## 22 December 2014

### All the planets would fit between the earth and the moon

And what a view that would be.  At low tide.
I pulled my numbers from NASA’s Solar System Fact Sheets, and they’re a little different from the original infographic, but close enough that the comparison is still valid.

 Planet Average Diameter (km) Mercury 4,879 Venus 12,104 Mars 6,771 Jupiter 139,822 Saturn 116,464 Uranus 50,724 Neptune 49,244 Total 380,008

The average distance from the Earth to the Moon is 384,400 km.
Image and text from Universe Today, via reader Adrian Morgan's The Outer Hoard.

The question I have (for any budding planetary scientists out there) is how is it possible to measure the diameter of a gas giant planet like Jupiter?  I understand all the data in the table are expressed as "average diameter" because even the Earth is not round, but the Earth's diameter is measured on a solid.  How can data be obtained on a gas giant, where the gases would (presumably) gradually thin out as one gets further from the center.  It seems ridiculously presumptuous to express such data to a precision of 1 km.

That aside, the concept of all the planets fitting between us and our moon is still mind-boggling.

1. Gas giants are measured based on the altitude where the pressure from the gas equals 1 Earth atmosphere (1 atm = the atmospheric pressure at sea level at 0 degrees C), or 101.325 kPa.

2. Jupiter (for example) is not merely a solid surrounded by gas--its core (which is hardly a regular solid, and we know little about it) is surrounded by supercritical fluid, which is not the same thing as gas. There is no point at which it clearly transitions from gas to liquid, making the surface of gas giants more like a thermally complex ocean.

More here:

http://physics.stackexchange.com/questions/77674/how-does-one-calculate-where-the-surface-of-a-gas-giant-would-be

1. Or, as I put it in a song a few years ago:

The greatest of the planets are made of gas
You could drop the earth in like throwing out trash
We’d sink through the clouds then far below
There’s a dense kind of fluid like nothing you know.

Not relevant to determining a gas giant's diameter, though.

3. Every time I see this illustration I like to remind myself that the diameter of the sun is 1,391,684 km...

In other words, the whole conga line of planets can fit inside the sun over 3 and a half times.

The universe is big...

1. I agree. According to this book I have...

"Space," it says, "is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space, listen..."

4. I'm not sure about gas giants, but I recall from my astrophysics course that the "surface" (maybe diameter is a better word) of stars is typically measured by the photosphere. That is the region that would just barely allow you to see a laser shined from behind the sun toward the earth.

I wouldn't be surprised if a similar method is used for gas giants although I cannot easily verify this. Wikipedia offers an answer that agrees with Roy above. The rather oddly phrased "the base of its atmosphere is usually considered to be ..." which isn't exactly the same thing as diameter (although the error is likely exceedingly small).

You are exactly right to say that there is only a gradual diminishing and not a clear edge. There are those who argue that the diameter of the sun should be considered to extend all the way out to the heliopause. And while common sense seems to argue against them I cannot say that they are exactly wrong.