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A larger brother for Pluto?

Distant icy heavenly body is a strong candidate for the tenth planet of the solar system

ROBERT HURT/IPACArtistic representation of Xena and its moon, in the background: heavenly body is the largest object found in the solar system since the discovery of Neptune, in 1846ROBERT HURT/IPAC

A remote and icy body, still without an official name, may change the content of that science lesson in which one learnt that the solar system was made up of nine planets. Now, according to some astrophysicists, the list of worlds that spin around the Sun is ten. The very strong candidate for being a new member of the solar family, nicknamed Xena and technically called 2003 UB313, is very similar to Pluto, made up basically by rock and ice, but is one third bigger and is currently to be found two and a half times more distant from the Sun than the ninth planet, at the beginning of the periphery of our system. For precisely 160 years, since the discovery of Neptune, nothing of the size of 2003 UB313 has been found in the solar system. For the first time in decades, there is in fact a celestial body with real chances of being hoisted to the condition of a planet in our system, a hypothesis that stirs popular imagination and scientific knowledge.

Celestial objects that have already claimed the condition of a planet in our system have not been rare since 1930, when Pluto was discovered, for the time being still recognized as the most distant of the solar worlds. But all the aspirant planets did not fulfill one of the requirements that are usually demanded of the bodies that desire this condition: to be bigger than Pluto. In this test at least, 2003 UB313 has now passed. If there was any doubt about the dimension of the new heavenly body, a study published in this year’s February 2 issue of the British scientific magazine Nature has undone a good deal of the questioning. In the work, astrophysicists from the University of Bonn were the first to confirm Xena’s diameter, around 3,100 kilometers, that is to say, about 35% larger than Pluto. Almost the size of the Moon. “Now it has become difficult to justify the use of the term ‘planet’ for Pluto, but not for 2003 UB313” explains astrophysicist Frank Bertoldi, the main author of the work, reviving the controversy about the status of Xena, which had begun some months ago.

The ratification of the diameter of the new heavenly body, which takes 560 years to do a complete trip around the Sun, was necessary. When the existence of the candidate to tenth solar planet was announced to the public in July 2005, astrophysicist Mike Brown, from the California Institute of Technology, who had the main responsibility for the discovery of the celestial body, stated that the cold and distant object had an advantage over Pluto, but did not show more precise data about its dimensions. In spite of the points in common with Pluto, 2003 UB313 has not yet been officially recognized as a planet.

The last word on its status will be given by the International Astronomical Union, the entity responsible for classifying and giving names to celestial objects, perhaps in August this year, during its general meeting in Prague, Czech Republic. Xena is 14.5 billion kilometers, or 97 astronomical units (AU) away from the Sun. One AU is equivalent to the mean distance that separates the Earth from the Sun, about 150 million kilometers.

More news about objects located on the periphery of the solar system came to light last month. In another article in Nature, this time in the February 23 issue, American scientists proved the presence of two new small moons revolving around Pluto, called temporarily P1 and P2, which came to add themselves to Charon, discovered in 1978, and until now the only known satellite of the ninth solar planet. The possible existence of P1 and P2 had been made public in October 2005, but only now has the first study come out ratifying the discovery. The first moon has between 60 and 165 kilometers in diameter and takes 38 days to go round Pluto once. The second is 20% smaller and needs 25 days to complete its orbit. They are tiny if compared to the 1,200 kilometer diameter of Charon, which boasts half the size of Pluto. Scientists believe that P1 and P2 were formed in the same way as Charon, from bits of Pluto that were ejected after an object collided with the planet. The terrestrial Moon may also have originated in this way.

The novelty came at the right time: in a US$ 650 million mission, NASA, the American space agency, launched on January 19 this year the New Horizons spacecraft, which, if everything goes according to plan, will, in 2015, become the first spacecraft to get a close look at Pluto, an icy world that is to be found 39 times more distant from the Sun than the Earth. Except that instead of having one moon to peep at, it will have three. After gathering data about the ninth solar planet, the probe will also try to observe one or two objects of the so-called Kuiper Belt, a peripheral region of the solar system, located after Neptune, where over a thousand frozen objects, like comets, asteroids, candidate planets and even Pluto have now been found. “We estimate that 20% of the objects of the belt may have satellites” explains astrophysicist Hal Weaver, from Johns Hopkins University, the main discoverer of P1 and P2. Incidentally, the tenth planet candidate, which was nicknamed Xena, also has a moon, so far without an official name, but informally called Gabrielle.

Broadly speaking, the solar system can be divided into three major regions. The first is made up of its innermost portion, which includes the so-called terrestrial planets, medium or small in size (Mercury, Venus, Earth and Mars). These worlds are located closer to the Sun and have a solid surface. The second part encompasses the giant gaseous planets (Jupiter, Saturn, Uranus and Neptune), which are to be found more distant from the mother star and do not show any rocky surfaces. They are basically made up of helium and hydrogen. There is also an asteroid belt between Mars and Jupiter, forming a sort of transition zone between the predominantly solid worlds and the mainly gaseous ones. The third region, extremely vast and little known, comprises everything to be found beyond Neptune, hence over the 30 AUs: small icy bodies, called generically transneptunian objects, such as Pluto and the tenth planet candidate. The Kuiper Belt, between 30 AU and 50 AU, arks the start of this region. The end of it is said to be the Oort Cloud, a theoretical proposal, still without irrefutable corroboration, but much accepted amongst astrophysicists.

Cradle of the comets
As the name indicates, the cloud is said to be a gaseous sphere that delimits the end of the solar system and also the cradle of the long-period comets, which take over 200 years to complete one trip around the Sun. By this proposal, what is inside the Oort Cloud, probably between 50 thousand AU and 100 thousand AU, still belongs to the solar system. What is outside has escaped from the gravitational influence of the Sun. Since 1995, astrophysicists have known that other stars, besides the Sun, may originate planetary systems. Over 150 extra-solar worlds have been discovered in recent years, but none of them has yet proved to be exactly the same as the Earth.

What is the importance of studying Pluto, the probable tenth planet and the other bodies located beyond Neptune? “This region is a sort of archeological site of the solar system” is the comparison by astrophysicist Enos Picazzio, from the University of São Paulo (USP). “For being distant from the Sun, and frozen, their objects are the best preserved ones in our neighborhood.” In other words, the chemical composition of these cold and faraway bodies must be the same today that they showed at the moment of the formation of the solar system, 4.5 billion years ago (the Universe, according to the Big Bang theory, arose long before, from a primordial explosion some 14 billion years ago). The very genesis of the Sun and of its planets can be better understood as human knowledge penetrates into the mysteries of the periphery of our system. There are those who also believe that clues about the origin of water and of organic matter, essentially carbon, which came from space and made life flourish on Earth, can be found in comets, asteroids or planets from the Kuiper Belt.

These are the underlying objectives that orientate the exploration and study of the confines of the solar system. Immediately, there are more prosaic and occasional questions – like deciding what a planet is and how many there are in our system. There is practically a consensus amongst astrophysicists that Pluto never ought to have classified as one. It was an error of judgment that resists the advance of knowledge. After all, Pluto does not look like the terrestrial planets, nor the gaseous ones. Its orbit is very elliptical (elongated) and follows a plane that is 17 degrees steeper than the other solar planets (the orbit of the 2003 UB313 is even stranger, 45 degrees steeper than the Earth’s). Rocks and ice must account for the basic composition of the planet, whose surface temperature must be lower than minus 200 degrees Celsius. In short, Pluto is a world apart.

The problem is that at the time of its discovery, in 1930, there was not yet any notion of the existence of cold transneptunian objects, and a race in search of new planets was holding sway. So, as people wanted to find a planet, and, to start with, it was believed that Pluto was larger than it really is, this body was labeled a planet. “Pluto was found before its time” says Rodney Gomes, from the National Observatory of Rio de Janeiro, who, in a series of three articles published in the May 25 issue of the Nature magazine, attributed the current architecture of the solar system to migrations undergone by the gaseous planets Jupiter and Saturn about 700 million after the birth of the king-star and the set of objects that surround it.

Quaoar and Sedna
Strictly speaking, Pluto was the first object of the Kuiper Belt to be located, although the notion of the true status of this body only became clear decades after its discovery – almost everybody, including the school books, had already incorporated the idea that there were nine planets in the solar system.

It is not easy to discover objects beyond Neptune. Several reasons explain this difficulty: the icy celestial bodies of this zone of the solar system are very small (smaller than the Moon), are very far from us, and hardly reflect the scant light from the Sun that possibly hits them. Getting a good image of these bodies has still not been achieved, as the rather unclear photos of Pluto attest. In spite of the snags, since the beginning of last decade, astrophysicists are managing to locate more and more transneptunian objects, with the assistance of new techniques or equipment.  After Pluto and Charon, the discovery of the first transneptunian object took place only in 1992, when a small asteroid of about 200 kilometers in diameter was found. Since then, over a thousand celestial bodies have been found in the Kuiper Belt. The majority of them are small in size, which makes it easy to classify them as asteroids, or something of the kind. However, some bodies discovered in this decade, like Quaoar and Sedna, show almost the same size as Pluto and went so far as to be considered as candidate planets. But, as they were smaller than Pluto, this kind of discussion did not prosper for very long. It is not going to be possible to do the same with 2003 UB313, which is a sort of augmented Pluto.

Bodies that move
In the course of time, the term “planet” – today, usually understood as a celestial body of spherical format, without any light of its own, and which rotates around a star – has changed slightly in meaning and acquired new nuances. Greek in origin, the word was used originally in Antiquity to discriminate in the sky the bodies that moved, as opposed to those that seemed fixed, the stars and constellations. It was a very generic definition, which fitted various types of celestial objects. “The first asteroids, like Ceres and Pallas, were even called planets, but afterwards were classified in another way” comments astrophysicist Daniela Lazzaro, from the National Observatory of Rio de Janeiro. It is not by chance that the asteroids – a term that currently encompasses bodies with an irregular format and varied sizes, with the dimensions of a stone or of up to a thousand kilometers in diameter – are informally called minor planets.

In some cases, like the planets properly speaking, the asteroids, almost always seen as rocks traveling alone in space, even have moons orbiting around them. The first satellite of an asteroid, Dactyl orbiting Ida, was found in 1993 by the Galileo probe. That is to say, the dividing line that separates what can be called a planet from what merits another designation is sometimes tenuous and arbitrary. “It is difficult to establish the borderline between what is a planet and what isn’t” says Daniela, who thinks, though, that all this discussion is secondary. “The important thing is to know what an object is like, and not how it should be called.”

Whatever the decision of the International Astronomical Union is about the official status of the larger objects of the Kuiper Belt, like 2003 UB313, the verdict is not going to please everyone. For reasons that are more historical than scientific, it will be difficult for Pluto to lose its condition as a planet. The question is to know whether other icy worlds from the Kuiper Belt, of a similar size to that of the new solar planet, will receive the same treatment. If he could choose an official name for his greatest discovery, Mike Brown would baptize the probable tenth planet as Persephone, the mythological wife of Pluto, the Greek god of the depths. “But an asteroid already has this name” says the astrophysicist from Caltech. “So I think that it is out of the question.”

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