Astronomers have found evidence of what may be the first planet to be discovered outside our galaxy.
So far, about 5,000 “exoplanets” have been identified – which lie outside our solar system, and orbit other stars – but all are located within our galaxy, the Milky Way.
potential planet The size of Saturn and detected by NASA’s Chandra X-ray Telescope, the US space agency. It’s in the Messier 51 galaxy, About 28 million light-years from the Milky Way.
The discovery is based on so-called 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 been used to find thousands of exoplanets.
Astrophysicist Rosanne Di Stefano and her colleagues looked for dips in the brightness of X-rays coming from a type of object known as a bright X-ray binary.
These objects often contain a neutron star or black hole that extracts gas from a nearby star. Matter near a neutron star or black hole heats up and glows at X-ray wavelengths.
Because the region that produces bright X-rays is small, a planet passing in front of it can block most or all of the rays, making it easier to detect traffic.
Team members used this technology to discover candidate exoplanets in a binary system called M51-ULS-1.
“The method we have developed and employed is the only currently feasible method for discovering planetary systems in other galaxies,” Di Stefano tells BBC News, part of the Harvard-Smithsonian Center for Astrophysics at Harvard University in the USA.
“It is a unique method, particularly suitable for finding planets around X-ray binaries at any distance from which we can measure the light curve.”
The future of planet hunting
This duo It contains a black hole or neutron star orbiting a companion star with a mass 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 decreased to zero. Based on this and other information, astronomers estimate that the potential planet would be roughly the size of Saturn—and would orbit a neutron star or black hole at twice the distance that Saturn is from the Sun.
Di Stefano claims that the techniques that have successfully found exoplanets in the Milky Way don’t work when looking at other galaxies.
This is partly because the large distances involved reduce the amount of light reaching the telescope and also mean that many objects are crammed into a small space (as viewed from Earth), making it difficult to identify individual stars.
Galaxy Messier 51 is located about 28 million light-years away from the Milky Way – Image: NASA/ESA/S. BECKWITH/HHT
Using X-rays, she explained, “there may only be a few dozen sources scattered throughout the galaxy, so we can identify them. Some are also so bright in X-rays that we can measure their light curves.”
“Finally, the massive X-ray emission comes from a small region that could be either massively or (as in our case) completely blocked by a passing planet.”
The researchers expressly acknowledge that more data is needed to verify this interpretation.
One challenge is that the candidate planet’s large orbit means it won’t cross in front of its binary companion again for about 70 years, nullifying any attempts to make a short-term follow-up observation.
Another possible explanation astronomers have 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 this is unlikely because the properties of the event do not match those of the gas cloud.
“We know we’re making an exciting and bold statement, so we hope other astronomers will look at it very carefully,” said study co-author Julia Berndson of Princeton University.
“We think we have a strong argument, and that process is how science works.”
Di Stefano said that the new generation of optical and infrared telescopes will not be able to compensate for clumping and dimming problems, so observations at X-ray wavelengths will likely remain the main method for detecting planets in other galaxies.
However, she said, a method known as microlensing could also hold promise for identifying planets outside our galaxy.
The study was published in the scientific journal Nature Astronomy.
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