might fly a hundred of them to a hundred asteroids. That is a more distributed way of doing the science.

Interstellar visitors were not a working inventory until recently. Two objects have now come through, and the first confirmed one, Oumuamua, was strange in two ways. Natural bodies in this solar system do not come that long and skinny; they stay fairly close to spherical. It also accelerated as it left, which nobody understood at first. Could it be alien space junk? De Kleer will not, as a scientist, claim she can rule that out. She also reaches for Carl Sagan’s line that extraordinary claims need extraordinary evidence. There are ice-shard stories that fit known physics.

One hypothesis is a chunk of nitrogen ice knocked off a Pluto-like body in another system. The other is a leftover from a failed solar system: a molecular cloud that never made a star, only lumps of hydrogen ice, one of which got ejected and drifted through here. In both pictures the ices sublimate after perihelion. Gas leaving the sun-facing side supplies the extra push. The shape is the same process. A bar of soap starts roughly spherical, at least to a physicist, and daily use leaves a long thin shard. Sublimating ice does the same weathering job. Given those accounts, she wants deep skepticism before anyone reaches for aliens. Fridman still hates the missed chance. The cheapest interstellar mission is the one that comes to us, and we were not ready to land on it.

Astronomy is moving toward time-domain coverage, all sky, all the time, at many wavelengths, so more of these objects should show up, with a plan for which telescopes to point and whether a fast-turnaround mission could even launch. Fridman wonders if the first sign of another civilization will be junk, relay nodes, or speed-limit signs rather than a mind we can talk to. De Kleer replies that theorists are too good at inventing natural models. Maybe Oumuamua was a stop sign and the simplest physical story still won, because there was only one object and science prefers the skeptical fit. He compares the community to Steve Jobs trampling Jony Ive’s baby ideas. She agrees the method’s built-in skepticism is usually a gift, and that requiring three verifications will sometimes miss a true one-off. Planetary science, she adds, can afford more play than virology or terrestrial volcano work, because getting a speculation wrong does not put lives at risk. Fridman, speaking for AI, worries his field is sometimes too eager to flip the switch and see what happens.

De Kleer has read fiction obsessively since childhood and will not pretend her list is light. Vladimir Nabokov’s *Pale Fire* is formatted as an introduction, a long poem, and footnotes; you are well in before you realize the whole apparatus is a novel, a story and a mind revealing themselves through the cracks. That mix of book and puzzle sent her looking for other slow reveals. Kazuo Ishiguro is her contemporary example: not much happens, and you keep reading to find out what reality he is uncovering. Rainer Maria Rilke, especially the *Duino Elegies*, hit her emotionally and, she thinks, ties to astronomy. Everyone goes through life alone. The vastness and desolation of space made that inner loneliness feel companionable. Rilke turns the same desolation into something almost joyful. Olga Grushin’s *The Dream Life of Sukhanov* is another slow reveal, a Russian father coming to terms with himself and his past as he may be losing his mind. Ishiguro, she says, is the light version of this diet.

She will not give life advice to a young person staring at the stars. She will give a career note she did not appreciate going in: the questions and the techniques are of maybe equal importance. If the questions inspire you and the methods bore you, the job is miserable. If the techniques themselves are a pleasure, the days are fun. She is fascinated by the solar system and she loves telescopes, data, and new ways to use those telescopes. Dynamical evolution of orbits would have been a different kind of work, and a bad fit. Fridman hears David Foster Wallace’s line about being unborable. Dreams and passions are the poster. The hours, the repetition, the years of failure, are the life. If those bits are not boring, you are on the right path.

Fridman closes by reading Robert Frost’s “Fire and Ice” and asking which end of the world she wants. She chooses fire. She has always liked chaos. Things slowly going cold, stopping, and dying strike her as more depressing than explosions, burning, or a lava flow, because activity itself seems to endow an ending with meaning. That taste matches the worlds she studies. She wants a framework that can hold Venus and Io, not only the comfortable, habitable Earths. Fridman thanks her and leaves Carl Sagan on Titan: molecules raining down like manna for four billion years, still frozen, waiting for chemists from Earth.