They detect signs of what could be the first planet discovered outside the Milky Way

The finding stems from an X-ray binary: a neutron star or a black hole that extracts gas from a companion star.

Photo: ESO / L. CALÇADA / copyright

If the astronomers are correct, this would be the first planet discovered outside of our galaxy.

Until now, about 5,000 “exoplanets” have been identified – defined as worlds that orbit stars other than our Sun – but all of them have been located within our galaxy, the Milky Way.

The possible signal of the planet discovered by NASA‘s Chandra X-ray telescope is located in the Messier 51 galaxy, about 28 million light years from the Milky Way.

This finding is based on the so-called transits, in which the passage of a planet in front of a star blocks part of the star’s light and produces a particular decrease in its brightness which is detectable by telescopes.

This general technique has already been used to find thousands of exoplanets.

Astrophysicist Rosanne Di Stefano and her colleagues looked for the decrease in brightness of X-rays received from a type of object they call X-ray binary shiny.

These objects typically contain a neutron star or a black hole that draws gas from a nearby orbiting companion star. Material near the neutron star or black hole overheats and glows at X-ray wavelengths.

Because the region that produces bright X-rays is small, a planet passing in front of it could block most or all of the X-rays, making the transit easier to detect.

The team members used this technique to detect the exoplanet candidate in a binary system called M51-ULS-1.

“The method that we develop and employ is the only currently implementable method for discovering planetary systems in other galaxies,” Di Stefano, who is part of the Harvard-Smithsonian Center for Astrophysics, an astronomical research and educational institute in the United States, told the BBC. Harvard University, in the USA.

“It is a unique method, specially designed to find planets around x-ray binaries at any distance from which we can measure a light curve.”

Future planet hunting

This binary contains a black hole or neutron star that orbits a companion star with a mass approximately 20 times that of the Sun. A neutron star is the collapsed core of what used to be a massive star.

The transit lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, astronomers estimate that the planet candidate would be about the size of Saturn and would orbit the neutron star or black hole at about twice the distance that Saturn is from the Sun.

Di Stefano says that the techniques that have been so successful in finding exoplanets in the Milky Way are broken down by observing other galaxies. This is partly because the large distances involved reduce the amount of light reaching the telescope and cause many objects to crowd into a small space (if viewed from Earth), making it difficult to resolve individual stars.

Messier 51

NASA / ESA / S. Beckwith / HHT
Messier 51 is also called the Whirlpool Galaxy (Whirlpool Galaxy, in Spanish) because of its distinctive spiral shape.

With X-rays, he explained, “there can be as few as several dozen sources scattered throughout the galaxy, so we can define them. Also, some of them look so bright in X-rays that we can measure their light curves.

“Ultimately, the huge X-ray emission comes from a small region that may be substantially or (as in our case) totally blocked by a passing planet.”

Researchers calmly admit that more data is needed to verify this interpretation.

One of the biggest challenges is that the planet candidate’s large orbit will no longer cross in front of its binary companion for about 70 years, avoiding any attempt to make a short-term follow-up observation.

Another possible explanation that astronomers considered is that the dimming was caused by a cloud of gas and dust passing in front of the X-ray source.

However, they believe that this is unlikely, because the characteristics of the event do not match the properties of a gas cloud.

“We know we are making an exciting and bold claim, so we hope other astronomers will examine it very carefully,” said study co-author Julia Berndtsson of Princeton University in the US.

“We think we have a strong argument and this process is itself how science works.”

Di Stefano said that the new generation of optical and infrared telescopes cannot balance agglomeration and darkness problems, so observations in the X-ray spectrum will remain the primary method for detecting planets in other galaxies.

However, he claimed that a method known as microlensing could also hold promise for detecting planets outside of our galaxy.

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