4 ms·

At half the speed of light, I'd guess that the solar system would have its heliosphere stripped by interstellar or even intergalactic medium. The heliosphere is

At half the speed of light, I'd guess that the solar system would have its heliosphere stripped by interstellar or even intergalactic medium. The heliosphere is the region around a star where the pressure of the star's solar wind is great enough to push back against the ambient gas floating around in the galaxy. For some scale, we think Voyager 1 may soon exit the heliosphere of our solar system -- a first for man made objects. As sibling comments describe, any planetary orbits that happen to survive would continue operating as usual, but if they were outside the heliosphere they would be subjected to interstellar gas traveling at half light speed in the other direction.

pfdietz 6y ago
Also, each proton in the interstellar medium would become a cosmic ray with energy ~120 MeV. At 1 proton/cm^3, the intensity of this radiation on the planet would be 270 kW/m^2 (more than 100x the intensity of sunlight). If this can penetrate the magnetosphere (not clear to me it could, although I bet auroras would be extreme) it would erode the atmosphere and create large amounts of radioactivity.
- deleted 6y ago
  [deleted]
- andromaton 6y ago
  Cue Randall Munroe
- Retric 6y ago
  Interstellar medium has a huge range of densities from approximately 10^6 per cubic cm down to 10^-4. However, the intergalactic medium is between 10^-5 and 10^-6 in high density filaments and gets much less dense in the voids. As such in a cosmic void planets around such stars would be fine. Though it seems extremely unlike for such planets to exist.
  - infogulch 6y ago
    Even at a comparatively low density surely it's many orders of magnitude higher than how often we encounter such cosmic rays on earth. I mean if we scale down the energy output by 10^-6, 270 mW/m^2 would still be way more than any known biological system could sustain, right?
    - yaantc 6y ago
      > 270 mW/m^2 would still be way more than any known biological system could sustain, right? It's negligible compared to our sun. All this from the top of my head (so double check if you really care ;) but our sun in winter at surface level is roughly about 100W/m^2 and in summer about 1kW/m^2.
      - infogulch 6y ago
        Yes, but the point is that these are wildly different types of energy delivery. The sun delivers energy by gamma radiation ("plain old light") but here we're talking about alpha particles ("relativistic protons") which has a very different effect on biological systems. The fact that we are able to compare these two things by their total energy content at all is shocking to me. To make this comparison more concrete, maybe we could consider how much alpha particle energy is present inside of a nuclear reactor running at peak energy output. I'd be surprised if it was as much as 1 W/m^2.
        Retric 6y ago
        Alpha particles don’t penetrate even the thin atmosphere of Mars. It’s just a question of total heat, which in cosmic voids would be negligible. https://en.wikipedia.org/wiki/Void_(astronomy) https://en.wikipedia.org/wiki/Void_(astronomy)
        pfdietz 6y ago
        Protons at 120 MeV will deposit a significant amount of energy via nuclear collisions. The binding energy of a nucleon in a nucleus is about 8 MeV, so plenty of free neutrons will be produced, which will then create 14C by the (n,p) reaction on 14N. Back of the envelope gives this creating about 10 tonnes per second of 14C in the planet's atmosphere (assuming the proton density of 1/cm^3, and assuming the planet is Earth-like and all the protons struck the atmosphere). This is about 11 orders of magnitude higher than the production on Earth, and would result in such a high equilibrium concentration of 14C that the biosphere would likely be sterilized, even ignoring thermal effects.
- LorenPechtel 6y ago
  Doesn't matter if it can penetrate the magnetosphere, if it's stopped that energy has to go somewhere and some of it is going to hit the planet. Even if it doesn't get blasted it's still going to get too warm.
  - pfdietz 6y ago
    A charged particle deflected by a magnetic field loses no energy.

pfdietz 6y ago

Also, each proton in the interstellar medium would become a cosmic ray with energy ~120 MeV. At 1 proton/cm^3, the intensity of this radiation on the planet would be 270 kW/m^2 (more than 100x the intensity of sunlight). If this can penetrate the magnetosphere (not clear to me it could, although I bet auroras would be extreme) it would erode the atmosphere and create large amounts of radioactivity.

deleted 6y ago
[deleted]
andromaton 6y ago
Cue Randall Munroe
Retric 6y ago
Interstellar medium has a huge range of densities from approximately 10^6 per cubic cm down to 10^-4. However, the intergalactic medium is between 10^-5 and 10^-6 in high density filaments and gets much less dense in the voids. As such in a cosmic void planets around such stars would be fine. Though it seems extremely unlike for such planets to exist.
- infogulch 6y ago
  Even at a comparatively low density surely it's many orders of magnitude higher than how often we encounter such cosmic rays on earth. I mean if we scale down the energy output by 10^-6, 270 mW/m^2 would still be way more than any known biological system could sustain, right?
  - yaantc 6y ago
    > 270 mW/m^2 would still be way more than any known biological system could sustain, right? It's negligible compared to our sun. All this from the top of my head (so double check if you really care ;) but our sun in winter at surface level is roughly about 100W/m^2 and in summer about 1kW/m^2.
    - infogulch 6y ago
      Yes, but the point is that these are wildly different types of energy delivery. The sun delivers energy by gamma radiation ("plain old light") but here we're talking about alpha particles ("relativistic protons") which has a very different effect on biological systems. The fact that we are able to compare these two things by their total energy content at all is shocking to me. To make this comparison more concrete, maybe we could consider how much alpha particle energy is present inside of a nuclear reactor running at peak energy output. I'd be surprised if it was as much as 1 W/m^2.
      - Retric 6y ago
        Alpha particles don’t penetrate even the thin atmosphere of Mars. It’s just a question of total heat, which in cosmic voids would be negligible. https://en.wikipedia.org/wiki/Void_(astronomy) https://en.wikipedia.org/wiki/Void_(astronomy)
        pfdietz 6y ago
        Protons at 120 MeV will deposit a significant amount of energy via nuclear collisions. The binding energy of a nucleon in a nucleus is about 8 MeV, so plenty of free neutrons will be produced, which will then create 14C by the (n,p) reaction on 14N. Back of the envelope gives this creating about 10 tonnes per second of 14C in the planet's atmosphere (assuming the proton density of 1/cm^3, and assuming the planet is Earth-like and all the protons struck the atmosphere). This is about 11 orders of magnitude higher than the production on Earth, and would result in such a high equilibrium concentration of 14C that the biosphere would likely be sterilized, even ignoring thermal effects.
LorenPechtel 6y ago
Doesn't matter if it can penetrate the magnetosphere, if it's stopped that energy has to go somewhere and some of it is going to hit the planet. Even if it doesn't get blasted it's still going to get too warm.
- pfdietz 6y ago
  A charged particle deflected by a magnetic field loses no energy.