Trans-Neptunian object
Trans-Neptunian objects (TNOs) are celestial bodies that orbit the Sun beyond Neptune's orbit, located approximately 2.75 billion miles (4.3 billion kilometers) from Earth. These objects primarily consist of rock and ice, with Pluto and Eris being the largest and most well-known among them, measuring about 1,500 miles (2,400 kilometers) in diameter. TNOs are predominantly found within the Kuiper Belt, a disc-shaped region extending from 30 to 55 astronomical units from the Sun, and can have either regular or highly eccentric orbits. As of November 2024, over 3,000 TNOs have been identified, with sizes ranging from 30 miles (50 kilometers) to 1,500 miles (2,400 kilometers).
The discovery of TNOs has significantly influenced our understanding of the solar system, especially regarding the criteria for planetary classification. The 2005 discovery of Eris led to the redefinition of these criteria by the International Astronomical Union in 2006, resulting in Pluto being reclassified as a dwarf planet. Ongoing studies of TNOs hold the potential to reveal insights into the origins and formation of the solar system. NASA's New Horizons mission, which launched in 2006, aimed to explore TNOs, successfully conducting close encounters with Pluto and the Kuiper belt object Arrokoth, further expanding our knowledge of this distant region.
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Trans-Neptunian object
Trans-Neptunian objects (TNOs) refer to minor planets and smaller bodies of rock and ice that orbit the sun beyond the orbital path of Neptune, which lies some 2.75 billion miles (4.3 billion kilometers) from Earth. The largest known TNOs are Pluto, a dwarf planet that is about 1,500 miles (2,400 kilometers) in diameter, and the slightly smaller but more massive Eris. TNOs may have regular or eccentric orbits in and around the Kuiper Belt, a great whirling disc-shaped region of ice-rock debris that lies beyond Neptune between 30 and 55 astronomical units from the sun.
Background
The current model of the solar system—eight planets orbiting the sun—has undergone considerable revision over time. Despite the high resolution of contemporary telescopes on Earth, little is known about the physical makeup of the solar system beyond Neptune due to the dimness and small size of the bodies there. After the discovery of Pluto in 1930 and its moon Charon in 1978, the next TNO was not discovered until 1992. According to the International Astronomical Union's (IAU) Minor Planet Center, by November 2024, astronomers had discovered more than 3,000 TNOs, with diameters ranging from 30 miles (50 kilometers) to 1,500 miles (2,400 kilometers). TNOs are grouped into two major categories based on their distance from the sun: Kuiper belt objects (KBOs) such as Pluto have nearly circular orbits that lie between 30 and 55 AU, while scattered disc objects (SDOs) such as 90377 Sedna and Eris typically have highly irregular orbits that are much farther from the sun.
The presence of many TNOs was first predicted mathematically by astronomers analyzing variations in the orbits of known objects. For instance, before Pluto was discovered by astronomer Clyde Tombaugh (1906–97), astronomers had conjectured decades earlier about the presence of some object of considerable size that was causing perturbations in the orbits of both Uranus and Neptune. Along those same lines, scientists have also hypothesized that, based on eccentric orbital patterns of certain TNOs, a relatively large planet may orbit the sun beyond Pluto. Though not yet discovered, this hypothetical planet is known as Planet Nine. Other researchers believe the combined gravity of many smaller TNOs may account for the observed orbital characteristics.
Impact
Perhaps the most famous consequence of the discovery of TNOs has been the revision of the criteria for planetary status. Following the 2005 discovery of Eris, the second largest TNO after Pluto and the most massive, astronomers were prompted to clearly define the criteria for planets. As a result, in 2006, the IAU created the new class of dwarf planets and subsequently demoted Pluto from a planet to a dwarf planet, reducing the number of planets in the solar system from nine to eight.
The study of TNOs will likely yield new information about the origins and formation of the solar system. In January 2006, the National Aeronautics and Space Administration (NASA) launched a spacecraft called New Horizons, a satellite roughly the size of a dining room table. Its primary mission was to chart and send back data on TNOs. On July 14, 2015, New Horizons successfully made its closest approach to Pluto. It confirmed the existence of a hydrogen wall at the outer edges of the solar system in August 2018. It next flew by an object in the Kuiper belt, 486958 Arrokoth, on January 1, 2019. As of October 2024, New Horizons was traveling through the Kuiper Belt. NASA expected it to leave the Kuiper Belt between 2028 and 2029.
Bibliography
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