Planet Nine: is it a planet, a primordial black hole, or something else entirely?

Episode Summary

Executive Summary: This episode examines Planet Nine: the evidence for an unseen massive object in the outer solar system, the debate over whether it could be a primordial black hole, and experimental ideas to detect it. Mike Brown explains why the data still most strongly support a distant planet, while Ziv Rogozinski describes a proposed fleet of tiny probes to test for a massive perturber.

Main Topics: Mike Brown’s discoveries in the outer solar system (Priority: 5/5): Brown discusses his early Kuiper belt surveys, the discovery of Eris, Haumea, Sedna and many other minor planets, and how these finds set the stage for Planet Nine evidence. The observational clues for Planet Nine (Priority: 5/5): The transcript outlines how Sedna and later distant objects showed unexplained orbital patterns, first in argument of perihelion and then in longitude of perihelion, suggesting a large unseen perturber. Why Planet Nine is thought to be a planet (Priority: 5/5): Brown explains that a tilted, elongated, roughly six-Earth-mass body can reproduce the orbital clustering and other strange high-inclination objects seen in the Kuiper belt. Primordial black hole as an alternative hypothesis (Priority: 4/5): The podcast introduces papers proposing that the mass inferred for Planet Nine could belong to a primordial black hole rather than a planet, but Brown argues this is far less likely. Brute-force spacecraft detection ideas (Priority: 4/5): Ziv Rogozinski describes methods inspired by Breakthrough Starshot: sending many tiny, fast probes to the outer solar system and detecting perturbations in their trajectories. Search strategy, uncertainty, and naming (Priority: 3/5): Brown says the object may already be in archival data, current surveys are ongoing, and naming it now would be bad luck; Rogozinski suggests mythological naming conventions if it is found.

Key Arguments: Mike Brown argues the orbital clustering of distant objects is best explained by a large planet, not by measurement error or by a black hole. The shift from argument of perihelion to longitude of perihelion was the key insight that made the Planet Nine explanation physically plausible. Planet Nine also naturally explains other previously mysterious objects, including bodies on near-90-degree inclined orbits. A primordial black hole is mathematically possible but, in Brown’s view, an unnecessary and highly implausible solution. Rogozinski argues that if direct imaging fails, sending many lightweight probes could reveal a massive unseen object through tiny trajectory perturbations. Both speakers agree that what matters most for the probe tests is mass, not whether the object is rocky, icy, or a black hole. Brown believes Planet Nine may already exist in archived astronomical images and expects dedicated surveys, especially Vera Rubin/LSST, to be decisive.

Data Points: Minor planets discovered/co-discovered by Mike Brown: 29 - Brown’s total credited discoveries from Kuiper belt surveys Year Brown began surveying distant objects: 1998 - He started systematic searches for large Kuiper belt objects Distance of proposed Planet Nine orbit: averaging over 250 times Earth’s distance from the Sun - Introductory description of the hypothesized orbit Planet Nine mass estimate: about 6 Earth masses - Brown’s explanation of the inferred mass Possible mass range mentioned by Rogozinski: 5 to 10 Earth masses - Range used in discussion of the hypothesized object Sedna orbital period: 10,000 years - Brown describes Sedna’s very elongated orbit Planet Nine orbital period estimate: about 5,000 years - Brown’s estimate of the planet’s orbit Suggested probe mass: about 100 grams - Witten’s proposed tiny spacecraft design Alternative probe mass discussed: about 1 kilogram - Rogozinski says increasing mass improves resistance to perturbations Travel time to the outer solar system for probes: about 1 year to 10 years - Rogozinski contrasts gram-scale and kilogram-scale probe concepts Planned survey telescope: Vera Rubin Telescope / LSST - Brown expects it to be crucial for finding Planet Nine LSST first year of operation: 2022 (noted as delayed/uncertain) - Brown’s estimate of when the telescope might begin operation

Pivotal Quotes: "it could be a black hole it could be a planet it could be a six earth mass hamburger" — Mike Brown: Brown stresses that only the mass is known, not the nature of the object "the chance that it's a six earth mass black hole is pretty close to zero" — Mike Brown: Brown’s rebuttal to the primordial black hole hypothesis "we've shown that this particular perturbation can be calculated using the tools that we have here on earth" — Ziv Rogozinski: Rogozinski explains the proposed transverse-perturbation detection method

Implications: The episode shows that Planet Nine remains a serious scientific hypothesis, but the black-hole idea is speculative. Future progress likely depends on archival data, ongoing surveys, and next-generation telescopes rather than exotic missions.