Red dwarf

In our galaxy, the Milky Way, there are about 300 billion (300,000,000,000) stars! Our Sun is orbited by 8 major planets, and if this were a typical situation for even 1 in 100 stars, it would mean that there are as many as 3 billion planetary systems around us. Sounds cool ;), but so far, only about 6,000 extrasolar planets have been discovered.

Among all stars, red dwarfs are one of the most common star types in our universe. It's due to their long-term existence. They are low-mass, so they produce relatively low luminosity. That's why they can produce energy much longer than other stars.

Red dwarfs are located in the lower right-hand corner of the HR diagram. This means that they are relatively cool and have low luminosity. Due to the low temperature inside the star, the opacity of the star (which determines how difficult it is for radiation to flow) increases. This is why energy is transported in red dwarfs by convection. Convection causes a lot of activity on the surface of the star. Red dwarfs are characterized by high variability and produce bright flares.

As red dwarfs are the most common type of star, they are easy to spot when observing the night sky. Planets are often discovered around them. The most common method used to detect this type of planetary system is the transit method — when a planet passes in front of a star, reducing its observed brightness. However, simply being able to tell whether a planet orbits a star is not very satisfying. You want to know more about the planet in question! From studying the brightness variation curves, it is possible to estimate the radius of a planet and its orbit, and even determine whether the planet might have an atmosphere. It is also possible to estimate the masses of the individual components of the system (based on TTV, Transit Timing Variations). That is something!

We already know the distance at which the planet orbits the star, its mass, and its radius, so we know its density. It is also possible that a study of the brightness variation curves will reveal the presence of an atmosphere on one of the planets. This is where it gets interesting. This encourages further research and increases curiosity. How can we learn more about the atmosphere of a planet a few tens of light-years away? Transmission spectroscopy comes to the rescue. It involves taking spectroscopic measurements of the host star before and during the occultation. Then, by subtracting the two spectra, a spectrum of the planet's atmosphere can be obtained. From such a spectrum, it is already possible to determine its composition in some detail and to provide factual support for theoretical considerations of its habitability.

Often, however, the host star is too bright, and modern telescopes have too poor a resolution to carry out transmission spectroscopy effectively. It would seem that systems orbiting red dwarfs would be the best candidates for this type of measurement — they are low in luminosity and so would not interfere with measurements of the planet's spectrum. Also, the red dwarf has a high signal-to-noise ratio. But here is the problem: red dwarfs are very active stars. It flares are short-lived but release high amounts of energy, which could alter the photochemistry of the atmosphere, making it harder to isolate the planetary signal. So, if you subtract the spectrum of the star before the eclipse from the spectrum of the star during the eclipse, you will not get the pure spectrum of the planet. The star can change its brightness during this short period. As a result, flares or superflares can appear in the spectrum.

And so we come to another turning point — more is needed. One idea for obtaining an undisturbed planetary spectrum is as follows:

• Study the nature of red dwarf outbursts.

• See how the outburst affects the planet's atmosphere (if an outburst is observed on a star, and after some time it reaches a planet that happens to pass in front of the star's disk, you can try to detect the effect of the star's outburst in the planet's spectrum).

• Subtract the changes in the planet's spectrum caused by the star's variability.

  
  

The plan sounds simple, but the execution is certainly complicated. However, in the near future, this concept will be improved, and in 2025, the Pandora satellite dedicated to this technique will be launched. Pandora will help correct for stellar variability during transits, improving measurements of planetary atmospheres.

The study of planets orbiting red dwarfs offers a wide range of possibilities for discovering nearby planetary systems. Although their detection and description are not as easy as they might seem, they provide a wealth of information and open up new possibilities. While the high activity of red dwarfs complicates transmission spectroscopy measurements, it is proving to be a key factor in creating favorable conditions for life on distant planets.