I am working on a RPG setting which requires my planet having a nearby flare star. I understand from reading through other, similar questions and from doing some research, that flare stars are particularly difficult to pair with an Earth-like planet due to many reasons. The distance at which the planet should be placed would make it quite hard to combine with an Earth-like planet, similar in most aspects to our own.

Which is why I thought I could avoid/resolve this issue by having my planet orbit two stars: a binary system formed by a sun-like star and by the flare star. My question is about the placement of these bodies, and I am willing to have my Earth-like planet be an Earth-like moon on another planet (i.e., a Jupiter-like planet, for example?) if it would help with the placement of both stars.

Where should I place my planet in relation to the flare star for the flares to be "felt" on the planet but still be an Earth-like planet? I would like to have flares be a catastrophic event, not unlike some of the major tsunamis, earthquakes, volcanic eruptions or hurricanes that affect our planet with relative frequency: they can be tragedies of immeasurable proportions, but they will not end our civilisation.


1 Answer 1


In binary star systems, planets can either orbit one of the stars in a S-type or noncircumbinar orbit, or orbit both of the stars in a P-type or circumbinary orbit.


If the planet is habitable, it should orbit within the circumstellar habitable zone of one of the stars if it has a S-type orbit, or the combined circumbinary habitable zone of both of the stars if the planet orbits both stars in a P-type orbit.

If you want to be certain that the planet will remain habitable, you might want to give the planet a S-type orbit in the circumstellar habitable zoneof one of the stars, and give the flare star a very distant orbit. You need to find the rang ein the absolute magnitudes of flare stars. If flare stars of a certain type only have flares of a certain range of luminosity you can separate the stars far enough that the flars from one star will not harm life on the planet orbiting the other star.

But if flare stars are not divided into sub types with different luminosity flares, if any flare star might occasionally emit a flare at least as luminous as the most luminous flare ever recorded, then you have to calculate what would be the minimum distance to be safe from the greatest flare ever known, extrapolate how much larger the largest possible flare a star could emit over billions of years would be, and increase the distance between the stars to at least one where that level of flare would be survivable.

A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in the stars' atmospheres. The brightness increase is across the spectrum, from X rays to radio waves.


You need to know if stellar flares include large increases in the gamma rays emited from the star. Intense gamma rays would be stopped by Earth's atmosphere, but would breakdown ozone in the upper atmosphere, thus enabling the ultrviolet rays from the Sun to penetrate to the ground and slowly kill all life on Earth.

Alpha Centauri C, or Proxma Centauri, is about 13,000 Astronomical Units (AU) from Alpha Centuari A & B. That is about 0.063 of a parsec or about 0.205 of a light year. For many decades it wasn't known whether Proxima was gravitationally bound to A & B and orbited them, or was just passing close by them. In 2017 it was shown that Alpha centauri C does orbit around Alpha Centauri A & B.


Thus it is possible to have a fictional binary where the two stars are separated by as much as 13,000 AU.

And there is at least one possible example of a system with a much greater separation of the components. Fomalhaut,or Alpha Piscis Austrini, has two companions. Fomalhaut B, or TW Piscis Austrini, is about 0.28 parsec or 0.91 light year from Fomalhaut. Fomalhaut C, or LP876-10, is about 0.77 parsec or 2.75 light years from Fomalhaut A, and about 0.987 parsec or 3.22 light years from Fomalhaut C. They are considered to be a gravitationally bound trinary star. Of course Fomalhaut A has almost twice the mass of the Sun and can hold on to orbiting stars at a greater distance.

So you can put the 2 parts of your binary so far away that the flare star has no effect on the planet orbiting the other star.

The flare star might be so dim and so far away that it might be invisible from the planet, except when it is flaring.

Or flare star be bright enough and close enough that it is seen very dimly at normal luminosity, and only looks bright when it is flaring.

Or the flare star might be one of the brightest stars in the planet's sky when it is normal, and be super bright when it is flaring.

You can bring the flare star closer and closer for the purposes of your story, making it brighter and brighter as seen from the planet, in both its normal and flare conditions.

But the closer the flare star is to the planet, and the brighter it appears from the planet, the greater will be the probabaiity that the flares, especially the rarest and most luminous flares, will destroy the habitabiity of the planet and kill all life.

So if you want the flare star to be as close to the planet and appear as bright as possible from the planet, you will need calculations to show how close and bright that can safely be for the planet and its life.

Or maybe in your story the flare star is too close for the good of life on the planet. Perhaps over billions of years the flars have increased in power, and many of the stronger ones now damage life on the planet. So the people of the planet have been bulding spaceships and travelling to colonies in other star systems or in distant artificial space habitats. And maybe the build up to a flare many times stronger thant the strongest previus one is detected, and the characters will know they have just a short time to board spaceships and escape from the planet.

And possibly you might want to make the planet both of the stars in a P-type orbit around both of them.

Possibly both of the stars are flare stars, and their flares have never been synchronized before, but now they are not only increasing in power but happening closer and closer to being simultaneous.

Or maybe the other star is a type G star many times as luminous as the flare star, so that the flares from the flare star are comparatively weak compared to the luminosity of the main star.

And possibly when the main star is below the horizon, and the flare star is low above the horizon and can be seen clearly, its flares are considered to be important omens by the people on the planet.

Sunspots and solar flares usually originate in active regions of the Sun. The temporary active regions appear and disappear during the approximately 11-year solar cycle.

As each cycle begins, sunspots appear at mid-latitudes, and then move closer and closer to the equator until a solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram. Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots is calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-series diagram.


So if flares on the Sun, and on flare stars, follow the same pattern, and if their radiation and charged particles travel in straight lines away from the star, it would be safer to have one of the poles of the fole star star ponted toward the planet during a powerful flare.

And possibly a writer might be able to design a star system where the orits and axial tilts of the stars, and of the planets, are arranged so that the pole of the flar star is almost always poointed away from the habitable planet. But perhaps now the period when the pole of the flar star is almost always pointed at the habitable palnet is ending and the period when the equator is sometimes pointed at the planet is about to begin.

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    $\begingroup$ The flare star as a bright star that sometimes gets crazy bright was the answer I was going to post. And very seasonal too! $\endgroup$
    – Willk
    Dec 12, 2021 at 1:03
  • $\begingroup$ Thank you very much M. A. Golding! (I couldn't figure out how to tag your name) It is a very comprehensive answer, and it gives me some good starting points to figure this out. At a glance, it would seem the easiest option would be to have my Earth-like planet orbiting my sun-like star, and the flare star also orbiting it at a relatively safe distance. I liked the idea of having a civilisation-ending event as a possibility though! Thanks again $\endgroup$
    – Stivsko
    Dec 12, 2021 at 9:58

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