Batygin, Konstantin

planetary astrophysicist at Caltech

Konstantin Batygin: Planet 9 and the Edge of Our Solar System

Lex Fridman Podcast · Jul 19, 2021

• Konstantin Batygin
• Planet Nine
• Kuiper Belt and Oort Cloud
• Pluto and planet definition
• primordial black hole hypothesis
• planet formation and Jupiter migration
• Schrödinger equation and astrophysical disks
• planetesimal formation
• 'Oumuamua and hydrogen iceberg hypothesis
• chaos and three-body problem
• usefulness of useless knowledge
• Neptune's mathematical discovery

Core ideas

Batygin lays out the structure of the solar system beyond the well-known planets: past Neptune lies the Kuiper Belt, a much larger and more radially extensive second "icy asteroid belt" (of which Pluto is simply one large member — explaining why Pluto was progressively downgraded from an originally overestimated seven-Earth-mass planet to a small icy body, ultimately losing planetary status). Beyond that lies the Oort Cloud, a vast, roughly spherical, essentially collisionless shell of icy debris extending nearly halfway to the next star, whose existence is inferred indirectly through long-period comets (whose elongated orbits result from slow perturbation by the Milky Way's galactic tide over immense timescales). He explains that Kuiper Belt objects are discovered via parallax — tracking faint, sunlight-reflecting points across multiple nights to distinguish them from background stars — and that determining a stable orbit typically requires observations roughly a year apart, since three consecutive nights alone only reveal position and rough distance.

Batygin describes the "Planet Nine" hypothesis: gravitational evidence (not direct imaging) for a roughly five-Earth-mass object on a ~10,000-year, moderately tilted, moderately eccentric orbit far beyond Neptune. The core evidence is that a subset of distant Kuiper Belt objects unaffected by Neptune's gravity show statistically unusual orbital clustering — their orbits point in a common direction and share a similar tilt — which Batygin and collaborator Mike Brown interpret as the signature of a massive, distant perturber gravitationally "shepherding" these orbits into anti-alignment; independent lines of evidence include highly inclined (even ~90°) Kuiper Belt objects that standard planet-formation simulations don't otherwise produce, but which Planet Nine's gravity readily generates. He's careful to note the statistical evidence is nuanced — a naive test on raw sky-survey detections shows weak significance, but properly accounting for exactly which parts of the sky prior surveys have actually searched raises the "false alarm probability" analysis to their preferred, more confident calculation. He and Brown have also modeled how Planet Nine could dynamically "inject" objects from an otherwise dormant, more distant primordial Oort-Cloud-like population back into the observable distant Kuiper Belt — meaning the observed population isn't simply flowing outward from Neptune scattering, but partly cycling back inward too, which subtly shifts the best-fit orbital parameters for Planet Nine itself.

He explains that unambiguous confirmation would require direct telescope imaging (any object reflecting sunlight at the right implied distance settles the matter instantly), contrasting this with Neptune's 19th-century discovery, which was mathematically predicted from Uranus's orbital anomalies via celestial mechanics (Le Verrier's calculation) rather than found by search — Planet Nine's predicted orbit and mass are comparably well-constrained, but its exact position along that orbit isn't, partly because Kuiper Belt objects haven't been tracked through even one full multi-thousand-year orbit. He's genuinely open to the possibility that "Planet Nine" is instead a primordial black hole (a relic of the early universe's black hole mass spectrum, distinct from stellar-collapse black holes, and gravitationally indistinguishable at this mass from a planet) — acknowledging that current data (orbit and mass alone) cannot rule this out — but expresses a personal preference for it being an actual planet, since planets (unlike the comparatively "simple," structureless nature of black holes) offer rich, heterogeneous complexity (citing Pluto's surprising glacial CO2-ice geology revealed by New Horizons) that could offer unique insight into planets of a mass class (a few Earth masses) common elsewhere in the galaxy but entirely absent as a category within our own solar system.

On planet formation broadly, Batygin explains that Earth's habitability is a comparatively rare outcome (roughly a 1% event) largely because Earth formed too slowly (~100 million years) to accrete a thick hydrogen-helium atmosphere before the primordial gas disk dissipated — unlike the "hot Jupiter"-style short-period gas giants common around other stars — and hypothesizes, based on modeling, that the solar system likely once hosted a compact system of short-period, few-Earth-mass planets analogous to those found around most other stars, which was destroyed when young Jupiter and Saturn migrated inward and then back outward through the gas disk, sweeping the inner system's material onto the Sun and later reforming the terrestrial planets from a depleted remnant disk. He notes gas-giant "analogs" like Jupiter-Saturn pairs occur around only about 10% of Sun-like stars (and are essentially absent around the more common M-dwarf stars), underscoring how atypical the solar system's basic architecture is.

Discussing simulation and modeling, Batygin argues two-body gravitational problems are analytically solvable, but three-or-more-body systems are chaotically unpredictable regardless of computational power — meaning that simulating the solar system's full formation history in exhaustive detail, even if theoretically possible, would be scientifically unilluminating (indistinguishable from just observing reality, since a simulation is inherently a lossy compression of information) compared to designing targeted, hypothesis-specific numerical experiments that yield generalizable theoretical understanding. He shares a personal mathematical discovery — that treating an astrophysical disk as an infinite number of gravitationally interacting concentric rings, then taking the continuum limit, yields the Schrödinger wave equation — clarifying this isn't a fusion of quantum mechanics and gravity, since the Schrödinger equation is fundamentally just a general-purpose wave equation whose "quantum" interpretation is separate from its mathematical form, making it a useful (if conceptually surprising) calculational tool for disk dynamics at scales ranging from small Kuiper Belt object rings to Saturn's rings to protoplanetary and galactic disks.

He walks through how planetesimals first form from a gas disk: centimeter-sized solid particles aerodynamically "draft" behind one another (analogous to tumbleweed clusters or cyclists in a peloton) to minimize gas drag, clustering into locally dense clouds that eventually gravitationally collapse into the first planetary building blocks — a process qualitatively similar to how diffuse gas clouds collapse to form stars. He discusses evidence that the solar system did not form in isolation but within a cluster of thousands of co-forming stars — citing the presence of aluminum-26 (an unstable isotope from nearby supernovae) in early meteorites as direct evidence of this crowded stellar nursery environment.

On engineering approaches to detecting Planet Nine, Batygin discusses (and is skeptical of the near-term practicality of) Edward Witten's proposal to launch a fleet of radiation-pressure-propelled probes and infer gravitational perturbations from their deflected trajectories, citing real-world confounds like unpredictable solar flares, while expressing more optimism about the broader trend toward cheap, fast-turnaround CubeSat-style missions (a stark contrast to traditional multi-decade NASA mission timelines) potentially enabling PhD-thesis-scale missions within a single graduate program. On 'Oumuamua, he discusses a proposed explanation (from colleagues Darryl Seligman and Greg Laughlin) that it may be a fragment of molecular-cloud hydrogen ice — its elongation explained by sublimation (like a shrinking bar of soap) and its unexplained acceleration by an invisible hydrogen outgassing jet — favoring this over an alien-origin explanation specifically because it's falsifiable (predicting many more such objects will be found by the upcoming Vera Rubin Observatory) whereas an "aliens" hypothesis structurally resists disconfirmation. Speculating further on detecting alien civilizations, he suggests that searching for artificial "debris" or byproducts of large-scale computation (rather than radio signals, which humanity itself broadcasts far less than a few decades ago) might be more fruitful, while cautioning that any concrete prediction about alien technology is likely to be as limited by present-day imagination as 1870s predictions about transportation ("faster buggies").

Batygin discusses the enduring "usefulness of useless knowledge" (echoing Abraham Flexner's essay of that title, and Maxwell's equations enabling modern electronics despite no such intent) and endorses the view (also citing Neil deGrasse Tyson's writing on military-funded science) that open, exploratory, seemingly impractical research consistently generates outsized long-term societal value — arguing failure in individual research pursuits is not evidence the underlying exploration was worthless, and that a wide, generative portfolio of speculative ideas (echoing Linus Pauling's "have a lot of ideas") is preferable to conservative, narrowly justified research programs. He reflects on his childhood emigration from Russia (via six years in Japan before arriving in the US at 14) as formative preparation for navigating major life changes, and closes with career advice cautioning against optimizing purely for credentials/grades (a "template for success" he doesn't believe reliably exists), instead recommending people spend most of their effort on work they're genuinely passionate about that can "take on a life of its own" separate from themselves (citing code, music, or other externally legible creative output) while maintaining just enough conventional achievement (grades, certifications) as a baseline.

Excerpts