Pluto and definition of planet Vedran Vrhovac*

1. Introduction tory, was surveying the photos of sky used by gravitational forces and result taken through telescope. On Febru- of those forces is locking of orbital Until recently, Pluto was designa- ary 18th 1930. he was comparing two periods. While Neptune makes three ted as planet but new discoveries were photos of same part of sky, taken at orbits, Pluto makes just two. cause for changing his designation to different nights. He noticed that one All these non typical attribu- dwarf planet. His name comes from little dot has changed her relative po- tes where cause for suspicion about Greek mythology, where Pluto was sition between stars. It soon turned his planetary status. Until early 90’s god of the Underworld. For very long out that the small dot was 9th planet that suspicions where not argumen- time, Pluto was last known object in – Pluto. ted. There were not such large body Solar System. After discoveries of si- Average distance between Sun and like Pluto with similar properties in milar bodies in outer Solar System, Pluto is almost 6 billion miles and for that part od Solar System so he was Pluto’s status has became problematic one orbit around Sun, he needs 248 unique. Development of advance te- for astronomy community. Just like years. Diameter of Pluto si 2390km, chnologies, such as computers and accepitng heliocentrical model of Solar one third less than diameter of our CCD cameras enabled discoveries System five centuries ago was pivotal Moon. Almost 50 years after discovery of new small objects in outer Solar point in history maybe last years con- of Pluto, his largest satellite, Charon, System. First object discovered was ference in Prague and accepting new was discovered. Charon is just half the 1992QB1. After Charon, 1992QB1 definition of planet will be significant size of Pluto. was first objcet discovered at those moment in history of planetology. Pluto’s orbit is significantly diffe- distances. This discovery proved that rent than orbit of other planets. His space beyond Neptune is not empty 2. Discovery orbit is inclined for 17° and very ellip- and it also gave new enthusiasm for se- tical. Pluto’s orbiting around Sun is arch of undiscovered objects beyond Astronomer-amateur Clyde Tom- in orbital resonance with Neptune’s. Neptune. Until 2002. almost 400 new baugh, employed in Flagstaff Observa- Orbital resonance is phenomenon ca- objects where discovered but none of those with significant dimensions. In 2002. the Quaoar was discovered. Di- ameter of Quaoar is 1260 km, so he is larger than Charon. After Quaoar even larger objects where discove- red: 2005FY9, 2003EL61 and 90482 Orcus. Dis- covery of Eris, with dimension 5-35% larger than Pluto caused adi- ttional turbulences

Figure 1. Pluton imaged by Hubble Space Telescope. Source: NASA

[*] Vedran Vrhovac, Undergraduate study of Geodesy and Geoinformatics, Faculty of Geodesy – University of Zagreb, Croatia, e-mail: [email protected] Figure 2. Compared dimensions of Pluto, Sedna, Quaoar, Earth and Moon. Source: NASA

in astronomy community. Some astronomers consi- dered Eris as 10th planet but other gruop felt that Pluto and Eris didn’t qua- lify to be planets. The revision of definiti- on of planet was needed and regular conference of IAU1 in Prague was oportunity to solve this problem. V. Vrhovac: Pluto and definition of planet - Ekscentar 2007, No.9 3. Conference in Prague dwarf planets. According to these de- ly know that his diameter is 2390 km finition, objects in Solar System where and mass 400 times less than Earth’s. Last year (14th to 26th August divided in three groups: Pluto’s orbit is very elliptical and incli- 2006.) conference of IAU was held in 1. Planet is a celestial body that ned. When he is closest to the Sun, Prague. Definition of planets and cri- • is in orbit around the Sun Pluto is 39.5 AU (one AU has 149.6 mil teria to divide them from other bodies • has sufficient mass for its self-gravity km) form Sun and when farthest he is in Solar System were one of the thin- to overcome rigid body forces so that 49.3 AU from Sun. Consequences of gs that where debated on conference. it assumes a hydrostatic equilibrium this are large variation of average tem- First draft proposal of definition was (nearly round) shape perature on surface, from -240°C to - announced on August 16th. Accor- • has cleared the neighbourhood aro- 218°C. When going away from Sun, ding to the draft proposal, to existing und its orbit. Pluto’s atmosphere, composed from nine planets would be added Ceres, 2. Dwarf planet is a celestial body nitrogen, gets frozen. Pluto’s period of Charon and Eris. Later, the number of that rotation is 6 days and 9 hours and his planets would grow since there where • is in orbit around the Sun axial tilt is 119.6°. Consequences of already known bodies that fullify cri- • has sufficient mass for its self-gravity this are that some regions experience teria. to overcome rigid body forces so that long periods of exposure to Sun radi- Main criteria of this proposed defi- it assumes a hydrostatic equilibrium ation and heat. Pluto has three satelli- nition where: (nearly round) shape tes. Largest satellite is Charon, with 1. Planet is body that circles around • has not cleared the neighbourhood diameter of 1212 km and orbits close star and it can not be star itself around its orbit, and to the Pluto (on distance of 19570 km). 2. Planet must have enough strong • is not a satellite. Effect of such orbits is synchronized force of the gravity to be shaped 3. All other objects, except satelli- orbiting of Charon with Pluto’s rotati- approximately like sphere. tes, orbiting the Sun shall be referred on. Due to this phenomenon, observer All bodies larger than 800 km and to collectively as “Small Solar System on Pluto would see Charon standing with mass greater than 5x1020 kg fu- Bodies”. These currently include most still on sky. From other parts of Pluto, llify this criteria. Draft proposal also of the Solar System asteroids, most Charon isn’t visible. Charon’s compo- included a new class of objects in So- Trans-Neptunian Objects (TNOs), co- sition is similar to Pluto’s, it’s compo- lar System – Plutinos. Objects with mets, and other small bodies. sed of rocks and ice but his surface is orbital period greater than 200 years, reddish. Other two satellites have mi- large inclinations and great eccentrici- 4. Physical properties of Pluto nor dimensions, around 150km. They ties of orbits would carry that name. were discovered just recently, ina Draft definition didn’t pass voting and Pluto is mainly composed from rocks 2005. using Hubble Space Telesco- one week later (August 24th) new and and ice. His composition is the reason pe. They bear names Nix and Hydra. final definition has been announced. why surface of Pluto is very reflective Pluto’s surface is probably very simi- New definition demoted Pluto’s status which caused many wrong estimates lar to Triton, Neptun’s largest satellite. and introduced new class of objects – of his size in past. Today, we certain- Triton is bigger and more massive but

[1] IAU stands for International Astronomical Union equaly cold. Geysers that eject nitro- Twotinos. Orbits of these objects can to or other Kuiper Belt objects in spite gen in rare atmosphere are interesting be inclined to 50° which explains why of that that for spacecrafts have already phenomenon on Triton. Surface of they where not earlier discovered. Eris traveled through Kuiper Belt. In Janu- Triton is very reflective cause of the is not Kuiper Belt object since she is ary 2006. NASA has launched New V. Vrhovac: Pluto and definition of planet - Ekscentar 2007, No.9 geysers. Its believed thah Triton, just too far. On distances of 50 AU there is Horizon spacecraft. New Horizon will like Pluto, was circling around Sun un- sudden drop in number of Kuiper Belt fly by Pluto in July 2015. and continue til it was captured by Neptune. objects. This drop is known as Kuiper his journey to interstellar space. There Cliff. There are theories that object are possibilities that new bodies will be 5. Kuiper Belt with mass between Earth and Mars is discovered so that spacecraft can explo- orbiting on those distances. His gra- re than after she completes her task at Kuiper Belt is space beyond Nep- vitational influence would clear those Pluto. In mean time we can expect dis- tune occupied by numerous small bo- space from other objects. coveries of new objects which slowly dies and dwarf planets, including Pluto. circle in those distant cold regions of Existence of Kuiper Belt was predicted 6. Exploration of Kuiper Belt Solar System. Exploration of those dis- by Gerard Kuiper as source of short tant, cold worlds we will find out more period comets. Objects in Kuiper Belt Exploring and determing proper- about genesis of Solar System and once are remains from most early days of ties of such distant and small objects again convince ourself in hospitability Solar System, when planetary genesis is very challenging task, even for most and preciousness of planet Earth. was still lasted. By exploring those pla- powerfull Earth based and orbiting te- nets we will find out more about con- lescopes. Most detailed informations References ditions in early Solar System. of Pluto gathered Hubble Space Te- Today, more than 800 objects lescope in conjuction with moder te- • http://en.wikipedia.org/wiki/Pluto - where discovered in Kuiper Belt. Most lescopes on Earth’s surface. Images of slobodna enciklopedija, ožujak 2007. of those objects occupy space on dis- surfaces of those distant object we will • http://en.wikipedia.org/wiki/Kuiper_ tances between 39 AU and 50 AU from have to wait until new generation of belt - slobodna enciklopedija, ožujak Sun. Neptune, as most massive body telescope becomes functional with lar- 2007. on those distance, has significant influ- ger objectives and better technologies. • http://www.iau.org/ ence on their orbits. Because of that, For now astronomers determine pro- iau0602.423.0.html - web stranice IAU, bodies in Kuiper Belt are divided by perties of those objects using IR part izjava za tisak, kolovoz 2006. orbital resonance with Neptune. Bodi- of spectrum. From IR data they can • http://www.iau.org/ es with resonance of 1:1 are Neptune’s estimate size and composition. iau0601.424.0.html - web stranice IAU, Trojans, 2:3 are Plutinos and 1:2 are No spacecraft has ever visited Plu- izjava za tisak, kolovoz 2006. • Nature Publishing Group, 2001, »Encyclopedia of Astrono- my and Astrophysics«, Hampshire, UK Cambridge University Press, 2002, »The Story of the Solar System«, Cambridge, UK

Figure 3. Triton, Neptune’s larger satelite. Sourcer: NASA