Signals that may be planets outside the Milky Way were found using NASA’s Chandra X-ray Telescope.
Astronomers have found signs of a planet, which may be the first to be discovered outside the Milky Way Galaxy.
So far, nearly 5,000 “exoplanets” (planets that orbit stars far from the Sun) have been discovered, but all of them are within the Milky Way Galaxy.
Signals that may be planets outside the Milky Way were found using NASA’s Chandra X-ray Telescope. It is located in the Messier 51 Galaxy—a distance 28 million light-years from the Milky Way.
The new results are based on transits, when a planet passes in front of a star, blocking some of the star’s light and producing a distinctive flicker that can be detected by telescopes.
This method has been used previously to find thousands of exoplanets.
Dr Rosanne Di Stefano and her colleagues have been searching for flickering X-rays of an object known as an X-ray binary.
These objects usually contain neutron stars or black holes that draw gas from a nearby orbiting companion star.
Matter near a neutron star or black hole becomes very hot and shiny in the X-ray range.
Because the region that produces the bright X-rays is quite small, a planet passing in front of it or blocking it makes transits easier to detect.
The team members used this technique to detect candidate exoplanets in a binary system called M51-ULS-1.
“The method we developed and implemented is the only method currently applicable for finding planetary systems in other galaxies,” Dr Di Stefano, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, USA, told BBC News.
“This method is unique and suitable for finding planets in the vicinity of the X-ray binary at any distance. From there we can measure the light curve.”
Hunt for future planets
The X-ray binary contains a black hole or neutron star orbiting a companion star about 20 times the weight of the Sun.
Neutron stars are the remnants of the core of an unusually large star.
The transit process takes about three hours. During that period, X-ray emission was reduced to zero.
Based on this, plus other information, astronomers estimate the candidate planet could be the size of Saturn orbiting a neutron star or black hole about twice Saturn’s distance from the Sun.
According to Dr Di Stefano, this method is successful in finding exoplanets in the Milky Way, but not when observing other galaxies. Partly because the long distance reduces the amount of light reaching the telescope. This also causes many objects to fill a small space (which is monitored from Earth) making it difficult to sort and determine one star from another.
Using X-rays, says Dr Di Stefano, “there are probably only a few dozen sources scattered throughout the galaxy that we can sort through. In addition, subsets of these objects are so bright in X-rays that we can measure their light curves.
“Finally, [the determinant is] the large emission of X-rays from a small region that is partially or completely blocked by a passing planet.”
The researchers acknowledge that more data are needed to verify their interpretation.
One of the challenges is the size of the candidate planet’s orbit. Because the orbit is so large, the candidate planet will not cross its binary partner again in about 70 years. This means that it is impossible to make further observations in the near future.
Another challenge that has challenged the interpretation of candidate planets outside the Milky Way is the assumption by some astronomers that the flickering light is caused by a cloud of gas and dust passing in front of the X-ray source.
However, this can be ruled out because the character of the events that occur does not match the characteristics of the gas cloud.
“We knew we were making such a bold and compelling claim that we already knew other astronomers would be reviewing this very carefully,” said study co-author Julia Berndtsson of Princeton University, New Jersey, USA.
“We think we have a strong argument and the [review by other experts] process is how science works.”
Dr Di Stefano said the new generation of optical and infrared telescopes would not be able to deal with the problems of multiple objects and fading light flickers, so observations of the X-ray range may be the primary method for detecting planets in other galaxies.
However, he says, there is a method called microlensing that holds promise for identifying planets in other galaxies.
This study has been published in the scientific journal Nature Astronomy.
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