So this planet enters our solar system from the 'south' pole at 30 kilometers per second. A bit faster than the first Interstellar asteroid ever discovered.

Of course, it plunges into the Sun's photosphere at a final speed much much higher than this.

Some things to consider...

Will the Sun get brighter due to the kinetic energy of the collision? Or will the Sun dim as the planet is vaporised and its composition added to the solar atmosphere? Would the iron content affect the Sun's magnetic field? Would the Sun throw out any dangerous (to Earth) solar flares? Would the Sun burn detectably hotter once the planet is added to its solar mass? Or would the Sun now burn dimmer, due to the iron and heavier-than-iron elements?

On Earth, will people notice any color and/or intensity change to the normal sun-light? (Short-term and long-term)

Of course, people will be screaming 'Nibiru' and running for the hills. Overall, is this event catastrophic, or merely something to tell our grandchildren about?

  • $\begingroup$ Are you by chance referring to a planet with 12x Earth's mass? $\endgroup$
    – RonJohn
    Apr 30, 2018 at 3:10
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    $\begingroup$ Also, how is this WorldBuilding and not Physics? $\endgroup$
    – RonJohn
    Apr 30, 2018 at 3:11
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    $\begingroup$ Where does it come from? This is the crucial part. The Sun itself won't care at all about the puny additional mass and momentum, but, depending on where it comes from and on the path it takes towards the Sun, the effect the rogue planet will have on the rest of the Solar System, Earth included, will range from minimal to devastating. $\endgroup$
    – AlexP
    Apr 30, 2018 at 3:50
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    $\begingroup$ You might profit from splitting this into multiple questions. Please don't take this the wrong way, not everyone has the luxury of an education in science, but almost every single point you have is based on some misunderstanding(s) of how things work. If you split it into multiple topics, you might learn a lot about nature, so please consider it $\endgroup$
    – Raditz_35
    Apr 30, 2018 at 7:07
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    $\begingroup$ Sun is roughly 27 thousand times the mass of your planet and while your planet has considerable kinetic energy the sun is powered by nuclear reactions on a massive scale. Thus I doubt the planet would make significant difference. Although it would make waves and probably even cause some mass to be ejected the direction of the collision would probably make the effects observable but not significant from Eerth. $\endgroup$ Apr 30, 2018 at 7:18

4 Answers 4


The Sun is 333,000 Earth masses. It has an average density of 1408 kg/m^3. Its surface gravity is 274 m/s^2 and has an escape velocity of 617.6 km/s.

If a rogue planet is falling through the Sun's gravitational field, it will gain the equivalent of its escape velocity as it does so. Therefore, with an initial velocity of 30 km/s, the impact will be approximately 647.6 km/s. It will slam into a hot, dense medium. Heat released with the kinetic impact of the planet hitting the solar environment.

On its passage before crashing into the Sun, the rogue planet will be subject to radiant heating, it will be struck by solar flares, and it will have ploughed its way through the solar wind. These effects will be relatively minor compared to collision with the Sun, but the outer layers of the rogue planet will get very hot. Some material will be sloughed off its surface. The overall effect will depend on how it takes the planet to approach the bulk matter of the Sun.

The rogue planet will collide with a mass that is 27,750 greater than its own mass. The Sun is not a solid object. The planet will plunge into an extreme bath of hot plasma. The planet's kinetic energy will blast off its outer layers and it will be burning up like a meteor.

An interesting question may be whether the planet will survive sufficiently intact to slow down so that it sink into the depths of the Sun as it gradually evaporates. Once it is deep within the bulk of the Sun, after the kinetic ablation has stripped away its upper layers, it will be subject to a continuous plasma blow torch.

Will this collision have much impact on the Sun? Probably not. There are stars which have 'eaten' up to fifteen (15) Earth masses of terrestrial planets. This one too. The main difference is that these stars devoured their planets, they didn't undergo a collision.

The most likely outcome of a rogue planet collision with our Sun will be a spectacular display as the rogue planet plunges into the solar southern pole. Perhaps material sprayed up from the kinetic impact. Some effects on sunspots and solar flares. By and large, the mass differences will mean the Sun will come out unscathed, without any significant disruption. There will be changes in the Sun's composition. This will be interesting to astrophysicists.

Collisions like this sound more spectacular than they actually are. Stars like the Sun can easily swallow planets whole. A rogue planet with 12 Earth masses will be no exception.

  • $\begingroup$ I think you mistakenly placed sun's gravity as 274 km/s, when it's in fact 274 m/s. Even neutron stars have surface gravity up to, but not exceeding 12 km/s. $\endgroup$ Apr 30, 2018 at 18:25
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    $\begingroup$ You can't add escape velocity this way. It's going to hit at only a hair over the 617.6km/sec. Look up "Oberth effect"--this is the flip side of the same thing. $\endgroup$ Apr 30, 2018 at 22:10
  • $\begingroup$ +1 for getting the math squared away, but 30 km/s won't make much of a difference to the non-event $\endgroup$
    – pojo-guy
    Apr 30, 2018 at 23:07
  • $\begingroup$ @YoustayIgo Absolutely right! My brain and fingers weren't on the same page. My gratitude for your spotting my mistake. An edit coming up. $\endgroup$
    – a4android
    May 1, 2018 at 4:41
  • $\begingroup$ @LorenPechtel Good! Glad you're pointing me in the right direction. Oberth effect, you say. Even so an extra 30 km/s wouldn't make that much difference. I am grateful for having my thinking corrected. Good work! $\endgroup$
    – a4android
    May 1, 2018 at 4:45

While the sun is just an average G type star it is impossibly large for our human mind to comprehend. About a million times larger than the earth. While a 12 earth-masses iron sphere sounds very intimidating for puny earth's standards, it is nothing ... nothing for the sun.

Also notice that while the temperature of the sun's surface is around 6000 °C, the temperature of its corona (an atmospheric layer around the sun, around 500 km above the surface) is no less than 1 million °C. The iron chunk hurtling towards the sun will not just melt, it will boil away before coming crashing into the sun.

A lot of the mass of that ferrous chunk would simply radiate away from the sun as steam. Only a small part of it will come crash into the sun. Considering the impossible size of the sun, it won't even feel a thing at all. No solar flares, no increase in luminosity, no nothing.

One solar creature to another: What was that thing that just came hit us? My scientific instruments detected a 0.00001 scale quake.

Other solar creature: Idk man. Some space junk or whatever. My detectors didn't feel a thing.

First creature: Yeah. Your instruments aren't as precise as mine.

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    $\begingroup$ There is debate as to what the impact of a Jupiter merging with our Sun would be (100s Earth masses) and some of it leans towards - nothing much. As Youstay points out, this iron ball is just far too small. $\endgroup$
    – Windlepon
    Apr 30, 2018 at 8:04
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    $\begingroup$ Aaah, boiling iron, my favourite gas. $\endgroup$
    – Joe Bloggs
    Apr 30, 2018 at 8:12



The boiling point of iron is about 3000 K (5000 F) while the surface temperature of the sun is about 5500 K (10,000 F), so this comet-of-iron would evaporate en route to the sun's surface.

Thus, to your question "now what?" Apparently not much.

  • $\begingroup$ The linked answer was for a comet, not a very large planet - would a large planet evaporate? $\endgroup$
    – user39029
    Apr 30, 2018 at 3:58
  • $\begingroup$ That's two different questions, which must be answered before you assume it'll hit the Sun. #1 Would a 12M⊕ planet traveling at 30km/s evaporate before it hits the Sun? #2 If not, how much of the solid mass would be left? And for that, you need... Math! $\endgroup$
    – RonJohn
    Apr 30, 2018 at 4:04
  • $\begingroup$ One clarification: It is 30km/s entering the solar system, but not 30km/s on impact (if it impacts), similar to the interstellar asteroid which got up to 87.4km/s, I'm expecting a much larger impact speed. $\endgroup$
    – user39029
    Apr 30, 2018 at 4:18
  • $\begingroup$ "It is 30km/s entering the solar system". What are the Solar System's northern and southern boundaries? $\endgroup$
    – RonJohn
    Apr 30, 2018 at 10:34

Depends on how it hits.

Straight in, probably not much. It's going to punch deep and the energy release is thus going to be spread over quite a period of time. However, if it's a glancing hit you've got a problem:

Your rogue will be carrying about 1.37E+37 joules of energy. The sun only emits 3.83E+26 joules per second--thus it's going to deliver 11 centuries worth of energy. The sun is going to radiate that away--but it's going to be bigger and brighter while it's doing so.


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