An exoplanet in another galaxy? “It will take decades” to confirm this

Thanks to the Chandra Observatory, scientists have discovered an interesting clue: that of the potential existence of an exoplanet in a galaxy other than the Milky Way. But nothing has been confirmed: it is currently only a candidate.

We discovered a planet in another galaxy »,« An exoplanet discovered in another galaxy »: Are such claims really correct? These press titles accompanied the publication of a study in the journal NatureOctober 25, 2021, also disclosed by NASA on that day.

However, an essential word is missing, which appears both in the title of the study and in the press release: ” possible “.

“She is a candidate”

Have we just found an extragalactic exoplanet? For the astrophysicist Anne-Marie Lagrange from the Paris Observatory, Research Director at CNRS, ” Ithe simple answer is no “. Asked by Numerama, the expert points out that” what has been announced is a candidate. It will take a long time if we want to confirm it with the transit method: several decades. »

What exactly are the authors of the study publishing in Nature ? They indicate that they have identified traces of the possible presence of an exoplanet, in a galaxy other than the Milky Way, the galaxy M51 – also called the Tourbillon galaxy. As NASA’s statement sums up, it could be “ the first observed planet passing in front of a star outside the Milky Way “. This candidate was seen with Chandra, a space telescope that observes X-rays. The observatory discovered a short-term attenuation of this type of rays in a binary system. Scientists conclude that a possible scenario would be an exoplanet passing in front of an X-ray source, around a neutron star or a black hole, in orbit around a companion star.

An alternative access

We are not able, a priori, to look for very distant planets in other galaxiescomments Anne-Marie Lagrange. There was the art of not going and observing these stars as we usually do because their light would be too dim. The approach is interesting: go and observe in domain X. The transit detection method is often used in the hunt for exoplanets: when the star passes in front of its star, a small part of the light it emits is blocked by the planet. Specifically, we observe that the light signal from the star decreases temporarily. ” The transit method is based on repetitiondescribes Anne-Marie Lagrange. We see several times a decrease in the star’s flux, at regular intervals. The time difference between the two drops gives the period of the planet, therefore its distance. »

Here the method differs somewhat. ” An alternative accessreads the study, is to study the temporal behavior of X-rays emitted from bright extragalactic X-ray sources, where a planet in orbit will temporarily block the X-rays and cause a short eclipse. It is potentially one of the events they think they have seen in M51-ULS-1, a binary system in the Whirlpool galaxy. In their scenario, the potential exoplanet would have a size similar to the source of the X-rays: its passage could completely or almost completely block these X-rays (hence the use of the word eclipse). Such a method will therefore help to identify transits at far greater distances than today.

70 years waiting time for next transit

In the case of this candidate exoplanet, the method still poses a great challenge, sums up Anne-Marie Lagrange. ” The farther the planet is from its star, the longer the period. Here the modeling tells us that it would be far enough, meaning it would take about 70 years to observe the next transit. In other words, impossible to hope to verify the hypothesis that it would be an exoplanet for decades.

That is why we need to be so much the more careful about this communication, which nevertheless remains promising and offers an interesting new method of detection. ” The hunt for extra planets [sic]orbiting planets outside the Milky Way has now become a realistic and practical undertaking “, the authors of the study conclude.

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