planet

All posts tagged planet

Credit SiBr4 – CC-BY-SA


We live on a rocky ball about 12,750 kilometers through and 40,050 kilometers around its widest diameter. It has a mass of about six quadrillion megatonnes, which is so ridiculously large compared to things we’re used to that it doesn’t really mean anything to us. Our size compared to Earth’s size is roughly equivalent to the size of viruses compared to us.

That is not to imply that humans are like germs living on Earth. It just provides some perspective on where we are. To go a little further, the next level at the same ratio compares the size of the planet Earth to the size of the whole Solar System. Give or take a few billion kilometers. It’s not an analogy that can stretch forever. (Here’s a link to a video that goes from the very small to the very large – 10 minutes)

And it doesn’t imply that humans are merely an unimportant example of a repeating theme. After all, why is our size one of the levels? We could just as easily be included with all of life to fall between the very small and the very large. As in – subatomic particles – living things – planets and stars. But that’s another analogy that shouldn’t be pushed too hard.

We choose humans as a level because we’re human. We tend to look at things relative to what we’re used to, so we don’t think twice about our size being one of the steps on the ladder. Besides, it was necessary to use something familiar as a point of reference in such a wide array of dimensions.

The dimensions of the physical universe are measured by such huge numbers, both hugely big and hugely small, that they don’t convey much meaning on their own. The very large is measured in billions of light years. Light years are trillions of kilometers each. Even a single kilometer is big compared to us. The small is measured in fractions of meters, changing the numbers from positive to negative right about our size. The very smallest things are such small decimals of a meter that we don’t write them out in full, using special mathematical shorthand instead.

In all that vastness, all that range of realities and possibilities, what is most amazing is that part of it is conscious. A narrow band in the middle has produced something that can look out and try to understand the whole thing.

I think we can justify being a little self-centered.

And will the Moon soon be one too? As mentioned in my post Size – Solar System, Pluto lost its planetary designation over ten years ago. In 2006, the International Astronomical Union demoted Pluto from planet status to that of Kuiper belt object. Pluto didn’t meet all of their criteria to be called a planet. While it orbits the Sun without being the moon of another object, and it has enough gravity to form itself into a spherical shape, without having so much that it ignites a fusion reaction like a star — two of their criteria — it hasn’t cleared its orbital zone of most other bodies — their third criterion. There are a lot of other trans-Neptunian bodies out there, especially in the region called the Kuiper Belt. Pluto was relegated to the status of just another Kuiper Belt object (KBO). I agreed with them. Even though their criteria are incomplete and somewhat arbitrary, I think Pluto should be grouped with the other KBOs, rather than with the major planets. Rather than having a nearly circular orbit on or near the plane of the ecliptic, it has a very elliptical orbit canted at 17.16 degrees to the orbits of the planets. I think it should be called a minor (dwarf) planet, like Eris, another trans-Neptunian object, or Ceres, the largest asteroid belt object.  Many people disagree with me, though.  When the International Astronomical Union demoted Pluto, a great howl went up in defence of the little planetoid.  Now there is a movement afoot to change the definition of planet so Pluto can regain its previous status.  If they are successful, then there could be more than a hundred more planets added to the Solar System.

The key change the team is hoping to get approved is that cosmic bodies in our Solar System no longer need to be orbiting the Sun to be considered planets — they say we should be looking at their intrinsic physical properties, not their interactions with stars.

They want each body to be assessed on its own attributes, and not its relationship with other bodies. So what it orbits or how it does so would not come into it. Based on this, the Moon could become a planet, as could many of the moons of other planets. Here is their definition:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

Put simply, anything massive enough to be round, but not as massive as a star. Anything up to and including what were formerly called brown dwarf stars (About 15-75 times as massive as Jupiter. See my post Size – Stars.) This would include a large and growing number of objects in the Solar System, and in other extrasolar systems, as well as any qualifying bodies that are not associated with any star. Those so-called “rogue” planets.

What do you think? Is it worth all that just to get Pluto its planetary status back?

Source: NASA scientists have proposed a new definition of planets, and Pluto could soon be back – ScienceAlert

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