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Jupiter is a behemoth. It’s the largest planet in the solar system by far — by volume, all the other planets could fit inside it — and very well studied by planetary scientists.

So you’d think we know its size — that is, its diameter — very accurately. However, the best measurements of its width have an uncertainty of about 4 kilometers. That’s actually pretty good considering Jupiter is about 140,000 km across! But models of its interior depend on the measured diameter, and getting it as nailed down as possible is a big deal.

The thing is, getting this number isn’t easy. For one, Jupiter is far away, and measurements made from Earth just have an inherent uncertainty to them. Also, Jupiter is a gas giant, which means what we see isn’t a solid surface, but the top of its atmosphere. That means there’s no easily observed benchmark (like sea level) to use for the measurements. Worse, in some places we see deeper in to its atmosphere, and in others clouds block that view. To get around that, scientists use a somewhat arbitrary benchmark as a baseline — the 1 bar level, where the pressure of the atmosphere is roughly equal to Earth’s pressure at sea level.

How do you measure that? One way is to observe stars as Jupiter passes in front of them from our point of view; as the starlight passes through the atmosphere it gets bent depending on the density of the gas, and this can be used to find the 1 bar level. Scientists did this decades ago when the Pioneer and Voyager probes passed by the planet, observing radio waves from stars (because visible light is easily blocked by gases in Jupiter’s air, so radio waves give a better view deeper down). 

The problem there is that the uncertainty was large, that 4 km number I mentioned earlier. Not bad, but we need better.

And we can do better. In a recent study, scientists used data from the wonderful Juno probe, which has been orbiting the huge world since 2016. They looked at radio wave observations of stars as the spacecraft saw them pass behind Jupiter (an event called an occultation) and, including effects from winds on the gas density, were able to measure the 1-bar diameter of Jupiter, knocking down the uncertainty by an order of magnitude (a factor of ten)! [link to journal paper]

They find Jupiter’s polar diameter is 133,684 km, and the equatorial diameter is 142,976 km, ±0.4 km.

Note that the planet is much thicker through the waist than through the poles^* . That’s mostly due to centrifugal force; Jupiter rotates in 9h 55m, which is the fastest of any planet, and, being the largest, that means the centrifugal force outwards at its equator is pretty big. On the equator you’d feel that counteracting the planet’s strong gravity pulling you down, which in turns means the planet is oblate, non-spherical. It’s flattened by over 6%, which is actually pretty obvious when you look at it through even a small telescope. In fact, the new measurements indicate Jupiter is 24 km smaller through the poles and 8 km smaller through the equator than previously thought, so it’s even more oblate than previously measured.

To account for that, sometimes scientists use an “average” value for its diameter, which, using the new measurements, is 139,772 km. For comparison, Earth is about 12,742 km (note that’s an average too; Earth is also oblate but only by 0.5%, or roughly 22 km). By the way, 11 Earths could fit across Jupiter.

Jupiter is immense. 

And now we know just how immense, with far less uncertainty than before. This will improve our understanding of its interior, how the winds blow, and how gas giants in general behave. Not bad, just by using a better meterstick.

^*  I feel ya, big guy.

My wife is a fantastic cook, and loves to experiment and make meals. I am the extremely fortunate recipient of this gift, and never fail to appreciate it.

She’s also pretty busy during the day, especially this time of year since she gardens, so she looks for ways to make dinner easier and quicker. Not too long ago she stumbled on a brilliant idea: naan pizza.

We always have naan around — though my computer programmer brain always loves to see it as NaN — because it’s fantastic. Naan is an Asian flatbread, generally somewhat oblong in shape, quite soft, and very yummy. Like many great breads it’s very simple and not hard to make, but it’s also widely available in grocery stores (not an ad, but we get ours at Costco and it’s delicious). I like to simply warm it up in the toaster oven and put a small amount of butter on it for a quick snack. 

But naan also makes for a fantastic pizza base. We just put on some tomato sauce (usually a marinara sauce that’s been simmered for a while to reduce the water content, which prevents the naan from getting soggy), cheese, and then whatever toppings we have around. Slap it in the oven on bake for a few minutes at, say, 350° F (175°C) until the cheese melts or the naan starts to brown (I like my crust crispy) and that’s it. You get your own personal-sized handheld pizza, simple and easy.

Also a lot healthier than frozen pizza, which is loaded with salt and other stuff my doctor recoils at when I tell her what I eat. When you make it yourself it’s really no harder than dealing with a frozen pizza, and you can control the salt and other things that are bad for you but make food glorious. I’ll note that it’s likely less expensive than frozen pie, too, depending on which one you get.

Speaking of which, as I write this it’s lunchtime. Hmmmm. I have an idea…

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