Question: Who was the American astronomer involved in the search for a planet thought to explain apparent discrepancies in the orbits of the giant planets?

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Five-planet Nice model: The five-planet Nice model is a recent variation of the Nice model that begins with five giant planets, the current four plus an additional ice giant, in a chain of mean-motion resonances.  After the resonance chain is broken, the five giant planets undergo a period of planetesimal-driven migration, followed by a gravitational instability similar to that in the original Nice model.  During the instability the additional giant planet is scattered inward onto a Jupiter-crossing orbit and is ejected from the Solar System following an encounter with Jupiter.  An early Solar System with five giant planets was proposed in 2011 after numerical models indicated that this is more likely to reproduce the current Solar System.

Planetary migration: Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis.  Planetary migration is the most likely explanation for hot Jupiters: extrasolar planets with jovian masses but orbits of only a few days.  The generally accepted theory of planet formation from a protoplanetary disk predicts such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals.  It has also become clear that terrestrial-mass planets may be subject to rapid inward migration if they form while the gas disk is still present.  This may affect the formation of the cores of the giant planets (which have masses of the order of 10 Earth masses), if those planets form via the core accretion mechanism.

List of Halley-type comets: The following is a list of Halley-type comets (HTCs), which are periodic comet with an orbital period between 20 and 200 years, often appearing only once or twice within one's lifetime.  The majority come from between the orbits of Saturn and Neptune; Due to the nature of their orbits, their orbits can be perturbed by the giant planets and sent into orbits too far from the Sun to outgas, and vice versa.  Minor planets in comet-like orbits similar to HTCs that never come close enough to the Sun to outgas are called Centaurs.  HTCs are named after the first discovered member, and the first discovered periodic comet, Halley's Comet, which orbits the Sun in about 75 years, and passing as far as the orbit of Neptune.

Centaur (minor planet): Centaurs are small solar system bodies with a semi-major axis between those of the outer planets.  They have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years.  Centaurs typically behave with characteristics of both asteroids and comets.  They are named after the mythological centaurs that were a mixture of horse and human.  It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.

Jumping-Jupiter scenario: The jumping-Jupiter scenario specifies an evolution of giant-planet migration described by the Nice model, in which an ice giant (Uranus, Neptune, or an additional Neptune-mass planet) is scattered inward by Saturn and outward by Jupiter, causing the step-wise separation of their orbits.  The jumping-Jupiter scenario was proposed by Ramon Brasser, Alessandro Morbidelli, Rodney Gomes, Kleomenis Tsiganis, and Harold Levison after their studies revealed that the smooth divergent migration of Jupiter and Saturn resulted in an inner Solar System significantly different from the current Solar System.  The sweeping of secular resonances through the inner Solar System during the migration excited the eccentricities of the terrestrial planets beyond current values and left an asteroid belt with an excessive ratio of high- to low-inclination objects.  The step-wise separation of Jupiter and Saturn described in the jumping-Jupiter scenario can allow these resonances to quickly cross the inner Solar System without altering orbits excessively, although the terrestrial planets remain sensitive to its passage.  The jumping-Jupiter scenario also results in a number of other differences with the original Nice model.  The fraction of lunar impactors from the core of the asteroid belt during the Late Heavy Bombardment is significantly reduced, most of the Jupiter trojans are captured during Jupiter's encounters with the ice giant, as are Jupiter's irregular satellites.  In the jumping-Jupiter scenario, the likelihood of preserving four giant planets on orbits resembling their current ones appears to increase if the early Solar System originally contained an additional ice giant, which was later ejected by Jupiter into interstellar space.  However, this remains an atypical result, as is the preservation of the current orbits of the terrestrial planets.

Nice model: The Nice ( ) model is a scenario for the dynamical evolution of the Solar System.  It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice, France.  It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary gas disk.  In this way, it differs from earlier models of the Solar System's formation.  This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies including the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.  Its success at reproducing many of the observed features of the Solar System means that it is widely accepted as the current most realistic model of the Solar System's early evolution, although it is not universally favoured among planetary scientists.  Later research revealed a number of differences between the original Nice model's predictions and observations of the current Solar System, for example the orbits of the terrestrial planets and the asteroids, leading to its modification.

Giant planet: A giant planet is any massive planet.  They are usually primarily composed of low-boiling-point materials (gases or ices), rather than rock or other solid matter, but massive solid planets can also exist.  There are four known giant planets in the Solar System: Jupiter, Saturn, Uranus and Neptune.  Many extrasolar giant planets have been identified orbiting other stars.

Sudarsky's gas giant classification: Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined by David Sudarsky and colleagues in the paper "Albedo and Reflection Spectra of Extrasolar Giant Planets" and expanded on in "Theoretical Spectra and Atmospheres of Extrasolar Giant Planets", published before any successful direct or indirect observation of an extrasolar planet atmosphere was made.  It is a broad classification system with the goal of bringing some order to the likely rich variety of extrasolar gas-giant atmospheres.

Planets beyond Neptune: Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit.  The search began in the mid-19th century and culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.

Carl Otto Lampland: Carl Otto Lampland (December 29, 1873 – December 14, 1951) was an American astronomer.  He was involved with both of the Lowell Observatory solar system projects, observations of the planet Mars and the search for Planet X.

Supporting paragraphs: Carl Otto Lampland Planets beyond Neptune
Answer: Carl Otto Lampland