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A definition of planet must be universal August 28, 2006

Posted by Jorge Candeias in Definition of planet.
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After writing a couple of pages where I explain the purpose of this blog and the definitions of planet that will be used in it (the one the IAU proposes and the one I’m using), linked from the header (go check them out) here’s the first post proper. It’s more an ideological post than a scientific one in the sense that I think that a true definition for what a planet is has to be universal. There’s not really too much science in the reasons for my thinking so and that’s why I’m saying it’s ideological. You could think otherwise and your thoughts would be just as meritable. Still, I hope to persuade you I’m right.

So, why do I think that a definition for planet must be universal? And what does it mean “universal”?

Universal means that it must be adaptable to and usable in any place in the Universe. That’s a common thing in science: when things are defined, they are usually defined for the whole wide wilderness out there. A prime number isn’t one thing here and something else an Alpha Centauri; An orbit as an ellipse here and in M31, the Andromeda Galaxy; A fatty acid is composed of the same atoms in your body and in the Orion Cloud Complex; A star is a star if it shines above your head in a sunny day or in some globular cluster in the galactic halo or in one of the Magellanic Clouds; The quarks in the nail of your left thumb are just like the ones produced or released in the Big Bang.

The only situation where we admit the possibility that something we know here may not be defined in such a way that it can be applicable elsewhere is when we don’t know any other example of it out there. Such is the case of life. We’ve only met life in our planet, and therefore our hands and feet are tied: we must define it within the limits, that may be quite narrow, of what we know on Earth. But once our understanding expands, assuming it ever will, we’ll probably gain a new insight of what life is and will have to adapt our definitions accordingly.

Now, we had a similar problem with the planets until some 15 years ago: we assumed that there must be tons of them out there, it was even a given in science fiction, but we really didn’t know because we hadn’t detected a single one. If this issue had arisen back then, we would have to look only to what we have here in our cosmic backyard in search for information and some orientation.

That, however, has now changed. The same technologies that allowed us to detect the planets in the outer system that led to the need for a redefinition of what a planet is gave us also the information that other planetary systems exist around other stars as well (well, some of the same technologies, at least). And that’s why I believe that any definition of planet has now to take into account not only the populations of planetary objects that orbit our sun, but also all the planets found around other stars… and other objects.

Here lies the first, but huge, flaw in the “IAU planets”: the definition adopted in Prague is limited to the Solar System and to the Solar System alone and cannot be applied to any other system, not only because the word “Sun” is explicitly stated in the definition, but also due to its very nature. That, alone, is more than enough to reject that definition as far as I’m concerned.

Convinced? Not yet? Then wait until I add more stuff to the blog. I strongly recommend the RSS feed, in case you’re interested. See ya.

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