For millennia the only known celestial objects were those visible to the naked eye. In the Solar System these are limited to the Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn, and some comets and asteroids occasionally passing near our planet. Astronomy was revolutionized in the early seventeenth century, when Italian astronomer Galileo Galilei was the first to use a telescope to observe the sky.
Thanks to this new instrument, Galilei saw countless celestial objects that no human had ever seen before, such as the four largest moons of Jupiter (Io, Europa, Ganymede, and Callisto). Over the following decades, the observations of Huygens and Cassini led to the discovery of five satellites of Saturn (Titan, Iapetus, Rhea, Tethys, and Dione). Nearly a century followed without new large solar system objects being confirmed until March 13, 1781, when English astronomer William Herschel observed by chance what he initially thought to be a comet.
Discovery of Uranus and Neptune
The measurements of the orbital parameters of the new object, carried out by Anders Johann Lexell, led to the conclusion that this was not a comet but rather a planet, later called Uranus, whose orbit has a semimajor axis of about 19 AU (astronomical units), placing it far beyond Saturn, which since ancient times had been determined to be the most distant of the known planets. Subsequent observations of the new planet made by Herschel identified the two satellites later called Titania and Oberon in 1787. The rings surrounding the planet were only discovered in 1977.
Over the following years, more precise measurements of the orbit of Uranus were made, and in 1821 French astronomer Alexis Bouvard noticed that the new planet deviated significantly and abnormally from its expected path, so he hypothesized that another large body was influencing the orbit of Uranus through gravitational interaction.
In 1843, British astronomer John Couch Adams began working on the description of the hypothetical orbit of a new body beyond the orbit of Uranus, and the following year he gave a first rough estimate of its position, providing more precise measurements. In June 1846, independently of Adams, French astronomer Urbain Le Verrier calculated the orbital elements of the hypothetical new planet, obtaining results that were very similar to those of Adams.
Adams then asked the director of the Cambridge Observatory James Challis to observe the predicted position in the sky in search of the new planet, but he found nothing. In September of the same year Le Verrier sent his results to the Berlin Observatory, from which Johann Gottfried Galle was the first to recognize the planet later known as Neptune on the evening of September 23, 1846. The discovery of the new planet, which was mathematically predicted before being observed, was considered the greatest achievement of celestial mechanics. A few days later Triton, the larger moon of Neptune, was discovered by William Lassell, who also spotted Ariel and Umbriel, two moons of Uranus, in 1851.
Images of Uranus (left) and Neptune (right) taken by Voyager 2 in 1986 and 1989 respectively (NASA, NASA).
Discovery of Pluto
Immediately after the discovery of Neptune and the calculation of its orbit, which has a semimajor axis of about 30 AU, astronomers began to hypothesize the existence of one or more planets on even larger orbits, but the research made at the end of the nineteenth century produced no results. In 1906, American astronomer Percival Lowell began an extensive search for the body he named Planet X from the Lowell Observatory that he founded in Arizona. Independently of Lowell, in 1908 William Pickering, another American astronomer, announced that he had proved the existence of the ninth planet by analyzing the irregularities of the orbit of Uranus, which were not completely explained by the existence of Neptune alone. These results convinced Lowell to intensify the research, which was abruptly interrupted by his death in 1916.
Research at the Lowell Observatory resumed only in 1929 and the assignment was given to the young Clyde Tombaugh. On February 18, 1930, Tombaugh noticed that two images taken on January 23 and 29 of that year showed the movement of a faint object, confirmed by another picture taken on January 21. The semimajor axis of the orbit of this object was calculated to be about 40 AU. This newly discovered body, called Pluto, was initially believed to be the much sought after Planet X, but it was soon realized that the object was small and had a very elliptical orbit compared to the other planets.
The mass of Pluto was initially estimated to be similar to that of the Earth in 1931, but this value was revised downwards several times, down to one tenth of the Earth’s mass according to Kuiper in 1950, one hundredth according to Baldi and Caputo in 1974 and finally two thousandths according to Christy and Harrington in 1980. This last estimate was calculated thanks to the discovery, made by James Christy in 1978, of Pluto’s largest satellite, Charon. Four other moons of Pluto have been discovered since, Nix and Hydra in 2005, Kerberos in 2011, and Styx in 2012.
Beyond Pluto
The discovery of Pluto and its physical characteristics immediately led to the hypothesis that the new planet was nothing more than one of the many bodies located beyond the orbit of Neptune. Among the first to explore this possibility was independent Irish astronomer Kenneth Edgeworth, who between the 1930s and 1940s hypothesized the presence of a vast number of comets beyond Neptune, remnants of the protoplanetary disk that were not condensed into planets due to the lower density of the disk here compared to the innermost regions.
In 1950, Dutch astronomer Jan Oort explored the idea, already hypothesized by Estonian astronomer Ernst Öpik in 1932, that long-period comets originate in a region located at the limits of the solar system, which was later called Oort cloud. Starting from this hypothesis and his calculation of the mass of Pluto (that he estimated to be one-tenth of the Earth’s mass), Gerald Kuiper, another Dutch astronomer, argued that the gravitational influence of a large body would wipe out any leftover objects from the formation of the Solar System in the region located a few tens of astronomical units beyond Neptune, sending them toward the Oort cloud.
American astronomer Fred Whipple hypothesized in 1964 that a large population of trans-Neptunian objects were responsible for the irregularities in the orbits of Uranus and Neptune. In the following years, studies on short-period comets led the Uruguayan astronomer Julio Fernández to conclude that these objects could not have come from the Oort cloud, but from a closer region, located between 35 and 50 AU, which was more concentrated toward the plane of the ecliptic. This region was later called the Kuiper belt.
The first extensive search for trans-Neptunian objects was carried out by Charles Kowal between 1976 and 1985 from the Palomar Observatory in California. However, his studies only led to the discovery of Chiron, a body that shares characteristics with both asteroids and comets. Chiron, located between the orbits of Saturn and Uranus, was the first of a new class of objects now known as centaurs.
Around the same time, other similar searches were unsuccessful until, in 1992, David Jewitt and Jane Luu from the Mauna Kea Observatory in Hawaii discovered an object provisionally called 1992 QB1, which was renamed Albion in 2018. Subsequent observations revealed a semimajor axis of about 41 AU, making this the first trans-Neptunian object discovered after Pluto and Charon. 1992 QB1 then gave its name to the category of trans-Neptunian objects called cubewans, which have low eccentricity orbits and are not in orbital resonance with Neptune. Over the following years, more trans-Neptunian objects were discovered, confirming the presence of the Kuiper belt.
The discovery of 1996 TL66, a trans-Neptunian object with a semimajor axis of 83 AU, led to the hypothesis of the existence of a new class of objects, located beyond the limits of the Kuiper belt, a region that was called scattered disc. This region is the result of the perturbations caused by the giant planets, especially Neptune, on Kuiper belt objects, which are driven into wider, more eccentric and inclined orbits. This theory was confirmed by the discovery of even more extreme objects, especially Sedna, a planetoid identified in 2003, whose orbit has a semimajor axis of over 500 AU and a orbital period of about 12,000 years. Sedna remains to this day one of the furthest objects ever discovered.
Comparison of the size of the inner Solar System, the outer Solar System with the Kuiper belt, the orbit of Sedna, and the Oort cloud (NASA).
A new definition of planet
Other notable Kuiper belt objects with sizes comparable to Pluto were discovered around the same time, including Haumea in 2004, and Eris and Makemake in 2005. These discoveries sparked a heated debate in the scientific community regarding the classification of these objects, including Pluto, and whether it was correct to consider them as planets.
A final decision was made by the general assembly of the IAU, the International Astronomical Union, in Prague in August 2006. Under the new definition, a planet is a celestial body orbiting the Sun that has sufficient mass to achieve hydrostatic equilibrium, meaning that it has a spheroidal shape, and has “cleared the neighborhood” around its orbit.
This last point marks the difference between a planet and the new category of dwarf planets, which do not have enough mass to dominate their orbit and “clean” it from other objects located in the same area. Due to the introduction of this new parameter, Pluto was reclassified as a dwarf planet. The other objects initially identified as dwarf planets were Ceres, the first object discovered in the asteroid belt between Mars and Jupiter in 1801, and Eris, located in the scattered disk at an average distance of 68 AU from the Sun and believed to be more massive than Pluto. In 2008 Haumea and Makemake, which have a semimajor axis of 43 and 46 AU respectively, were also officially recognized as dwarf planets by the IAU. Today it is believed that there are dozens of other similar objects that can be potentially classified as dwarf planets. The most likely candidates are: Quaoar, Sedna, Orcus, Gonggong, and Salacia.
The study of the orbital parameters of some of the most extreme trans-neptunian objects has led some astronomers to suggest that one or more planet-sized objects could be located on the farthest edges of the Solar System. However, no large body has been found yet.
Exploration of the outer Solar System
Because of the huge distances of Uranus, Neptune and the trans-Neptunian objects from Earth, exploration missions to these bodies are extremely long and difficult. The first probes to be launched toward the outer Solar System were Pioneer 10 in 1972 and its twin Pioneer 11 the following year. Pioneer 10 was the first probe to pass the orbits of Uranus and Neptune, but both missions did not include close approaches with the two planets.
In 1977, two more twin probes, Voyager 1 and 2, were launched. Voyager 1, after meeting Jupiter and Saturn and overtaking the two Pioneers, became the most distant artificial object. In 2012 the probe detected an increase in the presence of cosmic rays and a significant decrease in the number of solar wind particles, an event indicated by NASA as a signal that the probe exited from the heliosphere, the region of space dominated by the Sun, and entered interstellar space. However, the probe will reach the Oort cloud only in 300 years and it will take 30,000 years to cross this region.
Voyager 2 was instead the first, and to this day the only, probe to reach Uranus and Neptune. Many discoveries were made thanks to this mission, such as the presence of a magnetic field, two new rings and as many as ten new satellites of Uranus during the of the planet flyby in 1986. During the close flyby of Neptune in 1989, five moons and a system of four rings, previously only hypothesized, were discovered around this planet. Like its twin Voyager 1, this probe also exited the heliosphere and entered interstellar space in 2018.
The most recent mission launched with a trajectory leaving the Solar System is New Horizons, which departed in January 2006. After a journey that lasted more than nine years, this was the first probe to reach Pluto in July 2015. During its flyby of the distant dwarf planet, the probe produced the first high-resolution images of Pluto and made the most accurate measurements regarding its size and mass, together with those of its moons. After passing Pluto, the New Horizons mission made a close approach with trans-Neptunian object Arrokoth on January 1, 2019, at a distance of 43.4 AU from the Sun. New Horizons will continue to cross the Kuiper belt and could potentially study other objects if these are close enough to the trajectory of the probe. As of 2023, various proposals for missions toward Uranus, Neptune, and the trans-Neptunian objects have been made, but none has been approved.
Image of Pluto taken by New Horizons in 2015 (NASA).