Why it’s so complicated to discover an exoplanet in another galaxy

Astronomers have not yet succeeded in discovering an exoplanet in a galaxy other than the Milky Way. This research is particularly complex.

The existence of an exoplanet in another galaxy has never been confirmed. Recently, scientists have identified a possible candidate in the Tourbillon galaxy, but it will take decades to verify its presence. It is particularly complex to identify an exoplanet inside a galaxy other than the Milky Way with certainty.

Different methods for detecting exoplanets

To understand this, we need to remember how these stars are currently being discovered. There are several detection methods and they have all a privileged parameter space “, Explains astrophysicist Anne-Marie Lagrange from the Paris Observatory, Research Director at CNRS, to Numerama. In all cases “The two important parameters are distance and mass of the exoplanet we are trying to discover, the astrophysicist continues.

  • The transit method (indirect method, based on the occultation of a star when its exoplanet passes in front of it) rather makes it possible to identify exoplanets of ” very small masses (once or twice the mass of Earth) and very close to stars (which they revolve around).
  • The method of radial velocities (indirect method of detecting the motion of a star) makes it possible to ” discover planets indisputably up to about 5 astronomical units is the distance from Jupiter to the Sun. ” Beyond that, it is often difficult and ambiguous says Anne-Marie Lagrange.
  • The direct imaging method makes it possible to reaches only planets if they are further away than 10 astronomical units from their star “.

… And their boundaries

Problem: These methods have limits when we seek to observe a very distant exoplanet candidate – which therefore includes cases where we want to search in other galaxies. ” When you go outside the galaxy, the stars are just too weak for these methods to work. sums up Anne-Marie Lagrange. In other words, there is not enough light reaching us.

“The radial method is interested in parent stars very close to us, in the inner suburbs of the Sun, up to a hundred parsecs. It is based on spectroscopy, that is, the separation of light by wavelengths. You must have many photons [ndlr : particule de la lumière] to be able to spread them “, The specialist continues. As for the transit method, you also need bright stars – so it also stays limited to stars in our galaxy.

Another method seems interesting to use to discover more distant and potentially extragalactic exoplanets: the gravitational lens (the presence of a massive body between a light source and the observer, which distorts the images we perceive from this source). But it also has its own limitations. ” The difficulty is that once we have discovered the planet, we can not study it in more detail. We know there is a planet around a star, but we will never see it again. We saw it because it and its star passed in front of another star, which was much more distant. It is an indirect observation and we can not make scientific follow-up on that planet. », Details Anne-Marie Lagrange.

This method therefore makes part of coincidences, which does not mean that it is not useful to scientists: this chance ” allows for statistics. The idea is not to study a particular planet, but by following many systems we can find the occurrence rate of the planets according to the distance. »

“Exploring a diversity we did not suspect 30 years ago”

Despite the difficulties, the search for extragalactic exoplanets, or simply farther from us in the Milky Way, is promising. Identifying such worlds would be very interesting to better understand the diversity of the ways in which exoplanets are born and evolve. ” If we succeed in getting an idea of ​​the distance to the star, this gives clues as to the processes of formation or survival of the planets. One of the most interesting things in exoplanetology is to know how planets form and to study the diversity of these formations. The planets that exist in the universe may not all be formed as the planets in the solar system “, Develops the astrophysicist. For scientists, it would be valuable to know the properties of such exoplanets, as well as their relationships with their parent stars.

Discovering exoplanets in other galaxies would therefore be a wonderful way to make comparisons explore this diversity that we did not suspect 30 years ago and which we still have not finished exploring today The prospect of discovering exoplanets outside the Milky Way, or even in more difficult-to-access environments in our own galaxy, could help identify possibly different physical conditions and see how this would affect exoplanet formation.

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