Neptune Visited and the Outer Solar System Revolutionised, 1989–2019

Abstract

Chapter 9 sketched the labours of elite mathematicians across a century and a half from 1846 to about 1990, discarding Bode’s Law and seeking to refine celestial mechanics in order to narrow the search area and discover further planets in the Outer Solar System. All, including the long-held Laplacian Theory for formation of the Solar System, would be swept aside, as Solar System studies based on Isaac Newton’s most exacting science have been rejuvenated by supercomputers.

The Voyager 2 spacecraft’s stunning flyby reconnaissance of the Neptune system in August 1989 revealed astonishing detail of the planet itself and of its large satellite Triton, a planet-sized body now believed to have been captured from the Kuiper belt, of which Pluto and its moon are also members. The flyby explained away the small residuals of Gaillot’s tables of Uranus (1909) which inspired Percival Lowell’s quest for Planet X and tempted later investigators. Meanwhile, resonances such as the near-2:1 resonance between Uranus and Neptune have proved to be key to understanding the present arrangement of planets in the Solar System. Also, the discovery since 1992 of many more bodies populating the Kuiper belt—a region previously regarded as empty and devoid of interest—has utterly changed our ideas about the Outer Solar System and its evolution.

Also, discoveries far beyond the Solar System have helped to increase knowledge of our own. Since the first exoplanet system (Pegasi 51 b/Dimidium) was recognised in 1995, thousands of others been discovered, many of them multi-planet systems and most very unlike our own Solar System. The sheer dissimilarity has challenged computer modellers to explain how such different arrangements arise, and in turn have enriched our understanding of the past evolution of our own Solar System leading to its present arrangement. The latter is not God-ordained, or a clockwork system as some of the 18th century followers of Newton (though not Newton himself) imagined. The Outer Solar System in particular has proved to be an important byproduct of this evolution, and attests to past migrations of the giant planets in the early history of the Solar System. Their interactions with planetesmials in the circum-solar disk sculpted the Kuiper belt, and scattered a vast number of asteroids and potential comets into the far outer Solar System where they form the spherical Oort cloud halo round our system.

Intense scrutiny including by the new generation of large 10-metre telescopes has led to the discovery of numerous dynamically and physically distinct categories of Kuiper belt objects (e.g., “cold” KBOs, “hot” KBOs, etc.) The understanding of how these categories came to be has helped shape a general picture of the Solar System’s evolution; though details are disputed, there is overall agreement around a concept known as the “Nice Model”, in which Jupiter was the first planet to form, initially migrated inwards through the inner Solar System like a wrecking ball, possibly destroying a first-generation of inner planets. However, the formation of Saturn, and a resonance with Jupiter, caused a reversal of Jupiter’s inward motion and subsequent migration outward to its current position (Jupiter’s Grand Tack), while also pushing the embryonic ice giants—Uranus and Neptune—outward.

There, Neptune—the “star not on the map”, in Heinrich d’Arrest’s arresting phrase—is the most massive and dominant body shaping its distant icy realm. At present, it is the outermost of the “major planets”, but it may not be the last. Astronomers have found subtle hints in the way the orbits of some distant Kuiper belt objects are arranged, and surmised the existence of a possible “Planet Nine.” Completing this book’s underlying theme, we close with a look forward to a future in which celestial mechanics, much evolved and altered, but also much more powerful than it was in the hands even of Adams and Le Verrier, continues to play an indispensable role in our attempts to understand the motions of the bodies of the Solar System. That history has yet to be written….