550 million years ago, aliens erected a spherical barrier 1/6th of a light-year (~10,000 AU) in radius around the solar system, centered on the solar barycenter. Things outside the barrier can enter, while things inside the barrier cannot exit. Only massless particles and gravity can move freely both ways. Massive particles trying to exit are reflected away. An interstellar comet, for example, entering on a hyperbolic path can pass through the barrier's exterior unharmed, but upon exiting will impact the interior spectacularly (especially at ISVs). This means the barrier is totally transparent and can't be viewed directly, as it neither absorbs nor emits light of its own. (An interior spacecraft could still gauge its range to it by e.g. shooting ionized hydrogen at it and looking for infrared backscatter.)
little side notes... The barrier is non-rotating and station-keeps with the solar barycenter via magical means.

The barrier reflects all the massive particles that try to escape, so all electrons, protons, and alpha particles of the Sun's solar wind are trapped inside. Interstellar crosswinds are also similarly trapped. The barrier makes an exception for stars on close fly-bys, opening a very temporary exit hole for any that may enter the ~20,000 AU spherical region (which happens every ~10 Myr or so).

If this cosmic pressure cooker really came to be around our Sun 550 MYA, would humans up until our current time period be able to deduce its existence, given its behavior (the built-up density of our interplanetary medium)?
if so, how recent would the erection need to be for the barrier to be undetectable; and if not, how far back would the construction date need to be pushed?

This is related to a question of mine Aliens englobed the Solar System: will we notice? except I've tweaked the original behavior to suit a number of story-telling aspects. This tweak fundamentally changes what the alien barrier does, which I believe warrants a new question.


  • The checked answer to the above-mentioned question suggests that nuclear disintegration forced by the (old) barrier, as well as Hawking radiation, would make it shine like a supernova from all directions. That's no longer the case lol

  • There is a potential statistical bias in orbital energy distributions of long-period objects with aphelions greater than ~10,000 AU. It's not clear whether we have enough astronomical data currently to see that bias, but it would exist.

  • The barrier would act like a bucket under a faucet in the interstellar medium, scooping up dust on its trajectory through the galaxy and leaving a void in its wake. The ISM is awfully thin, so it's not clear whether we could detect that currently. The void would be a small fraction of the total distance light would've had to travel to reach us.

  • Over the 550 Myr barrier lifetime, near-parabolic Oort cloud objects would be gradually depleted. Future space exploration and "geology" may be able to detect the decline in high-energy bombardments, but it's uncertain. The inner Oort cloud would still be unperturbed, leaving lots of statistical noise to sift through.

  • Any spacecraft or interstellar lightsail probe fleets we send out will stop transmitting past the ~10,000 AU mark. This is really a moot point because it hasn't happened yet.

  • $\begingroup$ "impact the interior spectacularly" - how exactly spectacularly? Would we see the "glow" of the solar/star wind when it's impacting the barrier from the inside? $\endgroup$
    – Alexander
    Commented Sep 15, 2021 at 22:18
  • $\begingroup$ @Alexander not sure. May or may not be visible from Earth. Interstellar comets range from like 30-50 km/s iirc, that's a lot of momentum to give up in a split second. It'd be pretty spectacular from one vantage point or another lol. $\endgroup$
    – BMF
    Commented Sep 15, 2021 at 22:24
  • $\begingroup$ What is the exact mechanism of collision, especially for individual massive particles? Are they just bounce, or there is some complex interaction? $\endgroup$
    – Alexander
    Commented Sep 15, 2021 at 22:28
  • $\begingroup$ @Alexander the barrier conspires to give each interior massive particle a perfect inelastic collision. Momentum is exchanged and kinetic energy is lost, but the barrier does not honor its part of Newton's deal as it always stays centered at the solar barycenter. (Yeah, particles basically just bounce.) $\endgroup$
    – BMF
    Commented Sep 15, 2021 at 22:38
  • 1
    $\begingroup$ @MolbOrg oh yeah whoops, I meant perfectly elastic. Nice catch. It should read: "Momentum is exchanged though some kinetic energy is lost due to the barrier self station-keeping with the solar barycenter." The lost energy would probably be very small, not worth mentioning. $\endgroup$
    – BMF
    Commented Sep 16, 2021 at 2:17

2 Answers 2


Sure we would notice (if we were to live to see that, which is unlikely)

If solar system is traveling in that manner, if scoops all interstellar gas and dust on its way. Given that period of time is quite considerable (550 million years), the total mass sucked in will dwarf the mass of our solar system.

Sun is cruising around galactic center at an average speed of 230 km/s. It's speed in interstellar medium is considerably slower, estimated at about 23-26 km/s. If we are setting up a sphere around the Sun, how much space its cross-section would capture during 550 million years of travel?

Travel volume = Vsun * T * PI * R^2

Actually, we'd like to know how much travel volume compares to the "cooker sphere volume"

Sphere volume = 4/3 * PI * R^3

Travel volume / Sphere volume = 3/4 * V * T / R = 200,000

Which means that by now, density of gas and other interstellar matter would be 200,000 times higher than the normal density. Assuming that normal density is 1 hydrogen atom per cubic cm:

New sphere mass = Travel volume * Density * Hydrogen atom mass = 4.68 * 10^30 kg

Our Sun's mass is only 1.989 * 10^30 kg, so our sphere would contain more than 2 times mass of the Sun. This should make our sphere, at the very least, a dense interstellar gas cloud, but more likely this gas will turn into stars and planets. Some gas will get absorbed by the Sun, increasing its luminosity and driving other gas away to the boundary, where new stars will be forming, so before the humans were to evolve, solar system would become a cluster of multiple stars, planets and asteroids. So called "late heavy bombardment" will become a permanent state in the solar system.

P.S. As @EdvinW had noticed, the galactic medium where Sun rotates is not static. All objects, including interstellar gas, are also rotating around galactic center. This way, we should base our calculations on Sun's speed in interstellar medium, not galactic rest frame. Not so much known about the interstellar medium, but Sun's speed is estimated at 23.2 - 26.3 km/s, which is about 10 times slower than its orbital speed (230 km/s).

  • $\begingroup$ Ah, damn, I think your math is right! That definitely shoots some holes in my story. Thanks for your answer $\endgroup$
    – BMF
    Commented Sep 16, 2021 at 1:40
  • 2
    $\begingroup$ This answer assumes that the sun moves through a stationary background, like a car running through still air. But with the whole galaxy spinning, not just our particular star, wouldn't that imply a considerable tailwind that could potentially reduce the "drag"? I might very well be wrong, but this is at least what my intuition tells me. Am I wrong? $\endgroup$
    – EdvinW
    Commented Sep 16, 2021 at 6:44
  • 1
    $\begingroup$ @EdvinW you are quite right! I should have been calculating speed with respect to interstellar medium, not galactic rest frame. Although this speed is much less known, I could find that it's about 10 times less than Sun's galactic speed. So my initial calculations are likely 10 times off, but unfortunately solar system is still doomed. $\endgroup$
    – Alexander
    Commented Sep 16, 2021 at 7:31
  • 2
    $\begingroup$ @BMF I corrected my calculations (based on EdwinW's comment and one of my own mistakes), and the numbers now look much better for you, though still not quite great. $\endgroup$
    – Alexander
    Commented Sep 16, 2021 at 7:45

Would this barrier affect the Solar sysem's heliopshere?

The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes shape in form of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuously "inflated" by plasma originating from the Sun, known as the solar wind. Outside the heliosphere, this solar plasma gives way to the interstellar plasma permeating the Milky Way. As part of the interplanetary magnetic field, the heliosphere shields the Solar System from a significant amount of cosmic rays, including hazardous ionizing radiation (e.g. gamma rays). Its name was likely coined by Alexander J. Dessler, who is credited with first use of the word in scientific literature in 1967.1 The scientific study of the heliosphere is heliophysics, which includes space weather and space climate.

Flowing unimpeded through the Solar System for billions of kilometres, the solar wind extends far beyond even the region of Pluto, until it encounters the "termination shock", where its motion slows abruptly due to the outside pressure of the interstellar medium. The "heliosheath", is a broad transitional region between the termination shock and the heliosphere's outmost edge, the "heliopause". The overall shape of the heliosphere resembles that of a comet; being roughly spherical on one side, with a long trailing tail opposite, known as "heliotail".

Two Voyager program spacecraft explored the outer reaches of the heliosphere, passing through the termination shock and the heliosheath. Voyager 1 encountered the heliopause on 25 August 2012, when the spacecraft measured a forty-fold sudden increase in plasma density.2 Voyager 2 traversed the heliopause on 5 November 2018.[3] Because the heliopause marks the boundary between matter originating from the Sun and matter originating from the rest of the galaxy, spacecraft that depart the heliosphere (such as the two Voyagers) are in interstellar space.


Voyager 1 crossed the heliopause at a distance of 121 AU, so the heliosphere has tiny dimesions compared to the barrier at a radius of 1/6 LY or about 10,000 AU.

If matter and energy enter the barrier freely, the heliosphere should behave much like it does in real life.

If matter cannot leave through the barrier, it would accumulate inside over 550 million years as Alexander's answer says.

It is possible the density of gas and dust inside the solar system would become much higher, and affect the orbits of the interplanetary dust inthe solar system, perhaps producing detectable results.

And it would tend to accumulate on the back side of barrier. Thus it might have formed a new solar system dragged along by the barrier and about 1/6 LY or about 10,000 AU behind our solar system.

Alexandaer calculated that the accumulated mass would be over two times the mass of the solar system. If a star did form in the trailing region, most of the later incoming matter would eventually fall into that new star and increase its mass.

A star with about 2 times the mass of the Sun would be similar to a spectral class A4V star with 2.03 times the mass of the Sun, and at a distance of 1/6 LY or 10,000 AU it would be the brightest object in the sky of Earth except for the Sun and the Moon.

Incoming matter that entered through the barrier and then hit the back side of the barrier would be prevented from exiting, and thus its velocity would be changed. That would impart some of its velocity to the barrier, which should transfer the velocity to the barrier generators, which should be tied to the Solar System in some way.

Some of the momentum of incoming matter would be transferred to the Solar Systme, acting as a brake to the motion of the Solar System. The motion of the solar system relative to the local interstellar medium would slow, lowering the rate at which matter entered and the rate of deceleration. As the solar system slowed with respect to the local interstellar medium, its velocity with respect to the galactic center of mass would also change, changing the orbit of the solar system around the galaxy.

As mass accumulated in the rear end of the barrier and formed a new star system, the gravitational attraction between the new star system and the Solar System would pull them closer together, altering the motion of the Solar System relative to the interstellar medium and also to the galactic center, changing its orbit to a degree.

It is possible that as matter accumulates to form the new star system, its gravity will become stronger on the Solar System than that of any much more distant object. The two systems might be drawn closer to each other and collide with disastrous results.

Interstellar space might be said to begin at the heliopause, but the Solar System extends many times as far as the heliopause.

The solar system is believed to have an Oort Coud of billions of comets orbiting the Sun at distances both inside and outside the 1/6 LY or 10,000 AU radius of he barrier.

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 au (0.03 and 0.08 ly)[9] to as far as 50,000 au (0.79 ly)[5] from the Sun. Some estimates place the outer boundary at between 100,000 and 200,000 au (1.58 and 3.16 ly).[9] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 au (0.32–0.79 ly), and a torus-shaped inner Oort cloud of 2,000–20,000 au (0.0–0.3 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[5] The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981.[14] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[14][15][16] it is seen as a possible source of new comets to resupply the tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[17]


So the barrier would let in comets from outside whose orbits were perturbed into highy elliptical ones by nearby stars. If those orbits had aphelions outside of the barrier, the comets would strike the barrier on their first outward leg of the new orbit. Depending on what happens when a comet strikes the barrier, astronomers on Earth might detect bursts of energy as that happens. And possibly different observatories detecting the same burst of energy could get parallaxes and determine that those hypothetical explosions are all 1/6 LY or 10,000 AU from the Sun.

And that is about all the effects I can think of.


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