- Why is Pluto not a planet?<\/strong><\/em><\/a><\/li>\n<\/ul>
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>An illustration showing Jupiter and its four Galilean moons, from left to right Io, Europa, Ganymede, Callisto. Moreover, it has an orbit which is much more eccentric and inclined than those of the other planets.<\/p>\n
It did not even look like a planet. So ought it to be classed as a maverick asteroid<\/a>?<\/p>\n
There was something of a parallel with the discovery of Ceres<\/a>, on 1 January 1801.<\/p>\n
Ceres was promptly hailed as a new planet, despite its smallness.<\/p>\n
Doubts arose when Pallas, Juno and Vesta were found between 1802 and 1807; the four became known collectively as asteroids, planetoids or minor planets.<\/p>\n
But after 1846, discoveries became fast and furious. The asteroids, even Ceres, were dismissed as nothing more than cosmic debris.<\/p>\n
Now, we have the same situation with regard to the Kuiper Belt<\/a>.<\/p>\n
Pluto is the brightest member of the swarm of Kuiper Belt objects (KBOs), but it is not the largest, and others no doubt lurk in those far reaches of the Solar System.<\/p>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Dwarf planet Ceres, as seen by NASA\u2019s Dawn mission. Credit: NASA\/JPL-Caltech<\/span><\/figcaption><\/div>\n When I had to leave the IAU commission, Pluto had officially clung onto its planetary status, but I think we all knew that there was a certain degree of sentiment about this.<\/p>\n
Certainly I felt it, as an old friend of Clyde Tombaugh<\/a> and junior co-author of his book Out Of The Darkness.<\/em><\/p>\n
I am afraid that the time for sentiment has passed. If Pluto is a planet, then so are Varuna, Quaoar, Sedna and the rest.<\/p>\n
We have to come to a decision; the longer we leave it the more difficult it will be.<\/p>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Once the 9th planet in the Solar System, Pluto is now classified as a Kuiper Belt Object. Credit: NASA\/JHUAPL\/SwRI<\/span><\/figcaption><\/div>\n Pluto: planet or no?<\/strong><\/h2>\n
So let us look at the possibilities. First we have to decide whether Pluto really is exceptional, or whether it is simply a normal, if over-large, KBO.<\/p>\n
It has a satellite, Charon, more than half its diameter, and two tiny attendants have been discovered; but other KBOs have satellites.<\/p>\n
Pluto is the brightest of the KBOs, but not the largest. It has an extensive \u2013 if tenuous \u2013 atmosphere, but the same may well be true of other KBOs.<\/p>\n
There does not seem to be anything really remarkable about its spectrum.<\/p>\n
So it really begins to look as though Pluto is merely the senior KBO, just as Ceres is the senior asteroid.<\/p>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Pluto\u2019s moon Charon. Credit: NASA\/JPL-Caltech<\/span><\/figcaption><\/div>\n Incidentally, there is another point here. The asteroids (at least those of the main Asteroid Belt<\/a>) are sandwiched in between the orbits of Jupiter and Mars, and though it is true that Mars is vastly smaller than Jupiter, it is still much larger and more massive than any asteroid.<\/p>\n
In fact, all the Main Belt asteroids combined would not add up to Mars. It is conceivable that KBOs are sandwiched in between Neptune and another giant planet much further out.<\/p>\n
Before the discovery of the Kuiper Belt I was convinced that Lowell\u2019s Planet X was real.<\/p>\n
I am not so sure now, but I am still not a total sceptic.<\/p>\n
I was excited when one of the Pioneer spacecraft was not moving quite as expected. Could it be being perturbed by Planet X?<\/p>\n
Alas, the other Pioneer, on the opposite side of the Solar System also showed perturbations, and another attractive idea had to be jettisoned.<\/p>\n
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