The first evidence that there may be a planet outside the Milky Way

This graphic shows the M51 using the left panel moon (purple and blue) X-rays and NASA optical light. Hubble Space Telescope (Red, green and blue). The box indicates the location of the candidate planet, an X-ray binary file called M51-ULS-1. An artist’s chart in the right panel shows an X-ray binary and a possible planet. The asteroid object (white and blue in the image) is drawn on the neutron star or black hole, forming a disk around the dense object (highlighted in red and orange). The object in the vicinity of the dense object becomes very hot and glows with X-ray light (white). The planet appears as it passes this X-ray source.

Possible paths. Photo credit: NASA / CXC / M. White

The search for its darkest light when a star passes in front of it is called a transport technique. For years, scientists have been using optical telescopes to find exoplanets that capture part of the light that humans can see with their eyes, and more. This includes both ground-based telescopes and space telescopes such as NASA’s Kepler mission. Optical transit detection requires very high sensitivity because the planet is much smaller than the star moving ahead of it, so only a small fraction of the light is blocked.

This light curve shows how X-rays from M51-ULS-1 fall to zero for a while during Chandra’s observations. Source: NASA / CXC / SAO / R.Distepano, et al.

The transit view is different in X-ray binary. Since a potential planet is the size of a neutron star or X-ray source orbiting a black hole, a planet passing through Earth’s line of sight can temporarily block most or all of the X-rays. This makes it possible to detect transport at a greater distance – even beyond the Milky Way – than photovoltaic studies with current transits. The diagram (above) shows the X-rays of M51-ULS-1 temporarily dropping to zero during Chandra’s observations.

Although this is a confusing study, the condition of an exoplanet in the M51 is not severe. One challenge is that the planetary candidate’s best orbit in M51-ULS-1 will not pass for nearly 70 years in front of its binary partner, which has thwarted all attempts for confirmation for decades. There is also the possibility of an X-ray reduction caused by a gas cloud flying close to M51-ULS-1, although the researchers firmly believe that the data are in favor of the planet’s interpretation.

Note: “Potential candidate planet in outer galaxy discovered by X-ray transit” October 25, 2021, Natural astronomy.
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The paper describing these results will appear in the latest edition Natural astronomy. Authors Rosenne DiStefano (CfA), Julia Berndsson (Princeton), Ryan Urkuhard (Michigan State University), Roberto Soria (University of Chinese Academy of Sciences), Vinay Kashab (CfA), Theron Carmichael (CfA) and Nia Imara (currently at UC Santa Cruz). NASA’s Marshall Space Aviation Center manages the Chandra program. The Chandra X-ray Center of the Smithsonian Astrophysical Observatory controls science from Cambridge, Massachusetts, and aviation from Burlington, Massachusetts.