Completion of first orbit by Pluto

 When Pluto will complete its first full orbit since its discovery  

Astronomers will have to wait a while to celebrate the first complete orbit of Pluto since its discovery. Pluto was discovered on 18 Feb, 1930, using the Lowell Observatory in Flagstaff, Arizona. American astronomer Clyde Tombaugh found a moving object clearly beyond the orbit of Neptune. The object was later called Pluto, the ruler of the Greek underworld in their culture's mythology. There's a long-running debate about whether Pluto is a planet or a dwarf planet. Concerning its orbit, however, astronomers don't disagree that the world has yet to complete a single orbit since Tombaugh first spotted Pluto in imagery. It takes Pluto 248.09 Earth years to complete one orbit around the sun. Use this information along with its discovery date, and you'd find that Pluto will complete its first full orbit since its discovery on Monday, 23 March, 2178. Sadly, we doubt you'll be around for Pluto's NYE party.

Of the five dwarf planets in the Solar System, including Eris, Ceres, Makemake, and Haumea, Pluto is easily the best-known due to its brief categorization as a regular planet. Long before it could even make one turn around the Sun, it was demoted again. A surprising number of astronomical bodies have been discovered by observing Uranus. Observing the planet in the 19th century, astronomers noticed that Uranus was moving in unexpected ways, according to the Newtonian theory of gravity. Though the discrepancies were small, there was a difference between the observed orbit of Uranus and the way Newtonian physics predicted its orbit to be. In 1846, astronomer and mathematician Urbain Le Verrier suggested that an unknown planet beyond Uranus was altering its orbit, and made predictions as to the orbit of this unknown object. The object turned out to be Neptune, but Uranus still had some secrets hidden away up there. American astronomer Percival Lowell noticed that there were still unexplained movements in the orbit of Uranus, as well as Neptune. In 1905, he suggested these were caused by the pull of another unknown planet, and made predictions as to its orbit. Based on the predictions from Lowell and other astronomers, Pluto was eventually located in 1930. American astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona looked at areas of the sky where astronomers predicted Pluto would be. He used a blink comparator, a device which quickly flipped between two images of the same area of sky, allowing any slight differences to be noticed more easily, and found an object that had moved between 23 and 29 January, 1930. This was soon confirmed to be a planet (for a while, anyway), which was named Pluto by Venetia Burney Phair, an 11-year-old girl who wrote to the Lowell Observatory.

But there is far more to this intriguing world's orbit than the amount of time lapsed since its discovery, as astronomers said. Pluto's environment changes depending on how close or far it is from the sun. The larger worlds of our solar system tend to orbit near the ecliptic, which is the plane of the solar system. Pluto, however, is quite inclined compared with Earth and many others at 17 degrees. The world Eris exceeds that at 44 degrees, noted the Lowell Observatory's Will Grundy, co-investigator on NASA's New Horizons mission which encountered the Pluto system in 2015 and Arrokoth in January 2019. The craft is now moving into the Kuiper Belt. "Inclinations of smaller planets tend to be higher, as a result of dynamical interactions between planets early in solar system history: Mercury's is seven degrees, Eris' is seven degrees, Makemake's is 29 degrees, and Haumea's is [28.2] degrees," Grundy said. The same is true of small world eccentricity, which describes how far an orbit is from a true circle. While Earth's orbit is almost circular, Pluto's is elongated with an eccentricity of 0.25. By comparison, Mercury's is 0.205, Eris' is 0.44, Makemake's is 0.16 and Haumea's is 0.20. The deviations present among these various worlds, he said, " are valuable clues to dramatic events which happened early in solar system history, calling for a complicated history of planetary migrations and close encounters and maybe even some planets being kicked out of the solar system entirely to become free-floating rogue planets." Such perturbations are still dynamic enough which complicated calculations are required to navigate a spacecraft through these orbits, he said.

For a time, Pluto is included in the Solar System alongside actual planets like Jupiter and Earth. But before Pluto had even spent a plutonian year enjoying its planet status, it was reclassified as a dwarf planet and kicked off our diagrams once more. The International Astronomical Union (IAU) downgraded Pluto in 2006 as it did not meet all the criteria for what astronomers call a planet. Though it orbits the Sun, and has enough mass to make it into a nearly spherical shape, it does not dominate its own orbit like the actual planets do. Pluto has not tidied up enough of its orbit of other space rocks in order to qualify as a planet, sharing its orbit with other large objects which do not orbit the dwarf planet. Despite this, it is capable of holding onto moons. New Horizons principal investigator Alan Stern, of the Southwest Research Institute, noted there are four things to consider about Pluto's orbit. The first two are its inclination and eccentricity. The third is Pluto's resonance with Neptune. This large gas giant is locked in a gravitational dance with Pluto which keeps both worlds in a consistent pattern of orbits with each other (which we also see at places like Saturn, which have a lot of moons in resonant orbits). The fourth is what happens because of that resonance. Pluto comes closer to the sun than Neptune does for about 20 years of every orbit. "It is like clockwork," Stern said, explaining that when this happens, Neptune is always on the opposite side of the sun. "The two can never collide, because they're in this resonance." Grundy added a fifth element to the list of important features of Pluto's orbit: 

Pluto and its largest moon, Charon, are relatively close in size; Charon is about half the mass of Pluto. He said that rather than thinking of Pluto as a world to itself, "think of that common center of mass" which Pluto and Charon share as they orbit the sun. Grundy said, "Pluto and Charon are really a double planet," and that should be considered when mapping the orbit of the system. Pluto is orbited by five known moons, the largest of which is Charon," NASA explains. "Charon is about half the size of Pluto itself, making it the largest satellite relative to the planet it orbits in our Solar System. Pluto and Charon are often referred to as a 'double planet'. Pluto's days, though they drag on a bit at around 153 hours, are nothing compared to its years. Pluto's orbit takes 248 Earth years. Its orbit, as well as being long, is unusual in its elliptical shape and tilt. Complex terrain on Pluto includes "snakeskin" region. Pluto is a resident of the region which astronomers call the Kuiper Belt and filled with icy worlds collectively known as Kuiper Belt Objects (KBOs). Stern noted that these objects are very different from one another, and there are at least four known types, so the best way to think about KBOs are as worlds occupying "the same zip code" in the outer reaches of the solar system. As Pluto proceeds in its orbit, at times it is closer or farther from the sun and will react to the stronger or weaker sunlight. For example, Stern said, when the heating is stronger, Pluto will have a more massive atmosphere. "The increased sunlight is increased energy, and that energy sublimates more ice into gas, making for a thicker, more massive atmosphere," he explained. On the surface, changes also occur. "Ices are moving around from place to place, just like on the Earth," Stern said. "With the seasons [on Earth], it snows in the winter, and if you took pictures of the Earth from another planet, you would see more bright zones than you would in the summertime on a given hemisphere. Then in the summer, you [see] the ice retreats. It's the same thing on Pluto and on Mars and on other objects as well." 

"From 1979 to 1999, Pluto was near perihelion, when it is closest to the Sun. During this time, Pluto was actually closer to the Sun than Neptune." As interesting as its orbit is, it does mean that the planet was demoted long before it could even celebrate its birthday as a planet. New Horizons has enough power available to run for the next 20 years, and it is focusing on helio-physics (studying the sun) and astrophysics as it continues its mission, Stern said. Of course, the data from its Pluto encounter in 2015 continues to be available and analyzed for future science investigations, too. As for Lowell, the observatory continues to celebrate its role in Pluto research with programs, exhibits of artifacts used in the discovery and tours of the telescope. The observatory also hosts an annual "I Heart Pluto Festival" to honor its historic connections to the distant world in our universe.