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## Five top facts about Jupiter – no. 2

Continuing with my list of the five top facts about Jupiter which BBC Five Live tweeted a few weeks ago, today I am going to talk about the fact I listed as number 2 –

all the other planets in the Solar System together would fit into it [Jupiter].

The tweet from BBC Radio 5 with the five most interesting facts about Jupiter.

The list of the five facts

I think this gets across very clearly just how big Jupiter is compared to the other planets! I should add that this statement does not include Saturn’s rings, which extend a huge distance from Saturn, but if you take just the planets themselves without any rings (Saturn, Jupiter, Uranus and Neptune all have ring systems) they would all fit comfortably together into Jupiter.

In terms of the Earth, the Earth would fit across the diameter of Jupiter about 13 times, which means that it would fit into it $13^{3} \approx 2200 \text{ times }$!! The great red spot, which we believe is a giant storm system, could swallow up the Earth three times. Astronomers have been seeing the great red spot ever since telescopes were good enough to see it, which is about the middle of the 1600s. Although it has changed a little in size and appearance over these 400-odd years, it has remained a prominent feature of Jupiter’s cloud tops.

The comparative sizes of Earth and Jupiter. Earth would fit into Jupiter over 2000 times!

Comparing Jupiter to the next largest planet Saturn, we can say that Saturn would fit into Jupiter nearly twice (1.73 times to be more precise). This is, of course, not including Saturn’s large ring system. In terms of mass, Jupiter’s mass is 318 times the mass of the Earth, whereas Saturn’s is 95 times. Both are much less dense than the Earth, because they are mainly composed of gaseous hydrogen and helium.

When we look at Jupiter, we are seeing the cloud tops. If there is a solid surface we have never seen it, but we do infer its presence from our understanding of planetary formation. A solid, rock-like core probably exists and it may be similar in size to the Earth, but $99.9 \%$ or so of the volume of Jupiter is mainly hydrogen and helium gas, or hydrogen and helium under high pressure in some kind of liquid metallic state. The famous bands of Jupiter are caused by gases of different temperatures. The light-coloured bands (actually called “zones”) are regions where the gases are warmer are rising, and the dark coloured bands are where the gases are cooler and are falling.

Before I finish this particular blog, I will say a brief word about exoplanets (planets around other stars). I have blogged about this topic several times before, but with regard to Jupiter’s size and mass, most of the early exoplanets found were, in fact, larger and more massive than Jupiter. This is because our early detection techniques were not sensitive to find less massive exoplanets, but this has changed a lot with the advent of the Kepler Space Observatory. This interesting graph below shows the masses of exoplanet discoveries as a function of year. Note, the y-axis (vertical axis) is in terms of mass of Jupiter, so 10^0 means the same mass as Jupiter, 10^1 means ten times more massive, 10^-1 means one tenth etc. As you can see as we come closer to the present we see that smaller and smaller mass exoplanets are being discovered, and we have even found several now which are similar in mass and size to the Earth.

A graph showing the masses of exoplanet discoveries compared to the mass of Jupiter. The y-axis is logarithmic, so 10^0 means the same mass as Jupiter, 10^1 means ten times more massive, and 10^-1 means a tenth of Jupiter’s mass, 10^-2 means one hundredth, etc.

Next week I will talk more about Jupiter’s rotation and how long it takes to orbit the Sun.