The transit method: the best way to discover exoplanets

The search for planets beyond the solar system started only some decades ago, and in November 2022, the number of discovered exoplanets (planets that are found beyond the solar system) exceeds 5,250. Nowadays, various techniques are used in order to detect exoplanets. The most used and the most successful of them is the one based on the transit of exoplanets around other stars.

From more than 5,000 confirmed exoplanets until now, about 3,500 of them have been observed using the method of transit. In this post we will see in what this method consists of, its advantages over other methods, and what it can tell us about the observed exoplanets.

What does the transit method consist?

When a planet passes between us and its star, the latter blocks a small quantity of the light that reaches us from the star, causing a very small fall in the brightness of the star (in the order of 1/10,000). The period of time during which the planet passes in front of its star is what we name a transit. Transits normally last between 2 and 16 hours.

Effect of a transiting exoplanet in the brightness of a star

By observing the brightness of the star in relation to the time we are capable of detecting these small drops in the brightness of the star, which indicates the existence of a planet around the star.

The advantages of this technique

One of the biggest advantages of observing the transits of exoplanets is that we can precisely obtain the size of the planet. This is possible by observing how deep is the drop in the brightness of the star: whereas a small planet will cause a small change in the brightness, a big planet would provoke a much more pronounced change.

Combined with another method called “radial velocity” (which gives us the mass of the planet), it is possible to determine the density of the planet. From this, we can also deduce the inside structure of the planet and its composition (for example, if it is a rocky or gaseous planet). The planets studied using these two techniques are by far those we have more information about.

Moreover, exoplanet transit also enables us to study the composition of the planet’s atmosphere. When the light of the star goes through the atmosphere, the obtained spectra can be analysed to determine which elements are present in it, which can give us clues about the chemical composition of the atmosphere.

Finally, by analysing the duration of the transit it is possible to obtain the distance at which the planet orbits its star.

Which are the disadvantages?

In spite of all the previously mentioned advantages, detecting exoplanets through transit also has some issues. For example, planetary transits are only observable when the orbit of the planet is perfectly aligned with the observer’s sight line. This means that the planet has to precisely pass between the star and the Earth to be observable, because, if it wasn’t the case, we wouldn’t see any type of drop in the brightness of the star. It is estimated that only 10% of the planets that are near their star can be observable using this technique. This percentage gets smaller as the planet is farther away from its sun. It is for this reason that the detection of exoplanets through transit is carried out by observing hundreds of thousands of stars at the same time.

Moreover, this method also gives out a great number of false positives: between 30% and 40% of the detections end up resulting false. To confirm that it is really a planet, the drop in brightness has to be periodic, and, if the same planet produces the transit, the change in brightness has to be always the same. So, the confirmation of a planet detected using this method requires of a lot of time, and, on many occasions, other techniques are also used to verify that it is really a planet.

Finally, another problem that has the exoplanet transit is the activity of the star. There are stars that are very active, and that regularly have string changes in their brightness. This makes really difficult the detection of exoplanets around this type of stars. Therefore, we usually observe stars that we know don’t have much activity.

In conclusion, the detection of exoplanets using transits presents some disadvantages, and is much more effective combined with other methods. However, it is still one of the most used techniques. We have detected more exoplanets with it than with all the other methods combined.

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