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?

<P0-S0> Carl Otto Lampland (December 29, 1873 – December 14, 1951) was an American astronomer. <P0-S1>  He was involved with both of the Lowell Observatory solar system projects, observations of the planet Mars and the search for Planet X. <SEP> <P1-S0> The Nice ( ) model is a scenario for the dynamical evolution of the Solar System. <P1-S1>  It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice, France. <P1-S2>  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. <P1-S3>  In this way, it differs from earlier models of the Solar System's formation. <P1-S4>  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. <P1-S5>  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. <P1-S6>  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. <SEP> <P2-S0> 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. <P2-S1>  Planetary migration is the most likely explanation for hot Jupiters: extrasolar planets with jovian masses but orbits of only a few days. <P2-S2>  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. <P2-S3>  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. <P2-S4>  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. <SEP> <P3-S0> Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. <P3-S1>  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.