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Discovered a possible extragalactic exoplanet outside the Milky Way

NASA's Chandra telescope has detected an extragalactic exoplanet, the first planet belonging to another galaxy.

Categories Astronomy

The extragalactic exoplanet discovered would be among the first planets outside our galaxy to be detected. It is similar in size to Saturn and is located on the galaxy Messer 51, 28 million light years from the Milky Way. Its discovery belongs to a group of scientists led by Rosanne De Stefano of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, USA. The researchers used an innovative method that studies X-rays observed by Nasa’s space telescope.

extragalactic exoplanet
Chandra Telescope. Credits: NASA

The discovery of this galaxy was made by Charles Messier back in October 1973. The Messier 51 galaxy is one of the most famous spiral galaxies known to date. Among the observable galaxies, it wins the primacy also for its brightness, with an amplitude between 50000 and 100000 light years. Its conformation is characterized by a central bulge surrounded by a disk. The central bulge is populated by old stars, while the disk is characterized by new star clusters or gas clouds.

In 2020, the group of scientists observing the eclipses of a X-ray binary star realized that minor eclipses were present due precisely to the presence of the exoplanet.The magnitude of this discovery lies in the fact that this is the first confirmed extragalactic exoplanet (or nearly so) orbiting a star outside our galaxy. Just think that until 2016 there were no planets to which it was possible to attribute the adjective extragalactic due to their difficult identification. To date, there are more than 5000exoplanets belonging to the Milky Way at a distance equal to or no greater than 3000 light years from Earth.

How did they detect the extragalactic exoplanet?

The technology that led to the discovery of this planet is based on the transit method which definition is not very far from the famous “sun eclipse” of our solar system. The definition of the method is as follows: every time the planet comes between the observer and the star that hosts it, it causes a lowering of the brightness of the latter. In fact, as stated in the study published in Nature Astronomy, the Chandra telescope showed a decrease in the light flux measured in X-rays. Figure a shows the eclipse lasting about 20 ks (about 334 minutes). It is an enlargement of figure b which shows the spectrum for the entire duration of the observation.

extragalactic exoplanet
Chandra X-ray light curves. Credits: Natureastronomy

By comparing the distribution of the radius produced by the exoplanet with that of other planetary objects, it was possible to trace the nature of the transiting object to a planet. In particular, the probability that such an object has a smaller mass than the planet Jupiter is very high.

The discovery needs confirmation: it could also be a cluster of dust and gas. The latter hypothesis has little certainty to exist, as stated by Julia Berndtsson of Princeton University. Confirmation, however, may come only in 70 years, when the new Saturn will return to transit in front of its star. In addition, although the ranking puts the new planet on the same plane compared to Saturn, in terms of size, the former has a very long period of revolution being at a distance twice the relative position between Saturn and our star, the Sun.

The method used by scientists

The CCD spectrometer (ACIS) of the Chandra telescope from 2000 to l 2018 produced 16 observations, two of them were neglected because they related to a time interval too small compared to the duration of the entire observation. For each observation, the count rate of the presumed planet was calculated, taking into account the decreasing sensitivity of the ACIS-S detector.

Extragalactic Exoplanet Has a Companion

In 2009 a group of researchers from the University of Salento and Zurich had already identified the first extragalactic planet, located in the Andromeda galaxy at about 2538 million light years from our galaxy. For the study was made a computer simulation that mathematically treated the different effects recorded during the observation of Andromeda with the microlens gravitational. This is a smaller companion than the exoplanet under consideration, having a mass of 6 or 7 times that of Jupiter.

The search for exoplanets began about 25 years ago and only now have the appropriate technologies been achieved for their detection. The discovery of the extragalactic exoplanet, in fact, could prove to be the starting point. Moreover, the study required not only the use of advanced observational technology but also collaboration between experts in the field of X-rays and astronomers with expertise in binary stars and planets.

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