Why are the bigger planets made mostly of gas with just a small solid core?
This Solar System formed out of a vast cloud of gases (primarily hydrogen) and solid materials mixed with ices.The Sun had gobbled up much of the gases and material in the Solar System and then ignited and blasted the remaining gases far out into the Outer Solar System. To where the gas giants were forming.Because the area in which these planets formed still had a tremendous amount of gaseous material in their surrounding areas, the planets forming in the Outer Solar System had greater and longer access to this material. Moreover, the orbits of the four outer gas giants continued to move around and change in the outer part of the Solar system. Allowing them to scoop up much more of this material.Jupiter was well on its way to becoming a star when the sun ignited first and then deprived Jupiter of enough material to also become a star.The inner Solar System where Mercury, Venus, Earth, Mars and the Asteroid belts formed contained mostly rocky materials and water ice left over after the sun ignited. However, there was just enough of the necessary material still there to accrete and form the planets of the inner Solar System. And these planets were also built up by continuous collisions with other rocky bodies.One basically has to picture the early Solar System as a pinball machine. One where the farther out a planet was from the Sun, the larger it could potentially become. And because there was still so much gaseous material further out for the planets to accrete to huge sizes and consist mostly of gaseous material.
Do planets get larger or smaller or remain constant as they get older?
During the first several million years of their formations planets undergo rapid growth (and heating) due to accretion. Then, once almost all the dust and gas of the early Solar System has been cleared, the planets undergo very gradual shrinking— both from solar winds ripping off gas from the upper atmosphere, and from planetary cooling due to thermal radiation (until reaching thermal equilibrium).Image: Small graben, or narrow linear troughs, have been found associated with small fault scarps (lower white arrows) on Mercury. They suggest that Mercury is contracting even today. [From The Incredible Shrinking Mercury is Active After All]
Why aren't all planets made of basically the same material?
They are, for the most part. They all have iron cores, surrounded by rocky material. The gas giants were far enough away from the Sun to not have most of their heavy atmospheres blown off by the solar wind (or the inner planets never formed heavy atmospheres like Jupiter and Saturn, but for the same reason of the solar wind). Pluto and the dwarf planets in the Kuiper Belt are rocky and icy; they're far enough away for water ice to stay frozen without dissipation. Plus, the heavy element debris that triggered the Solar System's initial collapse would've been strewn throughout the hydrogen cloud rather haphazardly. Perhaps more iron collected in some areas, perhaps more silicates in others. Mercury was too small and had too little a magnetic field to hold on to any atmosphere. Mars held on to a thin one, while Venus held on to a thick one. The inner cores of all the gas giants are probably balls of iron several times Earth's mass, with some nickel and silicates thrown in, like the terrestrial planets are made of.
In terms of planets, are gas giants literally just giant balls of gas?
On a very basic level, yes, gas giants are “giant balls of gas” i.e. Hydrogen and Helium (mostly)Though, this description makes them sound a lot less complex or even dangerous as they are for humans. It makes them seem like they might be like an entire planet that is like our atmosphere, maybe with some nice clouds…but no. Maybe when talking about the part of the planet that is nearest to space, like near the cloud tops, it might be somewhat like this, but dive into the planet some more and you’ll find howling winds and high temperatures, lots of lightning for some of them and for others merely crushing pressures. The calmest of them all is probably Uranus, but even there go far enough down into this ball of gas and you’ll surely be crushed or destroyed via heat, whichever comes first.So yes, while in a very basic way, they are giant balls of gas, that also isn’t exactly the best description.And if you wanted even more of a headache, I could tell you about all the impurities that add up to huge masses or the fact inside some gas giant the hydrogen is crushed into itself so forcefully it acts like a metal would even though hydrogen obviously isn’t one, or that there seems to normally be a small “rocky” core that is under even more pressure to the point it probably couldn’t been be called rock…need I continue?Gas giants are pretty complex, but yes, when you boil it all down (pun…I have no idea?) gas giants as mostly giant balls of mostly gas that usually acts weird, mostly.Hope I could help :-)
Why do planets start smaller (Mercury), and get bigger (Jupiter) and then again get smaller?
The rocky inner planets and the gas giants are divided by something called the Frost Line. This is the distance from our Sun beyond which volatile compounds like water, methane, ammonia, CO2 etc. can condense and form small grains.Let’s start with the inner rocky planets. The predominant building blocks in the inner part of the solar system were the solid silicate compounds along with iron and nickel. The volatile compounds that I mentioned above were not suitable enough to form planets primarily because they were gases that close to the Sun. Thus, the first four planets turned out rocky and denser. They are very small compared to the giants because these heavier elements and compounds were not available in large amounts.Beyond the Frost Line, the basic matter available to form planets (and in a large quantity since it wasn’t used in the inner planets) was hydrogen, helium and these volatile compounds which as I said could condense into small grains so far away from the Sun. The accretion of these led to the formation of the gas giants. They were larger because hydrogen and helium are extremely abundant in the solar system. Jupiter is the largest probably because it was the first to form and by the time Saturn came around, there was not enough gas available.So, that answers the question to why the planets get larger and partly why they get smaller as the gas to form them became scarce over time. There is another reason why the last two planets in the system, Uranus and Neptune, are smaller. Their atmosphere is similar in composition to Jupiter and Saturn since they all contain hydrogen and helium predominantly. But Uranus and Neptune also have a higher proportion of condensed water, methane and ammonia. That is why they are denser and smaller than the former two gas giants and are sometimes called “ice giants” to distinguish them a little.Hope this answers your question! Keep asking!
What planet is made of entirely gas?
Likely there are no planets anywhere made entirely of gas. Some dust and water droplets are likely in the upper atmosphere and some vapor. It depends somewhat on the definitions of gas and planet. How about 1/2 gas and 1/2 other stuff by weight? Sometimes we call plasma and vapor gas, but I prefer to think of them as not gas. Neptune and Uranus are likely 99% plasma, gas and vapor by weight. You can not fly though them, except the outer edge as the density is too high a few hundred kilometers below the cloud tops, and not much lower they are too hot. Likely there are smaller cousins of Neptune somewhere, but my guess is 1/4 the diameter is not possible without a mini black hole or cool core of liquid or solid. Likely the universe is not old enough to have cool cores in bodies with more mass than Mars. I'll guess 500 c = 832 F for the center of Mars and center of Earth's moon. Has any one seen any estimates? Neil
Can a gas giant form theoretically entirely out of gases, with no rocky core?
If you are building a planet in an environment that contains only gaseous components (like hydrogen and helium) then, sure. This would only happen in very early solar systems.Solar systems like ours, that are forming out of material enriched with heavy elements from the supernovae of christmas past, will create planets that contain solid rocky material , even if it is in very small amounts.
Can planet earth transform into a gas giant? If so, how?
Planets form from the same material stars do, a cloud of gas (hydrogen and helium) and tiny solid grains of rock, metal and ice, and at the same time. When astronomers look at newborn stars, we often see discs that could collect to form planets around them. As we look at older and older stars, the discs go away — or collect into much harder to see planets. (Think about which is more noticeable — your laundry strewn all over your house, or sitting in the laundry basket in one place.)Astronomers think gas giants form first, and get big enough to grab onto the abundant hydrogen and helium gas that is still hanging around our newborn Sun. As the Sun shines, the gas is heated, and escapes into the space between stars. So planets that for slower or never get very big, like Earth, can’t hold onto that level of gas.Now, this gas does exist in space, but really only concentrates in the region a star is forming in, thanks to gravity and the fact the star itself is still ‘turning on’. So a planet like Earth couldn’t collect enough of it to change into a planet like Neptune (a smaller gas giant). it’s basically got be born a gas giant.(Scientists have hypothesized about gas giants being stripped of their gas, either as their stars become giants as they age and die, or because the planets move inward and end up heating up too hot to hold their gas. These are known as Chthonian planets, from the Greek ‘from the Earth’ used for things associated with the underworld. So you could end up with a solid planet that used to be a gas giant, but it doesn’t seem to work the other way around. It’s a lot easier to scatter gas away than collect it in one place, rather like the laundry example. [Hat tip to Mary Sullivan who noted the name for such planets]