Flare Stars

Flare activity is very common among the M-Type Stars, which is the most common type of stars in the Universe. M - type stars which lack flare activity are rather uncommon. Some stars, like UV Ceti, are known to be very active, others, like BL Ceti (which together with UV Ceti forms the system Luyten 726-8), have an average level of activity, while others, like Barnard's Star, are moderate. There are extreme cases of flares, but also a few M - type stars which only produce microflares.

Cause of flares

M - type stars, especially the smallest among them, are known to have convective currents, which permanently mix the core with the outer parts of the star. While this mixing process ensures that helium does not accumulate in the core and increases the lifetime of the star until basically all hydrogen is fused, it has a side effect: uneven magnetic fields. On the surface of these stars, large convection zones, associated with huge sunspots, often form, evolve and vanish. Just like in case of the Sun, sunspots influence the magnetic fields.

A flare is similar to a coronal mass ejection.

Overview

In most cases, a flare around an M - type star is similar to a Solar flare. Hoever, because these stars are so dim and any Habitable Zone Planet is so close, flares have a much stronger, even devastating effect on the planet.

Types of flares

There is no universally accepted classification of flares among the scientific community, even if terms like microflares, superflares or megaflares constantly appear on scientific papers. Strictly for the purpose of analysing the behaviour of flares on a terraformed planet, let's classify the flares based on their magnitude:

• Microflares. They increase the brightness of an average M4 star (like Barnard's Star) by an order of visual magnitude (2.5 times).
• Miniflares. They increase the brightness of a star 7 times.
• Average flares. They outshine the star 16 times.
• Big flares. Outshining the star 33 times, they are powerful.
• Superflares. Outshining the star 75 times, they are similar to the superflare detected on UV Ceti.
• Giant flares. They outshine the star 200 times.
• Megaflares. The brightest detected, they outshine the star 1000 times.

Evolution of a flare

The life and evolution of a flare can be divided into a few successive steps.

1. Grow. During the first 20 seconds, the flare forms and gradually increases in brightness until it reaches the maximum.
2. High plateau. This phase lasts 40 seconds, during which, brightness remains around maximum. Brightness fluctuates dramatically during this phase, with frequent dims and sparks, similar to a lightning during a thunderstorm.
3. Second plateau. After this, light drops to 40% fast and remains constant for a minute, without significant fluctuations. During this time, the flare expands in space.
4. Planet impact. Minutes after the flare erupted, the coronal mass ejection hits the planet. Charged particles (ions and electrons) penetrate whatever magnetic field exists and impacts the atmosphere, creating giant auroras, visible from the entire surface of the planet, magnetic storms and destroying electronics just like an electromagnetic pulse.
5. Final glow. Lasting half an hour to an hour after the initial flare, during this phase, the flare has 10 to 20% the initial luminosity as it dissipates through space.

Radiation hazard

A flare has many devastating effects on surrounding planets. Flare stars are known to glow in X rays with a similar intensity like the Sun. This happens because of flare activity, their corona is very hot, hotter then Sun's corona. However, because habitable planets are located 20 times closer to the star then the Earth is to the Sun, X radiation is 400 times stronger and strongly fluctuates with flare activity.

M - type stars are cooler then the Sun. Based on blackbody radiation models, their planets witness 100 times less ultraviolet radiation. However, flares are able to increase UV radiation levels, especially in the far UV part of the spectra, tens of thousand times above what the Sun releases at Earth's orbit.

A third hazard is represented by the Stellar Wind. M - type stars, while not releasing flares, have very weak stellar winds, which would create tiny heliospheres reaching to 10 or 20 AU away from the star. However, during flares, this changes dramatically. Because of flare activity, Proxima Centauri has a stellar wind 20% as strong as the Sun's. Flares propagate through space and extends the heliosphere of such stars up to 100 AU or even further. Because of flares, the environment around flare stars is dynamic, with plumes ejecting into interstellar space and gaps where cosmic radiation can advance close.

For a habitable zone planet, the stellar wind is 80 times stronger then the solar wind at Earth's orbit, with huge variations depending on flare activity. There is no way a magnetic field like Earth's can withstand this. The stellar wind will impact the atmosphere, especially during flares.

It is clear that flare activity will erode the atmosphere of habitable zone planets within millions of years. However, since a human civilization needs centuries or at best millennia to develop, an artificially created atmosphere might be breathable for long enough.

Types of stars

Flares are known to occur much often around young stars. Barnard's Star, which is older then our galaxy, has a less intense flare activity. Also, bigger stars are far more stable then smaller ones.

• M0 stars are safe, with only exceptional microflares.
• M1 stars are safe, with rare microflares.
• M2 stars are relatively safe, with miniflares.
• M3 stars are relatively safe, with miniflares and occasional average flares.
• M4 stars are active. Old ones are relatively safe.
• M5 stars are active, with average and big flares.
• M6 stars are unsafe, with frequent superflares.
• M7 stars are known for frequent superflares and even megaflares.
• M8 stars are known for the biggest flares recorded.
• M8 brown dwarfs are active, with occasional average flares.
• M9 brown dwarfs are less active, with occasional miniflares.

Flare simulation

Will a flare completely destroy life on a terraformed planet? Should we exclude planets like the ones around Proxima Centauri from future colonization programs? The best way to find out is to run a computer simulation. Depending on the magnitude and frequency of flares, results can be surprising.

As seen from Earth, flares occur without a warning. However, a solar observatory in orbit or on the surface of the planet would be able to spot early warning signs.

The simulation is done for a habitable zone planet orbiting Barnard's Star, which is a M4V star. The effect of flares can be milder or worse depending on star and how far the habitable zone is. In case of brighter stars, like Lalande 21185 (which is a M1 star), the habitable zone is 4 times further and the effect of flares is milder. By opposite, in case of DENIS, which is a dim M8 star and has the habitable zone much closer, the effect of flares is devastating.

Microflare

The most common are microflares, they erupt all the time. During a microflare, brightness of a star increases by one order of visual magnitude (2.512 times). Certain stars, like Proxima Centauri, produce microflares almost continuously. Settlers must routinely get used to them.

At 20 seconds after start, a microflare reaches its maximum brightness, sunbathing the surface with temperatures of 115 degrees C. This lasts for 40 seconds, then, for a minute, temperature drops to 65 degrees C. The radiation is enough to cause effects like sunburns, but it is not deadly. Plants, in most cases, can survive this heat. It is known that on Earth, in hot areas like the Sahara desert, sand can be hotter then 100 C, people can boil eggs or make coffee on the sand. So, a microflare will mot trigger an extinction event.

A few minutes after the flare, if the planet is in the direction of it, auroras can be seen both on the illuminated hemisphere and on the dark hemisphere, reaching as close as the equator. This is because charged ions and free electrons from the plume pierce through the magnetic field and reaches the atmosphere. Electronics will be affected and should be turned off.

After this, for nearly an hour, temperature will be 40 degrees C (assuming that before the flare it was 15 degrees C). This will trigger a few storms in the coming days, as the planet tries to reach a state of equilibrium.

During a microflare, it is enough if people go in the shade and wait a few minutes. Plant life will survive, animals will face a few sunburns but will not die. Microflares are expected to happen all the time. People should have with them protective gear, like sunglasses and metal-foil umbrellas.

Miniflare

A miniflare is about 7 times brighter then the star. Scientists should be able to detect changes in magnetic fields and be able to warn the population to take cover.

After 20 seconds of heating, for 40 seconds, temperature will be like 180 degrees C. In addition to that, organisms will face a huge dose of X and UV radiation. After this period, for a minute, illuminated ground will experience temperatures of 110 degrees C. However, if there are clouds, which can reflect a significant part of the radiation, the effect on the ground will be far milder. Then, within minutes, if the flare is in the direction of the planet, one will witness auroras everywhere. Charged particles will pierce through the magnetosphere and reach the atmosphere. For at least half an hour, temperature will be 60 degrees C.

Basically, the scorching heat and radiation of a miniflare works only for two minutes. If in this time humans and animals manage to get in the shadow, they can survive. The remaining will be like a heat wave that in the end will vanish. Still, a miniflare will affect plants and will cause significant losses to agriculture.

After a miniflare, storms are expected to occur. This is similar to what happens on Earth after a solar eclipse. The eclipse itself does not affect the movement of winds. It is the sudden dip in sunlight that cools an area which otherwise would be hotter, which creates a difference of pressure and temperature and triggers the formation of winds. A miniflare would create storms in a similar manner to an eclipse, by adding heat which destabilises the local climate.

Average flare

Outshining the star 15 times, an average flare occurs. Early warning systems tell all settlers to rush to underground shelters. Then, as a magnetic bubble collapses, in a flare starts and in 20 seconds reaches its maximum brightness.

For 40 seconds, the planet is scourged with temperatures of 280 degrees. Flashing lights, like a giant lightning, stretch out of the star, forming the flare. It is a bright, blinding light. This is not long enough to cause plants and most animals to die, but radiation can cause severe sunburns.

After this, for a minute, the effect of the flare is like being exposed to 180 degrees C for a minute. Life struggles to survive. Most tiny creatures, like insects and snails, are boiled.

The planet gets bombarded with charged particles, which create huge auroras. However, if the planet is lucky not to be in the way of the flare, only a few residual particles will impact the magnetosphere, which, if it is similar to Earth's, will be able to contain the blow.

In the last phase, lasting between half an hour and an hour, temperature is still high, 110 degrees C. Swedish experiments with sauna showed that people can survive hot dry air up to 180 degrees C. In the Sahara desert, during daytime, the sand heats-up over 100 degrees, so that you can boil an egg or make coffee on the sand. The air above the ground is also hot, but people survive. In hot air, moisture is the ultimate killer. If the air is hot and wet, human body cannot cool itself through sweating. So, it is questionable what animals might be able to survive. Plants will be severely affected. The impact on agriculture will be devastating. Still, trees are able to survive and they will blush again once the heat is gone.

The effect of radiation on living organisms will be devastating.

Another effect will be storms that will be created by the sudden increase of temperature on one hemisphere.

Big flare

If a big flare, outshining the star 33 times, is to occur, things do not look good for settlers. Everyone must go to underground shelters immediately. Once a magnetic field rearranges itself, the flare starts and in 20 seconds reaches its maximum.

For 40 seconds, the planet is scorched and heated to 390 degrees C. Animals get blind by the flashing light of the flare if they don't manage to take cover and fast. Grass and leaves get dry, but don't die yet. Animals experience heat burns on their skin, insects, worms and snails die at this point.

For the next minute, temperature drops to 260 degrees C. This is the point when many organisms die of hyperthermia. The same happens to humans which did not manage to take shelter. Clouds vanish as they heat and water droplets boil.

A few minutes later, the sky gets full of colour, with auroras visible all over the planet. Charged particles impact the atmosphere and cause a fantastic show of lights to start. High altitude lightning might also occur. As the flare vanishes, temperature is left to 160 degrees C for an hour. The planet will need time to cool down. Life will most likely die. However, seeds and roots will survive. Animals that managed to hide in burrows will come back out. Humans can step out in 50 hours and see what is left.

After the flare, the planet will experience massive wind turbulences as it tries to reach a new climate equilibrium. We must keep in mind that planets orbiting M - type stars are tidal locked, so one hemisphere is in perpetual daylight and one is in eternal darkness. We might see massive hot winds advancing to the dark hemisphere and melting the ice, while masses of cold air will advance towards the illuminated hemisphere, creating episodes of frost.

Superflare

Once an observatory noticed that a superflare is imminent, all alarms will be triggered, advising people to take shelter. And once magnetic fields rearrange, it takes only 20 seconds for the flare to reach its maximum brightness.

After the first 20 seconds, for 40 seconds, the flare outshines the star 75 times. Temperature rises to 500 degrees C. During this phase, clouds absorb light and vanish, as water heats and turns into gaseous phase. On the surface, the effect is like being in a fire for 20 seconds. Leaves dry, insects and snails are cooked, but tree trunks remain alive. Animals get massive sunburns, both from heat and radiation. Little birds die instantly, while big mammals survive, even with burns. The bright light, full with UV and X radiation, is enough to blind most creatures too. Humans get severe skin burns. Clothes get so hot that they ignite.

After this, light diminishes to 40%. Temperature decreases to 375 degrees C and stays like this for a minute. Dry grass and dry leaves ignite. Many animals lose consciousness, while many humans might experience heart failure. Due to massive UV and X radiation, brain activity in all animals is almost paralyzed. On the surface, paddles and small ponds might start to boil. Situation is different in deeper water, where only the upper layer is overheated. The air gets saturated with moisture.

During the next few minutes, the sky turns red or pink. Huge auroras dance all over the sky, all electronics breakdown or get burned, while fires rage all over. Temperature remains 250 C for an hour. The atmosphere becomes overheated and full with moisture, unbreathable, cooking all living creatures within it. Even if the flare event is over after an hour, the atmosphere needs time to cool down to breathable temperatures. On the surface, plants and animals die during this time, unable to breath and cool. Whatever life is left lies in lakes and seas. As hot water tends to rise to the surface, pockets of cold water remain inside lakes.

The only way humans can survive this is inside underground shelters. However, once they get out, a few days after the flare event, what do they find? All crops are burned, all life is extinct except for deep waters and all things in their houses are destroyed.

A human civilization can cope with a superflare occurring every few decades or centuries, but not with a star like UV Ceti which blows such flares yearly.

Climate will be severely affected. We must remember that planets around M - type stars are tidal locked, with one hemisphere facing perpetual day and one hemisphere of eternal darkness. As the daylight hemisphere will experience temperatures of 200 C, hot air will move to the dark side, causing significant meltdown, while cold air from the dark hemisphere will blow towards the daylight side, causing frost. Massive hurricanes can be triggered by the big difference in temperature.

Superflares are devastating, but they also help stop excessive accumulation of ice on the dark hemisphere. Without them, all or nearly all water will accumulate as ice on the dark side.

Giant flares

A giant flare outshines the star 200 times. They are common for dim M7 and M8 stars, but are also expected to occasionally happen around M5 and M6 stars once a few decades. They are devastating.

As the flare starts, it heats the ground to scorching 730 degrees C for 40 seconds and releases massive, lethal doses of UV and X radiation. One can notice that even for short exposures 700 degrees C is too high for life to survive. Everything hit by direct light is dead. Clouds heat-up and vanish, as water turns from liquid droplets into gaseous phase. Plants dry instantly, ponds evaporate and animals are killed, not only by heat which burns their skin and lungs, but also by radiation. Even in the sea, it is questionable if anything can survive except for the depths of the ocean.

Then, for a minute, temperature falls to 540 degrees C. Dead animals will be literally barbecued. Vegetation will ignite and massive fires will start all over the planet. All the moisture, evaporated, will act make the air unbreathable, killing also all organisms that managed to hide in caves and burrows. Surface of rivers will be boiling.

A giant flare is so powerful, that when its charged particles reach the atmosphere, no magnetic field can resist it. Sky will turn into purple, as nitrogen and oxygen will combine into nitrous oxide. The ionizing effect is so powerful that lightning becomes omnipresent, combined with auroras. Lightning occurs both on the hemisphere facing the star and on the opposite hemisphere. The flare will strip part of the atmosphere into space. While on the ground, atmospheric pressure might not fall more then 1%, this will trigger a planet-scale hurricane.

During the lasting hour, temperature will be 370 degrees C. Combined with all the moisture released previously, this is enough to sterilise life on the surface. The planet will need weeks to cool down until people will be able to step out of their shelters. They will find a dead world. The effect will be so strong that hot air will reach the dark hemisphere, melting large amounts of ice. Storms will come with heavy rainfall, which will alter some Geographic patterns. For many, the planet will be unrecognizable.

Megaflares

The largest flares detected outshine their stars 1000 times. They are really giant, able to destroy anything on a planet. What would happen on a terraformed planet hit by a megaflare?

After 20 seconds, when the flare reaches its maximum brightness, the illuminated hemisphere is bathed in lethal doses of UV and X radiation. Temperature on the surface reaches 1200 degrees C for 40 seconds. This is like the temperature inside a campfire. Then, for 60 seconds, temperature falls to 910 degrees C.

Life has no chance to survive in such conditions. The resulting hot air accumulates moisture and kills organisms sheltering in caves, microorganisms and seeds inside the ground, while life inside lakes and seas is destroyed by radiation. These flares are so massive, that they extend beyond the planet's orbit, heating the dark hemisphere too.

Temperature is just too hot for an atmosphere. Because of the huge doses of radiation involved, water breaks into hydrogen and oxygen. Even oxygen and nitrogen molecules in the atmosphere are split, creating a mass of charged ions. Because of the high temperature, ions achieve escape velocities, literally leaving the atmosphere. To make things even worse, minutes later, the planet is hit by a plume of charged ions and electrons, which are strong enough to penetrate any magnetic fields and further erode the atmosphere, breaking molecules and pushing ions away towards the interstellar environment.

After the plume hits the planet, for nearly an hour, temperature remains at 670 degrees C. This further helps degradation of the atmosphere. The dark hemisphere is heated by the flare as it propagates outwards and by hot winds from the illuminated hemisphere. All ice melts. The planet will need at least weeks to cool down until settlers can exit their bunkers.

After a megaflare, settlers will find their planet destroyed. The atmosphere might no longer be breathable and all life will be extinct, except maybe for the depth of the oceans.

Conclusion

Flares are violent events that have the power to destroy lives and human colonies. While a microflare might require only a bit of shelter, like hiding in the shade, a megaflare has the power to render a planet uninhabitable. Also, it is the frequency of flares that matters. It is good to have a superflare once a few decades or centuries, to melt the ice on the dark hemisphere. However, as shown above, even an average flare has the power to compromise crops and cause famine.

If flares are rare enough to allow crops to be harvested between them, superflares occur once in a few decades at maximum and megaflares don't occur at all, the star can be considered suitable for terraforming its planets. Most if not all M0 to M3 stars qualify this way, while some M4 stars qualify too. Most if not all stars dimmer then M5 are not suitable for terraforming their planets.

Life around a flare star is not easy. Flares have the power to destroy everything settlers have. Even for settlers that live not on a terraformed planet, flares have devastating effect, as they have the power to destroy equipment and passing spaceships.

In many aspects, like around a flare star is like gambling with death. Only the strongest will survive.