First extragalactic exoplanet discovered?

© Artist’s impression of the supposed exoplanetNASA/CXC/M. Weiss

Robert Klatt

All known planets revolve around a star in the Milky Way. Now astronomers may have discovered the first extragalactic exoplanet 28 million light years away.

Cambridge (U.S.A.). All known planets revolve around a star in the Milky Way, which is also Earth’s home galaxy. Scientists Harvard-Smithsonian Center for Astrophysics (CfA) have now discovered an object in the star system M51-ULS-1 that could be the first known extragalactic exoplanet. In your Message (PDF) the astronomers write that the object, which is about the size of Saturn, is 28 million light years away from Earth. According to data from German aerospace center (DLR), the previous record holder is “only” 22,000 light years away from Earth. The object that has now been discovered is therefore more than a thousand times further away from the earth.

As the scientists explain, they were able to discover the object because it is in an X-ray binary star system. Such systems are millions of times brighter than the sun, but have relatively little radiation in a small space. The passage of an exoplanet can therefore completely obscure the X-rays for observers on Earth.

Systematic search for X-ray binary star systems

The discoverers of the object searched with the NASA Chandra X-ray Telescope targeted for such X-ray binary star systems. They were successful with the M51-ULS-1 star system, whose X-ray emissions sank to zero for more than three hours. “Our discovery of the planetary candidate gives us the first insight into external populations of planetary systems and extends the range of the planet search to distances that are about 10,000 times further away,” explains study director Rosanne Di Stefano.

Verification of the exoplanet is problematic

Whether the object is actually an exoplanet cannot be verified by research with the available options. You would have to wait for another passage of the supposed exoplanet in front of the double star. In the long orbit, however, this will only happen in about 70 years. “And because of the uncertainties about how long it will take, we don’t know when to look,” said Nia Imara of the University of California at Santa Cruz.

Some astronomers also criticize the inference about a putative exoplanet, including the physicist Hugh Osborne, according to which the NASA TESS telescope never discovered a comparable orbital period, although the telescope observes more than 100 million star systems a day.

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