Is life possible on red dwarf planets?

Most of the exoplanets known today are located in red dwarf systems. However, many experts say that life is still impossible on them, even if we talk about worlds similar in size and light to the Earth. Other scientists disagree with them, which gives rise to disputes that have been going on for years.

Planets in red dwarf systems

One of the most controversial topics in modern astronomical science is the possibility of life in red dwarf systems. News that scientists have proved that it is very likely or, conversely, definitely impossible to find there appears quite regularly, which means that there is no consensus on this issue.

The primary source here is the fact that 75% of the stars in our Galaxy are red dwarfs. As a result, most exoplanet discoveries are associated with them. And the fact that these stars are dim further increases the chance of finding an Earth-like world in an orbit that provides similar temperature conditions on the surface, and often quite close to us.

And this kind of sensational news immediately leads to the idea that none of this matters because it is a planet in the orbit of a red dwarf… And here we need to go into a much more detailed analysis of all the arguments used by the parties in the dispute.

The first thing to realize is that red dwarfs are much smaller and colder stars, which means they are dimmer than the Sun. People who do not know much about astronomy may conclude that the problem is that their planets will be much colder than the Earth.

This would be true if a world of similar mass and chemical composition were located at the same distance from the star as the Earth. However, nothing prevents a planet from being so close to its star that even very weak rays heat it to temperatures at which life is possible.

Red dwarfs include very different stars: from Lacaille 9352 (spectral class M0.5V), which has 48% of the Sun’s mass and 3.7% of its luminosity, to DEN 1048-3956 (spectral class M8.5), which has only 7% of the Sun’s mass and 1.6 millionths of its luminosity. In other words, this name unites objects whose luminosity can differ by a factor of thousands.

And this fact means that the width of the planets’ orbits can also vary by tens or hundreds of times. But if astronomers say that a world is in the habitable zone of its star, it means that it receives enough energy from its star to allow liquid water to exist on its surface, regardless of how far the orbit goes from the star and how many Earth days a year it lasts.

Tidal entrainment, atmosphere, and hydrosphere

And this is where the first serious argument against the existence of life on red dwarf planets comes into play: tidal forces. The same ones that cause the rise of water in the oceans and the fact that the Moon is always turned to us with the same side. And they are proportional not to the force of gravity, but to its gradient, i.e., how much it changes.

And it changes the more, the closer the objects are closer to each other. Their most important characteristic is the “attempt” to slow down both objects so that they rotate synchronously, i.e., so that the rotation around the axis occurs at the same time as the rotation around the center of mass of the pair.

In the case of both red dwarfs and Earth-like planets, the “habitable zone” is located close enough to the star for tidal forces to do so in a few billion years. The result is that a year on a planet is equal to a day, and one side of the planet is always facing the star, while the other side is always hidden from it.

Even this statement is not 100% correct. The fact is that for the brightest red dwarfs, such as Lacaille 9352, the habitable zone is located far enough from the star that the massive Earth-like planet in it will not have time to synchronize its rotation in the time that the Earth exists. The change of day and night would be greatly slowed down, but not eliminated.

But most Earth-like planets in red dwarf systems should indeed have synchronous rotation or, as this situation is said to be, be tidally entrained. And it is from this that many people conclude that one side of exoplanets should be a desert heated by an endless midday, and the other side should be a permanent realm of winter and night. And only in a narrow strip along the terminator can life exist.

In expressing such views, however, the main condition for the existence of life on a planet similar to Earth is forgotten – the presence of large amounts of liquid water and oxygen. And water and air masses have one important feature. They actively absorb huge amounts of thermal energy, and after absorbing it, they begin to travel, trying to bring the entire system back to a state of equilibrium.

Earth’s cyclones and ocean currents are a manifestation of this very process. So why shouldn’t they balance the temperature between the lighted and darkened hemispheres of the tidally invaded planet, just as the Gulf Stream provides Europe with a milder climate than it would have had based on its geographic location alone?

How powerful these processes will be and whether they can make a big difference is the subject of very sophisticated simulations that take into account a large number of factors, from the composition of the atmosphere to the presence of land on the starlit side of the planet. They show that there are a large number of possible outcomes, but in general, we can say that there are options where most of the tidally trapped planet will remain habitable.

Planet orbit and libration

The next point to understand about the debate over the habitability of a red dwarf system is that the necessity of a planet’s transition to a tidally trapped state applies only if its orbit is close to perfectly circular, i.e., its eccentricity is close to zero.

If the eccentricity of the planet’s orbit is significant, i.e., it is elongated, it is impossible to perfectly synchronize its daily and annual rotation, since the speed of the planet’s orbit varies significantly throughout the year.

This will lead to libration – the planet “wobbling” in the east-west direction relative to the position in which it is supposedly always turned to its star. If its axis also deviates from the perpendicular to the orbital plane, then libration will also occur in the south-north direction.

How important this mechanism is is well illustrated by the example of the Moon. The eccentricity of its orbit is only 0.055, and its rotation axis is tilted by 6.58° relative to the perpendicular to the orbital plane. This is enough for 59% of its surface to be facing the Earth at least occasionally, and not 50% as one would expect.

If a planet in the habitable zone of a red dwarf has an orbit with a large eccentricity, it can synchronize its annual and diurnal rotation not in a 1:1 resonance, but in a 1:2 or 3:2 resonance, which means that at least sometimes the Sun will be visible over its entire surface. Of course, in this case, the day will be equal to several Earth days, and the elongated orbit will mean dramatic changes in illumination, but these conditions, although extreme, are still not a barrier to the development of life.

The situation when a planet is constantly facing its star with one side turned is quite rare and occurs only under close to ideal conditions, when the orbit is almost circular and the axis of rotation is perpendicular to its plane. In the professional community, discussions of planets in red dwarf systems do not focus on this aspect.

Flares on red dwarfs

The main argument against looking for life on red dwarf planets is their activity. Most of these small stars occasionally produce flares that far exceed the solar flares we are used to. And while a short-term increase in brightness in the visible range can still be tolerated, frequent and very powerful emissions of high-energy particles pose a truly great threat to both life itself and the conditions for its emergence.

This is complemented by concerns that tidally entrained worlds should have extremely weak magnetic fields due to the same tidal entrainment.

The latter statement is highly speculative and based on a not-so-correct understanding of the nature of tidal entrainment. According to modern concepts, the magnetic field of an Earth-like planet is formed by the presence of a solid conductive core inside it, which rotates at a slightly different speed than the liquid conductive medium around it.

And the fact that a planet is tidally locked does not mean that it does not rotate on its axis. It must do so, albeit much more slowly than the Earth. How this might affect the strength of the magnetic field remains a question. To do so, we do not know enough about how conductive flows of matter are formed in the outer core of our planet. Perhaps the magnetic field will indeed be weak, perhaps not.

However, the main argument given in favor of the impossibility of life on red dwarf planets is that the flares on them are so powerful that even the Earth’s magnetic field would not stop them. Over hundreds of millions of years, their numerous repetitions could not only kill life on the planet, but also destroy its atmosphere and hydrosphere.

The atmosphere of the planet

There are many studies with calculations that show that most Earth-like planets in red dwarf systems would have to turn into dead rocks without water or air in just a few hundred million years. But many studies show that dense atmospheres and powerful hydrospheres can absorb flares for billions of years without disappearing altogether.

The destruction of the atmosphere and hydrosphere of planets orbiting red dwarfs remains a real possibility that must be taken into account. But it is not a fact, because no one has ever seen the destruction of a planet’s atmosphere by solar flares. The closest analog is Mars, which for billions of years was left without its gas envelope in conditions when, in the absence of a strong magnetic field, it was exposed to much weaker high-energy particles than on Earth.

But it is impossible to say for sure whether they were the main cause of this phenomenon or the Red Planet’s low gravity. After all, Venus’ magnetic field is also 10-20 times weaker than Earth’s, but its atmosphere is 93 times denser than our planet’s. For some reason, the Sun’s influence has not destroyed it.

It is also known that throughout the history of the Earth, its field has changed and weakened tenfold. But there are no signs that life on our planet became worse during these events. There are traces of increased radiation, but there are no traces of mass extinctions or climate change that could be linked to the destruction of the atmosphere.

Why is it important?

So, there is no definitive answer to the question of whether life is possible on red dwarf planets. Most likely, it can exist there, but no one can rule out the possibility that these stars create unbearable conditions for it.

The reason why scientists are so interested in what is going on around those little red stars is actually because the answer to the question about them has a big impact on understanding how widespread life is in the Universe.

Because even the closest planet to us, Proxima Centauri b, is a tidally trapped Earth-like world in the star’s life zone, for which we are not very sure how often outbursts occur there. And there are many such systems around.

And in the next decades, the question of the suitability of red dwarf planets may become a little clearer. After all, the study of exoplanet atmospheres by spectrographs is just beginning. And the mere presence of a gas envelope in a world orbiting a red dwarf, which generates powerful flares, will mean that fears about the latter are unfounded, at least for some cases.

And if they find at least water and oxygen there, it will definitely mean that these worlds should be taken into account in our search for extraterrestrial life, just like the rest of the planets.