Signs of the first planet found outside our galaxy

Astronomers have found clues as to what may be the first planet ever discovered outside our galaxy.

Nearly 5,000 “exoplanets” – worlds orbiting stars beyond our Sun – have been found so far, but all of these have been located within the Milky Way galaxy.

The possible planetary signal discovered by NASA’s Chandra X-ray telescope is in the galaxy Messier 51.

This is located approximately 28 million light years from the Milky Way.

This new result is based on transits, in which the passage of a planet in front of a star blocks some of the star’s light and produces a characteristic drop in brightness that can be detected by telescopes.

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

Dr. Rosanne Di Stefano and her colleagues looked for dips in the brightness of X-rays received from a type of object known as an X-ray light track.

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

Because the region that produces light 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 spot.

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

“The method we have developed and employed is the only currently implementable method for discovering planetary systems in other galaxies,” Dr. Di Stefano, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, in the United States.

“It is a unique method, particularly suitable for finding planets around X-ray tracks at any distance from which we can measure a light curve.”

This binary contains a black hole or neutron star orbiting a companion star with about 20 times the mass of the Sun. A neutron star is the collapsed core of what was once a massive star.

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

Dr. Di Stefano said the techniques that have been so successful for finding exoplanets in the Milky Way break down when other galaxies are observed. This is partly due to the fact that the large distances involved reduce the amount of light reaching the telescope and also mean that many objects are clustered in a small space (seen from Earth), making it difficult to resolve individual stars.

With X-rays, he said, “there might just be several dozen sources scattered over the entire galaxy, so we can solve them. Also, a subset of them are so bright in X-rays that we can measure their light curves.”

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

Researchers freely admit that more data is needed to verify its interpretation.

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

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

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

“We know we’re making an exciting and bold claim, so we expect other astronomers to look into it very closely,” said co-author Julia Berndtsson of Princeton University, New Jersey.

“We think we have a strong argument, and this process is the way science works.”

Dr. Di Stefano said the new generation of optical and infrared telescopes would not be able to compensate for crowding and penumbra problems, so observations at X-ray wavelengths would likely remain the primary method for detecting planets in other galaxies.

However, he said a method known as microlensing could also hold promise for the identification of extragalactic planets.

The study was published in the scientific journal Nature Astronomy.

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