Besides the relative data to this planet that we have picked up of internet, we have centered ourselves more in Pluto's history, particularly in what the astronomers have thought since it was discovered in 1930; they thought it was a bigger planet than the Earth, and now thinks it´s an asteroid or comet of the belt of Kuiper. 

Using some books and articles of magazines has obtained the information. As it is a recent history, we have revived her almost in direct looking for in the magazines. Certainly the observation of Plutòn is outside of our reach, but we have built a model to illustrate the unique particularities of the system Pluto-Caronte and its relative movements. Finally we compare it with Newt, a heavenly body of similar physical characteristics, and we find the differences with the system Earth-moon.

Pluto has been the last planet in being discovered, concretely February 18 1930 for Clyde W. Tombaugh. The path that led to its discovery is attributed to Percival Lowell who founded the Observatory Lowell in Flagstaff, Arizona and it sponsored three separated searches of an unknown planet that explained the anomalies in the orbit of Uranus didn´t explained by Neptune. Lowell died without finding it and it was Clyde Tombaugh who carrying out pictures of the plane of the solar system (ecliptic) with a separation of one to two weeks looked for something that moved among the stars. This systematic process obtained its fruits and Pluto was discovered by this young laboratory assisting, 24 years old, on February 18 1930. In that moment it was thought that it would be bigger than the Earth and it was considered the ninth planet of the solar system giving him the name of the Roman god of the underground world. The current knowledge demonstrate that it is too small to be the "Planet X" that Percival Lowell had hoped to find. Pluto was an unsuspected discovery. At the moment it is considered that it is a member more of the belt of Kuiper which is formed by asteroid-comets located beyond the orbit of Neptune. 

Its orbit is something special; it is usually the planet more far from the Sun, but I eat their orbit it is it is quite eccentricy nearer of the Sun than of Neptune, concretely during 20 of the 249 years that it has their orbit. Pluto crossed the orbit of Neptune on the 21 of January of 1979, it made his nearer approach September 5 1989 and it remained inside the orbit of Neptune up to March 14 1999. This won´t happen again until September of 2226.  

As Pluto approaches to its perihelion it reaches his maxim distance from the ecliptic due to its orbital inclination of 17 grades. Therefore, it is very above the plane of the orbit of Neptune. In these conditions Pluto and Neptune cannot collide and they don't  come closer one to another more than 18 UA. It is said that it has a resonance 3:2 with Neptune, what means that for each two turns that it gives around the Sun, Neptune gives 3. So, the maximum approaches between both happen always in the same places. 

As it is so far, the limited information about the distant planet of which prepared delayed a realistic understanding of its characteristics. Pluto is the only planet that has not been visited by a space ship, soon, it´s planned the launching of the mission Pluto Express whose arrival to Pluto would be in 2006-2008, before that the increasing distance  from the Sun makes that its atmosphere freezes. The mission will consist of a pair of small and quick space ships, relatively cheap ships that weigh less than 100 kilograms. Anyway,lately is growing quantity of information is obtaining on this peculiar planet, as this picture obtained by the space telescope Hubble. 

Pluto's data are: 

Medium distance to Sun: 5913,5 million kilometers 

Maxim distance:  7375 million kilometers 

Minimum distance: 4425 million kilometers 

Duration of the day: 6,38 terrestrial days 

Duration of the year: 248,54 terrestrial years 

Diameter: 2.320 km (0.18 times the terrestrial one) 

Mass: 0.002 times that of the Earth 

Graveness: 0.4/9.8  

Satellites: Caronte 

It seems that Pluto has a highly reflective south pole, a more delicate polar cap in the north, and brilliant and dark areas in the equatorial region. Pluto's geometric albedo varies among 0.49 and 0.66 and it is more brilliant that Caronte whose albedo varies between 0.36 and 0.39.   The diameter of Pluto has been stimated in about 2,274 kilometers (1413 miles) and that of Caronte in 1,172 kilometers (728 miles). The half separation between both is of 19,640 kilometers (12,200 miles), more or less eight times Pluto's diameter.  

The half separation and the orbital period are used to calculate Pluto's masses and Caronte. Pluto's mass is about 6.4 x 10-9 solar masses. This is almost 7 times the mass of Caronte and approximately 0.0021 masses of the Earth, or the fifth part of our moon.  

Pluto's half density is of 2.04 grams for cubic centimeter. So, Pluto is between 50% and 75% rocks mixed with ice. The density of Caronte is of 1.2 g/cm3, that indicates that it contains few rocks. The difference of density tells us that Pluto and Caronte were created in an independent way, although the data of Caronte obtained from the images of the Hubble are contrasting with those derived of the terrestrial observations. Pluto's origin and Caronte still remain in the kingdom of the theory.  

Pluto's icy surface contains 98 nitrogen% (N2). methane (CH4) and present appearances of monoxide of carbon are also (CO). The presence of solid methane indicates that Pluto's temperature is inferior to the 70 grades Kelvin. The temperature varies very much during the course of its orbit because Pluto can come closer in the sun until the 30 UA and to go away until the 50 UA. It exists a fine atmosphere that freezes  and it falls on the surface of the planet as it moves away from the Sun.. The atmospheric pressure deduced for Pluto's surface is 100.000 times smaller than the pressure in the surface of the Earth.  

The period of rotation of Pluto is 6.387 days, the same as that of its satellite Caronte. Although it is very frequent that a satellite travels around its primary one following a synchronous orbit. Pluto is the only planet that rotates synchronously with the orbit of his satellite. Due to this tide effect, Pluto and Caronte always present the same face one to another during their trip through the space, and from a point of the surface of one of them will be seen always to the other one in the same place. From one Pluto's hemisphere you always leave Caronte, and from the other one never. 

On the contrary of the most of the planets, but equally as Uranus, Pluto rotates with the poles almost in his orbital plane. Pluto's axis rotacional is inclined 122 grades. When Pluto was discovered for the first time, his relatively brilliant south pole was the first thing that was seen from the Earth. Pluto seemed to fade as our point of view changed from the polar view in 1954 until an equatorial view in 1973. Pluto's equator is what one can see now from the Earth..  

Certainly we find that the beginning of the story very curious. An astronomer that wastes their life and illusions looking obsessively for a new planet that found never, another that continues working after the death of the first one, and receives discoverer's honors from something that hadn´t any relation with what was looking for, and the science that the more goes knowing about this heavenly body goes him considering.  

The most curious thing is as the size that has been given to Pluto it has been more and more smaller, and imprecise measures have been used. In two guides of the sky that we have in the institute, both published in 1982 appear like diameter of Pluto 14000 and 4000 km, although in both cases with query.  At the beginning it was thought that it would be bigger than the Earth (if it was really who affected the orbit of Uranus) but in 1950 Kuiper was able to measure the small disk and obtained a value of 5800 km. (less than Mars), but they didn't believe him. In 1965 the occultation of a star that didn't cover all and they deduced that indeed it was smaller than Mars. In 1978 it was discovered a satellite (Caronte) and with the movements of both their combined mass was deduced. Between 1985 and 1990 happened mutual eclipses that allow to know the exact sizes (2300 and 1186 km. Of diameter respect.) and to differentiate the spectra to obtain the composition of the two heavenly bodies. In 1992 observating with the telescope Hubble the movements around the center of masses the densities were obtained (2.04 and 1.2 g/cm3) 

With these characteristics many people think that it is not a planet (for the size, the density and the composition), but a member of the call belt of Kuiper, formed by comets or icy asteroids far from the Sun than from Neptune, and that it was predicted by this astronomer almost fifty years ago. There is a very curious fact that we have found on this. In the number of Julio-August of 1992 in a magazine of astronomy appears a titled article  "Where is the belt of Kuiper? " where it explains that some component of the belt should already have been found, and at the end it says that it is possible that simply it doesn't exist. One month later, in September of 1992 it´s found the first of these heavenly bodies: 1992 QB1, or rather the second (or third or fourth) if it is counted Pluto (and Caronte, and Tritón). But 40 years waiting, and when it is already doubted,  the first one appears. Although Pluto is much bigger that the other ones, at the end of last year one was found, with a diameter of more than 1000 km. 

But will there be more planets?  Pluto was  when they were looking for what altered the orbit of Uranus and Neptune didn't explain to it, and it is clear that it is not it. However some few years ago a news appeared,that they had made the calculations supported by meticulous mensurations of Voyager II again, and the interferences of Uranus were only due to Neptune. The calculations of Lowel were wrong. 

In all ways we have found many strange theories that relate Pluto and the satellites of Neptune.  One says that Pluto could be in another time a satellite of Neptune. Another that Pluto passed near Neptune and it took Caronte that before was a satellite of Neptune, and another that Triton was an independent heavenly bodyr of the belt of Kuiper  as Pluto, and it was captured by Neptune. 

As Pluto and Caronte they have such a special movement, we have made a scale model with two porexpan balls and a motor in that these peculiarities appear: 


- It is everything to the same scale: sizes, distances and situation of the center of masses..  

- Rotation of both around the center of masses that is outside of the planet (it is the only case of the whole Solar System) 

- Synchronous rotation of both (it is also the only case), and that it makes very easy the construction joined with a simple wire. 

- The plane of turn of Caronte and Pluto's rotation is almost perpendicular to the ecliptic. So that we placed it to it vertical 

- Reddish colour of Pluto and gray of Caronte and darker area in the equator of both. 

A heavenly body to compare similar to Pluto we have chosen Triton (satellite of Neptune) as Pluto it could have been a member of the belt of  Kuiper captured. Their characteristics are very similar to Pluto: 2700 km. of diameter, density 2,08 and reddish colour; surface of methane ice and a delicate atmosphere of nitrogene and methane. 

As different heavenly body, we chose our own planet: although the systems Earth - Moon and Pluto-Caronte they have similarities because they are the only planets with a single satellite, and in both cases the relationship between the size of the planet and the satellite is the most similar in the whole solar system, and they can be considered double systems. However the differences are very large. The Earth is much bigger, the density is much bigger, due to the distance from the sun the temperatures in Pluto are very much lower, synchronous rotation in Pluto and not in the Earth. Seen from the Earth the Sun and the Moon they have the same apparent size. Although the Moon in our sky already highlights enough, calculating what Caronte would occupy in Pluto's sky, it leaves much more: 3.6ş, that is to say more than 7 times more than our Moon. However from there the Sun would be seen only like a brilliant star because it would not get to appreciate the disk of less than a minute of arch. 

But we have just found something in the magazine of this month of June that could make us change the similar star to Pluto. It is that between more of 500 stars that have been found in the Belt of Kuiper, 7 are binary as Pluto and Caronte, and perhaps the most curious is 1998WW31 because their orbits around the center of masses are elliptic so that they can come closer to 4000 km one of other, or to go away up to 40.000 km 

With all this story of going discovering things about Pluto we have got excited because we thought that every thing was already known about the Solar System, and we have seen that right now they continue being discovered new things, or that all that is discovered is not always correct, or it is not accepted or known by all in the moment. We have used the magazines from 1986 that we have in the institute:  Tribune of Astronomy, Cosmos and Universe.  

To finish a paragraph of a very complete article published in 1995, and of the one that we have taken out some things.  "There are several bodies that at the moment populate those freezes regions. Of them Pluto and Triton are the most representative but, possibly, not the only ones and I even dare to risk that they will be discovered more about them in next years although, almost with all security, they will be of smaller size". Everything coincides, but at 1995 they had already been found several! 

And this doesn't finish. Today (October 8) when we are going to send the report we have read in the newspaper that it has just been found the biggest star in the belt of Kuiper: 2002ML60 Quacar, with a diameter of 1300 km. Maybe tomorrow they will find another bigger than Pluto (who takes the news to class, the professor will put him a good mark) 

We find that it cannot have a more motivational exercise to make in class that to be watchful of the news to complete the story of the Solar System.