Passage:
In 1859, the French mathematician and astronomer Urbain Le Verrier published a rigorous re-examination of Mercury’s orbital motions, synthesizing over a century of planetary transit observations recorded between 1697 and 1848. Le Verrier had previously achieved international renown by mathematically deducing the existence and orbital location of Neptune based solely on unexplained perturbations in Uranus’s orbit. Applying the same Newtonian perturbation theory to Mercury, Le Verrier accounted for the gravitational tugs exerted by all known planets, notably Venus, Earth, and Jupiter. However, his calculations revealed a persistent residual discrepancy: the perihelion of Mercury’s elliptical orbit was advancing at a rate approximately 38 arcseconds per century faster than could be explained by Newtonian interactions with known celestial bodies (a figure later refined to roughly 43 arcseconds by Simon Newcomb).
To reconcile this discrepancy with Newtonian mechanics, Le Verrier evaluated several competing physical hypotheses. His primary proposal posited the existence of an undiscovered planet—or a diffuse belt of asteroidal bodies—orbiting within the space between Mercury and the Sun. Le Verrier reasoned that an intra-Mercurial planet with a mass comparable to Mercury would be exceptionally luminous and readily detected; consequently, he argued that a group of smaller planetoids, possessing lower individual reflectivity, was more physically plausible. The hypothetical intra-Mercurial body was named Vulcan, and over subsequent decades, numerous amateur and professional astronomers reported tentative telescopic sightings of dark circular silhouettes transiting the solar disk. However, rigorous synchronous observations during solar eclipses, most notably those orchestrated by the American astronomer Asaph Hall and later by William Wallace Campbell, consistently failed to corroborate these claims, rendering the existence of Vulcan untenable.
Le Verrier also analyzed alternative explanations, including a hypothetical solar oblateness—the possibility that the Sun possessed a slight equatorial bulge capable of exerting a quadrupolar gravitational pull. Yet Le Verrier determined that the degree of solar oblateness required to generate the observed perihelion advance would distort the apparent solar disk to an extent that directly contradicted optical measurements. Another conjecture, modifying Newton’s inverse-square law itself by adjusting the exponent of distance slightly away from two, was proposed by Hall in 1894; however, this ad hoc alteration successfully resolved Mercury's advance only at the cost of introducing intolerable errors into the orbital calculations of the Moon and Venus. Thus, throughout the late nineteenth century, Mercury’s anomalous precession remained an intractable crisis within classical mechanics, awaiting Albert Einstein’s 1915 formulation of General Relativity, which accounted for the full 43 arcseconds per century without invoking unobserved matter.
Statement: According to the passage, Le Verrier favored the hypothesis of a belt of smaller planetoids over a single intra-Mercurial planet because a single planet of sufficient gravitational mass would have been luminous enough to be readily detected.
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