A solar eclipse occurs when the moon passes between the Earth and the sun. The result is a giant shadow that sweeps across the Earth's surface.

This world has the opposite phenomenon. Instead of a giant shadow, this world has a giant deadly "spotlight" sweep across its surface.

Must achieve these effects:

1. Must produce enough thermal power to kill humans (who are not native) and some (or all) non-native animals efficiently enough that full exposure to the light is (near?) certain doom.


2. Should have easily-observed warning signs allowing vulnerable creatures roughly one minute to find shelter. None of the lifeforms on the planet have developed its meteorology well enough to predict them except by visual/thermal observation.


3. Must be natural/meteorological/astronomical in nature. (Nothing like a giant orbiting laser).


4. Most or all plants, fungi, etc (non-animals) should be able to survive.

These would be a plus:

1. The starlight should normally be white, but the spotlight should be red (best case), blue (next best) or orange (third best). Otherwise, just brighter.


2. Should occur a few times per week in at least one area on the planet.


3. Should not be precisely periodic. If two occurrences are 36 hours apart, the next one might be 34 hours after, or 50 hours after. There may be a complex pattern, but it shouldn't be plausible to figure out over the course of a couple of weeks.

A "spotlight" that covers part of the planet is preferred, but if being larger than the planet is easier, that may be acceptable.

Here are a few ideas I was toying with, but I'm not sure how realistic they are:

1. There are multiple small (or distant) stars in the system, each white in color, and multiple red moons surrounding the planet. The stars are clustered around the center of the system so there is still a day/night cycle (is this possible?) with a roughly Earthlike luminosity. The red moons reflect light on the surface more or less constantly but the intensity is relatively small compared to the normal light from the stars... Until enough moons reflect light from enough stars to the same spot on the planet's surface! I don't know if they can really reflect enough light for this to work though.


2. Because of chaotic atmospheric conditions, atmospheric refraction causes the (single) star's rays to focus on a small area. I don't know if the color objective can be accomplished this way though.


3. There is very thick cloud cover, but sometimes a hole opens that allows the full light of the star through onto part of the surface. Also not sure if the color objective can be accomplished.

A wobbling pulsar will do the trick.

Pulsars emit a lot of energy in narrow beams that come from their poles. The slowest ones fire every few seconds; make its tilt wobble so that it is not pointing at the planet all the time. In addition, wobbling causes the pulsar to shoot at different points of the planet's orbit through time. The planet is hit when the pulsar's beam path just happens to be passing by the planet.

If the pulsar is firing every few milliseconds (as is normal for them), it will seem like a continuous beam for observers.

Finally, to make the beam small enough that it doesn't cover the entire planet and more, justify it with lensing from nearby nebulas, the planet's atmosphere, and maybe a black hole between the pulsar and the planet.

Not entirely sure about the feasibility of this, but it's an idea I had when I read your question. I wonder if something like this would be possible through Gravitational Lensing. Essentially, this is when black holes (with enormous gravitational pulls) bend light around them, causing telescopic effects. I've linked the Wikipedia page for gravitational lensing as well as an article from Space.com below which you could read up on to give you a better idea of how it all works.

My idea though is that what if, just outside the edge of what can be seen from the planet, there's a system of black holes which pull light in such a manner that it's focussed into a thin beam, which cuts across the galaxy and occasionally burns its way across your planet? This would explain the huge intensity of the light as well as allowing a 'natural' explanation for how it's focussed so tightly.

Having it at such a distance would also mean that the appearance of the beam of light can't be predicted, as the people on the planet don't have the technology to either see that far into space, or understand what they're seeing. Besides, at such a distance that the black holes don't mess with the solar system's structure, the light would take very long to reach the planet. So when the beam lines up, its effects are only seen on the planet later (how much later depends on the distance. Centuries, or even millennia maybe).

As for the colour of the beam, we can assume the source of light is moving away from the black holes and the planet, which would cause Redshift. This would make a white light source look red to the observer. Frequency wise, we could assume that other objects in space (planets, dust clouds etc) often block the light from hitting our planet, but occasionally it slips through the gaps (like when you see the sun for a couple of seconds through a clearing in the clouds before it gets blocked again).

For non-animals to survive, perhaps they've evolved to feed on the huge light intensity and maybe even need it every few days to live? Or (depending on the history of the 'humans' on your planet) maybe everything else is evolved to survive the intense light to an extent, while humans aren't. Perhaps this is similar to how we have to wear clothes - we can't handle Earth's natural climates without external help. Maybe those caught outside of their radiation booths are killed rapidly, while those who stay inside are fine?

There's a lot of ways you can go with this idea and I think the rest is up to you. I've included a few links at the bottom you might be interested in.

Further Reading:

Gravitational lensing:

https://www.space.com/39999-how-gravitational-lenses-work.html

https://en.wikipedia.org/wiki/Gravitational_lens

Red and blue shift:

https://en.wikipedia.org/wiki/Redshift

https://www.space.com/25732-redshift-blueshift.html

A transparent sphere works as a burning glass so a moon of (impossible) clear material should do the trick by concentrating the rays from the sun if it orbited at the right distance.

Trouble is that absorbtion would eat most of the light if the diameter was more than a kilometer. A thin ice-shell might work, but good luck with explaining the origin (and stability!!) of that ;-)

The remains of an ancient Dyson swarm

Not quite natural, but mostly non-technological. If a prior civilization had constructed a Dyson swarm around the system's star, the light coming from the star might be heavily occluded. Assume the sun is >10X hotter than ours (or the planet is much closer in), and there are enough collector bodies in the swarm to block some 90% of the sun's light at any time. If they are close enough in toward the sun, there will be enough diffraction around each collector that they wouldn't cast visible shadows, and could only be observed by direct observation of the sun, which requires a minimum level of technology to avoid blinding yourself.

The spotlight effect would occur when resonances in the orbital periods of the different bodies in the swarm cause gaps in coverage. The creating civilization could have arranged this purposefully to provide sunlight to further flung planets/stations, or be coincidental. The apparent brightness would grow gradually as more pieces of the swarm leave the "hole" in the field, so the warning sign would just be a rapid but gradual increase in brightness.

The spotlight color would probably be the same as the normal sunlight. However, if the star is very hot, heading toward blue spectrum, the swarm might occlude the blue/UV portion of the spectrum more and let redder light through (imagine if each collector is a giant solar array panel with no backing, e.g. microns of silicon). At the very least they would radiate heat in the infrared. If natives to the planet are used to these conditions, that might be their normal "white" light.

Any sort of passive light-focusing (with lenses, mirrors, etc) scheme is unlikely do more than to make slightly warm spots. The fundamental reason has to do with the conservation of etendue, and you can read more about it at Would a Moon made of water pose a threat to Earth during eclipses?

As such, if you want the spotlight to come from a moon, the moon would either need some kind of power source (which starts to sound like "giant lasers") or would need some natural mechanism to eject jets of energy or matter. As far as I know, all kinds of astrophysical jets would require something much more massive than a moon, so this seems like a dead-end.

I think the most feasible explanation is a planet which is ordinarily protected by its atmosphere and/or magnetosphere, but on occasion the weather aligns such that the protection is lost in an area. We to experience this to a small extent on earth: both the sun and earth have magnetic fields that vary over time. One trouble is if the earth's magnetosphere were periodically penetrated by the solar wind, the atmosphere would be stripped away. Though it could take a very long time -- perhaps it is interesting for your story to have a "dying planet".