- Paul Rincon
- BBC News Science Editor
If astronomers are correct, this will be the first planet discovered outside our galaxy.
So far, about 5,000 “exoplanets” have been identified – defined as worlds orbiting stars other than our sun – but all of them have been located within our galaxy, the Milky Way.
The possible signal for the planet discovered by NASA’s Chandra X-ray telescope is located in the Messier 51 galaxy, about 28 million light-years away from the Milky Way.
This discovery is based on the so-called transit, which A planet passing in front of a star blocks part of the star’s light and results in a certain decrease in its brightness which can be detected by telescopes.
This general technique has already been used to find thousands of exoplanets.
Astrophysicist Rosanne Di Stefano and her colleagues investigated the reduced brightness of X-rays received from the type of object they call Dual X-ray Bright.
These objects typically contain a neutron star or black hole that pulls gas from a nearby star in orbit. Material near a neutron star or black hole warms 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 X-rays, making transits easier to detect.
Team members used this technology to discover candidate exoplanets in a binary system called M51-ULS-1.
“The method we are developing and using is the only currently feasible method for discovering planetary systems in other galaxies,” said Di Stefano, who is part of the Harvard-Smithsonian Center for Astrophysics, a US astronomical and educational institute. BBC, Harvard University, USA.
“It’s a unique method, specifically designed to find planets around X-ray binaries at any distance from which we can measure the light curve.”
Searching for the future planet
This binary contains a black hole or neutron star orbiting a companion star 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 candidate planet will be the size of Saturn and orbit around a neutron star or black hole at a distance of twice the distance that Saturn is from the Sun.
Di Stefano says the techniques that have successfully found exoplanets in the Milky Way are disintegrating 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 be crowded into a small space (if viewed from Earth), making it difficult to resolve individual stars.
With X-rays, he explained, “there can be at least several dozen sources scattered throughout the galaxy, so we can identify them. Also, some of them appear so bright in X-rays that we can measure their light curves.”
“Ultimately, the massive X-ray emission comes from a small region that may be significantly obscured or (as in our case) completely obscured by a passing planet.”
The researchers quietly acknowledge that more data is needed to verify this interpretation.
One of the biggest challenges is that The planet candidate’s grand orbit will not pass in front of its binary companion for 70 yearsAvoid any attempt to make a short-term follow-up note.
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 that this is unlikely, because the properties of the event do not match the properties of the gas cloud.
“We know we’re making an exciting and bold claim, so we hope other astronomers will examine it very carefully,” said study co-author Julia Berndson from Princeton University in the US.
“We think we have a strong argument and that process in and of itself is how science works.”
Di Stefano said the new generation of optical and infrared telescopes cannot balance out clumping and darkening problems, so observations in X-ray spectrographs will still be the primary method for detecting planets in other galaxies.
However, he claimed that a method known as microlensing could also hold promise for the discovery of planets outside our galaxy.
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