University of Silesia in Katowice

Pluto is a celestial body widely considered to be a planet until 2006. It was discovered by the American astronomer Clyde William Tombaugh, employed at the Percival Lowell Observatory (one of the oldest astronomical observatories in the United States, located in the city of Flagstaff in the northern part of Arizona) as a technician and sky photographer. It was Lowell (also known for promoting the once famous idea of the existence of channels on Mars) who initiated a research programme aimed at finding a planet that could disrupt the motion of Uranus and Neptune. Tombaugh, when comparing photographs of the area of sky covering the projected position of the new planet, which were taken a few days apart in January 1930, found an unknown object changing its position against the background of the fixed stars. It soon became clear that this object was the planet he was looking for.  Its previous working name “Planet X” was changed to “Pluto”, in accordance with the tradition of naming planets after Roman gods (the first two letters of the current name are also the initials of the founder of the observatory where the discovery was made, Percival Lowell). The name seemed appropriate: according to ancient Roman beliefs, Pluto was a god of the underworld and the newly discovered object was located at the edge of the Solar System, where the amount of solar energy is minimal. The original estimates of Pluto’s mass turned out to be greatly overestimated. 

Theoretical calculations based on perturbations of the motion of Uranus and Neptune suggested that Pluto’s mass was several times greater than the mass of the Earth. In reality, the observed perturbations of the orbital motion of these gas giants were the result of a measurement error and not the gravitational influence of an additional planet. Therefore, one may be tempted to say that Pluto was discovered accidentally… Currently, based on the analysis of the gravitational interaction between Pluto and its moon Charon (according to Kepler’s third law), the mass of this object is estimated to be about 0.002 of the Earth’s mass.  

The discovery of other celestial bodies of similar mass to Pluto, such as Ceres from the asteroid belt (the area located between Mars and Jupiter and containing a huge number of small stone objects) or Eris, situated, like Pluto, outside the orbit of Neptune, made it necessary to reconsider the correct definition of a planet. According to the new definition, announced in Prague in 2006 by the International Astronomical Union, Pluto is classified as a so-called dwarf planet. Interestingly, it fails to meet only one requirement to be considered a planet: its mass is so small that Pluto does not gravitationally dominate the space around its orbit. The downgrade of Pluto has solved a number of problems associated with this object, which made it stand out as a planet when compared to other planets in the Solar System. Its mass is very small compared to the other planets in the Solar System; even many satellites like the Moon and Jupiter’s Galilean moons have higher mass than Pluto. Pluto’s orbit is strongly elliptical (Pluto’s orbital velocity at the perihelion is almost twice as high as at the aphelion!) and its plane crosses the ecliptic plane at an angle of 17° which is quite unusual for a planet (the planets of the Solar System move more or less in the same orbital plane). Currently, Pluto is the brightest of the so-called transneptunian objects – objects from the Kuiper belt, which is a torus-shaped region on the periphery of the Solar System extending over a distance of 30 to 50 astronomical units (an astronomical unit is the approximate distance from the Earth to the Sun). The Kuiper belt is most likely composed of small ice and rock objects that are remnants of the formation period of the Solar System. It is estimated that beyond this area there must be another one, acting as a reservoir of comets. Here we identify the so-called scattered disc, extending over a distance of hundreds of astronomical units (the source of short-period comets), and the spherical Oort cloud, extending up to hundreds of thousands of astronomical units (the source of long-period comets). Disturbances caused by the gravitational influence of neighbouring objects destabilise the orbits of bodies from these areas. As a result, they begin to head towards the Sun, and become comets: as they approach our star they begin to evaporate, producing a gaseous envelope and characteristic tail (gas and dust). 

Until the 1990s, Pluto was the only known Solar System object from beyond Neptune’s orbit. Today we know that beyond it extends a vast region filled with a huge number of small bodies. As it turns out, the size of the Solar System is much larger than we previously thought and our understanding of its formation process has since progressed. Currently, five space probes are leaving the Solar System: Voyager 1 and 2, Pioneer 10 and 11 (launched back in the 1970s) and the youngest of them, New Horizons, dedicated to the study of Pluto and its moon Charon and other Kuiper belt objects (launched in 2006; the first images of Pluto were sent in 2015). Three of them (Voyager 1 and 2 and New Horizons) continue to send signals towards Earth, which, from a scientific point of view, are extremely valuable.