I've just come across a subplot in Jack McDevitt's Firebird where the earth-like planet Villanueva and the rest of its planet system was moving towards an unspecified dust cloud that made the planets uninhabitable for three centuries, leaving only robots behind (which became hostile so the planet is still uninhabitable, but let's imagine this world without the robots).

Some time after the system it has exited the dust cloud, the nature continued to grow and everything was inhabitable again.

Contrary to this question, it's assumed that the temperature of the dust isn't relevant. I'm well aware that there are things like a nuclear winter, but that problem happens solely in the atmosphere, the outer space isn't involved.

That finally leads to my questions:

  1. Could a dust cloud block the light from a planet effectively for some decades or centuries, but not permanently?
  2. Are there special conditions a dust cloud has to fulfill to achieve this result? (Certain density, size, ...)
  3. Would there be other consequences beside the climate change?
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    $\begingroup$ Just a note that's not worthy of an answer...The sun possesses a Heliosphere, which deflects stellar particles the same way the earth's magnetic field does. This field is strong and routinely deflects protons flying at near speeds of light...for this dust cloud to get through the heliosphere, it has to be neutral charged dust. The movement of other particles tends to cause a flow like pattern around the heliosphere...I'm not convinced a dust cloud would actually be able to enter the solar system. Also remember how big space is...it might be more than a century for a dust cloud to pass by. $\endgroup$
    – Twelfth
    Feb 3, 2015 at 22:28

4 Answers 4


The cloud could not form naturally.

Naturally occurring dust clouds in the universe aren't dense enough to block out the sun by any noticeable amount. Even in huge nebulae, the stars inside the nebulae are visible from Earth. Furthermore, in a dust cloud of that size and density, the internal gravity of the cloud would lead to fairly rapid star formation. We see this in the darker parts of the Eagle Nebula.

But let's assume that some alien factory released a huge, dense dust cloud fairly close to our solar system, which was then slowly pulled into the sun, with gas spiraling in across the disk of the solar system and blocking out most of the light from the sun from reaching earth. What would happen then?

  • The Earth would have continuous spectacular meteor showers. That density of dust would quickly lead to the formation of agglomerations of dust, which would hit the atmosphere and burn up. This probably wouldn't produce enough heat to warm the Earth noticeably, but it would be quite pretty. Imagine a meteor shower brighter than any seen in the history of Earth happening continuously for hundreds of years.

  • Almost everything would die. Plants would definitely be gone, as well as all terrestrial animals. Life colonizing sea floor vents wouldn't notice the lack of sunlight, nor would the extremophiles colonizing other mineral hot springs.

  • Nothing else in the solar system would be visible from Earth. Likewise, everything outside the solar system would be effectively invisible.

  • The Earth would gain rings. So would all of the other planets that don't already have them. This would be a result of not all of the dust getting sucked up by the sun, and some of it falling into orbit around the planets.


"Is it possible to block temporarily (enough) sunlight to a planet with stardust"?

No, I don't think so. Any nebula dense enough for that would collapse and start stellar formation in short order; that's how the Solar System itself is thought to have been born. Same goes for a cloud of gravel. Unless (as pointed out by HDE 226868) the temperature of the dust cloud was high enough to prevent a gravitational collapse.

Moreover, the solar wind pressure from the star would simply sweep away the cloud much faster than the cloud could come in, creating a planetary nebula.

Such a phenomenon can be observed in some stars that happen to enter a dense nebula. It is the case of LL Orionis, a star not much larger than the Sun, which creates a "bow wave" between 20 and 60 AU far.

This means:

  • that even a planet as far as Saturn would likely be "protected" from the incoming nebula. The inner planets would never be reached and would suffer no ill effects. Actually probably no effect whatsoever, except maybe an intensifying of the harmless Gegenschein.
  • that little or no gas (or particle cloud) could have a chance to enter the much more intensely sweeped area inside the planetary orbit, and block the light of the Sun. Even if released artificially exactly on the ecliptic, the radiation and solar wind pressure would blow it away.

There might still be a problem due to high energy particles being generated in the bow shock area, but I don't think that it would be comparable to a solar storm, nor that it would be likely to affect a technological civilization (or, for opposite reasons, a non-technological civilization). Actually, I suspect that biological systems would be much likelier to resist the onslaught than electronic robots.

Science stations as far as Neptune or Pluto would probably need to be abandoned, since resupply flights in the direction of the bow shock would probably be unhealthy to both ships and astronauts (it depends on technology though. Given sufficiently powerful engines, you can load sufficiently massive shielding, I guess).

With much larger (and unlikely) particles or clumps, chances are very high that many of them would experience gravitational scattering or radiation pressure braking and fall inwards at an accelerated rate, so that the solar wind would not be enough to keep them at bay. In that case the effects would not go as far as rendering the planets uninhabitable, but it could well be enough (for gravel-sized particles) to trigger a catastrophic Kessler cascade in addition to filling the sky with a continuous dazzle of falling stars. This in turn could disturb or disrupt long-range radio communications due to the radio scattering of the ionized meteoric trails.

Update: the "artificial cloud in stable disk formation" scenario is actually exactly the device of astrophysicist Sir Fred Hoyle's novel, "The Black Cloud". But even there, the dramatic turn of events is triggered by the Cloud's ability to remain stable around the Sun, which no astronomer had foreseen (rather the contrary), which in turn is caused by the Cloud's particular... nature (wouldn't want to spoil a really interesting read). A similar effect is explored in the devices supplying Larry Niven's Ringworld its days and nights. Again, stability is achieved through "innatural" means inherent in the object's composition (i.e., not something you set from the outside and let go, such as the point, speed and temperature of release of a cosmic cloud of squid ink; rather something like the ink's viscosity).

  • $\begingroup$ +1 for mentioning the solar wind. The sun isn't just a big light bulb in the sky! $\endgroup$ Feb 4, 2015 at 11:11
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    $\begingroup$ Very good answer, +1. I do disagree with the idea that it would collapse - temperature plays into it as well. $\endgroup$
    – HDE 226868
    Feb 7, 2015 at 21:56
  • $\begingroup$ You're referring to a possible thermal stability of the cloud. The cloud would need to be near its Jeans radius and/or be "freshly" formed and/or still in the expansion stage. I'll try and do some calculations; it's an interesting consideration. Thanks! $\endgroup$
    – LSerni
    Feb 7, 2015 at 23:17

This would have to be one pretty dense dust cloud to block sufficient sunlight to make the temperature drop sufficiently to make the planet uninhabitable. However, the possibility does exist that such a phenomenon may be possible.

Yes, it would be possible to reduce the levels of sunlight reaching a planet sufficiently that the planet would no longer be inhabitable. The further out from the primary, the easier, as the effect of dust on light reduction would be cumulative over distance..

The dust cloud would not require any particularly special properties other than its existence. The dust would both absorb and reflect photons depending on the albedo of the dust, however the effect would be the same - a reduction in solar energy reaching the planet.

A dust cloud sufficiently dense to block sunlight enough to cause climate change to the point that the planet became "uninhabitable" would also be dense enough that significant amounts would be captured by the planet's gravity and would rain down onto the surface. As long as this wasn't "dust" in the 5cm+ size range, the consequences of this would simply be the accumulation of a layer of the dust upon everything exposed to the sky. Depending on the density of the dust cloud, this may be anything from a millimetre to many centimetres.

If more than a few centimetres of dust accumulated on the planet, combined with the prolonged winter, this would pretty much exterminate anything much bigger than a bacterium save around deep-sea hydrothermal vents. Some plants may be able to survive a prolonged freeze, but any significant amount of dust would bury them for good, and seeds cannot successfully germinate if buried too deeply.

So, if the planet was habitable after the dust cloud was cleared, then it may not have been all that dense, and hence the accumulated dust layer on the surface was probably not all that thick.

  • 1
    $\begingroup$ Wouldn't all of the dust burn up in the atmosphere before reaching the surface? $\endgroup$
    – ckersch
    Feb 3, 2015 at 22:04
  • 3
    $\begingroup$ @ckersch, no. No matter the size of a meteor, no matter how much of it is ablated by its passage into the atmosphere, any ablated matter ends up in the atmosphere, and if heavy enough, will eventually end up on the ground. Dust, by definition, has a low mass and a high surface area, so will not have enough energy to get hot enough to melt or evaporate in the atmosphere before atmospheric drag slows it to its terminal velocity, meaning that dust will just accumulate on the surface. $\endgroup$
    – Monty Wild
    Feb 3, 2015 at 22:10
  • $\begingroup$ Would the dust be made of things that would precipitate as solids, though? I was thinking it would be mostly organic compounds and ices, since those are made of the most common elements. I would think they would mostly form volatiles after impacting the atmosphere. $\endgroup$
    – ckersch
    Feb 3, 2015 at 22:28
  • $\begingroup$ @ckersch, if we're talking about a cloud of volatiles, I wouldn't really call it dust. Anyway, if our dense nebula was made up of that, we could get a significant change in atmospheric composition. Exactly what consequences we can expect would vary according to the particular volatiles you're talking about. $\endgroup$
    – Monty Wild
    Feb 3, 2015 at 22:32

It seems plausible.

This physics question includes a reference that objects would start to get "lost in the fog" in a Nebula at only 5800 KM - that's an incredibly small distance in astronomical terms. So assuming that's correct, even a relatively non-dense dust cloud would likely block significant sunlight.

Nuclear Winter estimates that a small-scale nuclear war (50 cities, and not huge bombs) would potentially cool the entire planet by several degrees for a decade. But a dust cloud wouldn't ever go away - it would just keep cooling.

This can easily be temporary if the dust cloud just "clips" your planet and sun, instead of passing directly through it. Most nebula are really insanely huge - light years and light years - so it would take a long, long time to pass through them. But if you just went through the edge of a cloud, it could last as short or as long as you want.

Secondary effects - I'm not 100% sure. I don't think there'd be any significant long-term effects, if you don't count 99% of the biosphere dying off. Short term, you'd probably get some really amazing auroras until you froze to death.

  • 4
    $\begingroup$ Mean free path (what's quoted in that question) isn't a good indicator of when vision starts to become occluded. It's the average distance you need to travel in a line to reach an atom in a random direction. The atmosphere, for comparison, has a mean free path of 68 nanometers at sea level. 5800 km qualifies something as 'extremely high vacuum'. en.wikipedia.org/wiki/Mean_free_path $\endgroup$
    – ckersch
    Feb 3, 2015 at 22:13

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