First of all it could NOT hit the sun at 100 km/s – the MINIMUM speed at which it could hit the sun would be 617 km/s.

If a Jupiter-sized planet hit the sun it would be a “Kill Everything” event.

Anything hitting the sun will hit at at least 617 km/s.

Which means with an energy of 1/2 x 617 x 617 megajoules per kg which is 190,000 megajoules.

The sun is 1000 times the mass of Jupiter so the impact will add 190 megajoules/kg to the entire sun.

190 megajoules/kg – that will raise the sun’s temperature just a wee bit!

At Earthly temperatures that would raise the temperature by about 90,000 degrees.

The sun operates on a different regime but still there would be a substantial increase in temperature which would cause the sun to swell up and which would massively increase the rate of the sun’s fusion reaction.

This would not destroy Earth but it would blast the atmosphere and the oceans into space and blowtorch the exposed surface.

An iron planet of that size would be about six times that mass – so six times that energy.

So “Kill Everything” many times over.

Here is another way of looking at it:

The impact would add the same amount of energy that the sun gets from the fusion in its core in 30,000 years.

That is 30,000 years’ worth of the whole sun’s output in one SMACK!

A number of people have asked about the 617 km/s.

This is the speed that something dropped onto the sun will impact at – and it’s the same as the sun’s “escape velocity”.

If you drop something onto Earth it will land at 11 km/s – it can land FASTER – if it was already traveling towards us then that speed would be added on but it can’t arrive slower – or at least hit the atmosphere slower.

The atmosphere will slow it down BUT it does that by taking energy from it – and that energy stays on the planet.

So anything falling onto the sun will hit at a MINIMUM velocity of 617 km/s.

Conversely if you were ON the sun and you wanted to escape that is the speed that you would need.

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I’m going to take your question at exactly face value, giant space aliens waved their hands and made it so. As such, a Jupiter sized planet, except composed of elementally pure iron at STP, appears right above the Sun’s corona, headed down at 100 km/s.

So first, a Jupiter sized iron planet would actually out mass the sun at a whopping 1.1259e31 kg – it’s almost six times heavier than the sun. That means it would start to undergo it’s own fusion, and fairly quickly, it would need energy for fusion though, so where does that come from?

The raw tonnage of the planet is more than enough to overwhelm the compressive strength of iron, so it begins to shrink – after all, we assumed STP so we could actually calculate how much iron there was . It stars with a gravitational binding energy of 7.26197641764 times ten to the power of forty three. (30% shy of being 1 Foe

– things are not likely going to go well) Gravitational binding energy is negative – it is the total energy required to lift every particle out of the gravity well.

So if this value goes up, it means that gravitational potential energy has converted into other forms, for example kinetic energy and heat. By the time it has contracted 1 millimeter, its binding energy is 7.26197641774 times 10 to the power of 43. Or a difference of ~1e33 joules.

The sun only outputs at 3.86e26 watts – so we are about 1 month of total solar output of energy unleashed.

How long does this take?

Well, this could be pretty difficult to answer exactly, so let’s do some approximations to see how accurate we should bother to be.

The surface gravity of this monster is 15,561 times that of Earth. That means that a layer of iron 33 centimeters thick is enough to overcome the compressive strength of iron. Our shell is tens of thousands of kilometers thick.

Given that, I feel it’s fair to say the outermost layer at least starts in near freefall. After falling 1 millimeter it will be contracting at 17.5 m/s, at the surface gravity of this behemoth, this will have taken 0.11 milliseconds.

After the surface has fallen 10 meters, about 1e37 joules have been released. About 100 years of solar output. Despite the rapid acceleration of the surface downwards, not much else should be noticable there yet, unless there is some factor I have forgotten.

The surface is falling at 1753 m/s (1/3rd of the way to the speed of sound in iron) and 11 milliseconds have passed. In less than 1 second, the surface gravity of this iron giant will have started crushing itself so fast that it is actually receding from the sun – or it would be, if its own local gravity didn’t massively overwhelm that of the Sun, and start eating our poor, poor star.

The GBE at this turnover this point will have added ~1e40 joules to the collapsing megastorm of iron.

We can’t keep this up indefinitely, too much energy is being released. We need to figure out what the ultimate fate of the situation is. This is actually easy! When a large enough supernova occurs, it produces tremendous amounts of iron! This much? Well…. hard to say.

They tend to form black holes in the center, so we can’t investigate the exact outcome very well. Is this big enough to form a black hole? Yes, yes it most certainly is.

It is already far heavier than the stellar mass limit for a dwarf star, or a white dwarf, or a neutron star, so the only question left is does the incoming hypernova (remember, the GBE started at Foe levels, then immediately shot up at a silly rate) leave enough mass left afterwards to still form a black hole. I don’t know. We wouldn’t be around to investigate.

Neither will the Sun, the hypernova will flat out destroy it. Not extinguish, not trigger a supernova of its own, the energy released will be enough to physically obliterate a star. They’ll never actually collide until the blast wave hits though, so I guess the answer is…

You can’t actually hit the Sun with an iron Jupiter.

So there you have it.

Edit: I’m getting notifications about comments on this answer, but I cannot actually see or respond to them, I’m not ignoring them.

Edit the second: I apparently slipped a digit on the calculations, my number for the mass of Jupiter is 1000 times too high. [Props for figuring out how to communicate via suggested edit! I should thank you directly but I clicked edit before writing your name down and now it’s gone 🙁 ]

Believe it or not, a mistake of this magnitude does not actually change the outcome. I am leaving the erroneous numbers above as a lesson for myself to triple check my work – ridicule me at your leisure. There is still however more than enough mass and potential energy to physically obliterate the Sun. Also it was requested that I clarify that fusion of iron does not produce energy – it consumes it!

All of the energy from the star obliterating hypernova comes from gravitational collapse, not the fusion of iron. That’s just something that also happens as a side effect. Jupiter also still never actually touches the sun, as the surface is still in free-fall (though it is now actually very important that we start at STP for this to hold!) – the initial forces involved just have so many orders of magnitude behind them that dropping a factor of 1000 doesn’t meaningfully change the end result.