28 million light years away: Planet outside the Milky Way detected


Astronomers have made a sensational discovery: the researchers, headed by the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge (USA), succeeded in detecting a planet in a galaxy other than our Milky Way for the first time. Using a new measurement method, they discovered it in the ” Whirlpool “Galaxy M51 (Messier 51) – at a distance of around 28 million light years!

In an interview with BILD, Ulrich Köhler, planetary geologist at the German Aerospace Center (DLR) explains how the scientists achieved this – and what it means.

“So far, planets have only been discovered in our own galaxy, the Milky Way, now 4000. But it has always been assumed that there are naturally planets in neighboring or more distant galaxies that orbit stars, like our earth the sun. The only problem was to find them – but that has now been achieved. “

For several years, according to Köhler, the study of stars in galaxies in our vicinity, in addition to M51 – also M101 and M104 -, a total of 238 systems.



The “whirlpool” galaxy takes its name from its spiral shape. The square shows the area where the new planet was discovered Photo: NASA

So now the breakthrough with M51 – with the help of the Chandra Observatory from NASA and the XMM-Newton space telescope from the European Space Agency Esa, two X-ray telescopes. They circle the earth in a wide ellipse at a distance that varies between 20,000 and 129,000 kilometers.


The X-ray telescope Chandra was put into earth orbit by NASA in 1999 orbits the earth as a satellite at great heights, without any influence from the earth's atmosphere

The X-ray telescope Chandra was put into earth orbit by NASA in 1999, orbits the earth as a satellite at great heights, without any influence from the earth’s atmospherePhoto: NASA


XMM-Newton is an X-ray satellite from Esa: It has also been in orbit around Earth since 1999

XMM-Newton is an X-ray satellite from Esa: it has also been in orbit around the earth since 1999Photo: dpa

The researchers’ new approach

“If a planet passes a star on its orbit, there is a brief drop in light – this can be measured in our galaxy by up to 0.01 percent,” says Köhler. The problem: This method cannot be used with other, more distant galaxies.

In the case of M51, the researchers observed a binary star system with a neutron star or perhaps a black hole (a bright light in the illustration). This first permanently removes gases from the second, significantly larger star (light blue in the illustration), which heat up extremely in the process. X-rays are emitted, which – unlike light – are shorter-wave and more energetic and thus enable higher resolutions. This can also be measured over long distances.


The newly discovered extragalactic exoplanet (black in the illustration) is named M51-ULS-1

The newly discovered extragalactic exoplanet (black in the illustration) is named M51-ULS-1Photo: NASA

“As the planet passes by, the view of this constant stream of gas is briefly weakened, a so-called ‘dip’ in the measurement curve. The researchers have now succeeded in demonstrating these weakenings, which recurred regularly. Subsequently, this could only mean that something must have passed by again and again – in this case the planet, ”says Köhler.

What is the significance of the discovery?

“The goal of astronomers is to find out why there is the earth, with life and water – and is that exclusive to us? There are 200 billion stars in our galaxy alone, ”says Köhler. “The galaxy closest to us is Alpha Centauri, around four light years away – and one light year is around 10 trillion kilometers. But there are billions of galaxies that are much further away and where life could have evolved. However, they will never be accessible to humans because the drive systems are not sufficient. “

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