Amateur Astronomers Association of Princeton (AAAP) Monthly Meeting

Most of the exoplanets (4,651+) discovered to date were detected by looking for tiny diminutions in a star’s visible light when a non-luminous object transits in front of it. Almost all exoplanets and exoplanet candidates are within 3,000 light years of Earth, well within the Milky Way galaxy. Dr. Di Stefano and her colleagues may have detected signs of an exoplanet transiting a star outside of the Milky Way, in M51, the Whirlpool Galaxy, about 28 million light years away.

They did this by examining data from the Chandra X-ray Observatory to look for dips in the brightness of X-rays received from bright X-ray binaries in galaxies M51, M101, and M104. Of the 238 binary systems observed, one exoplanet candidate turned up, now known as M51-ULS-1. This binary system contains a neutron star or black hole orbiting a 20-solar-mass companion star. Because the region glowing in X-rays is small, a transiting planet could block much or all of its X-ray emission, making it easier to spot. In the case of M51-ULS-1, the X-ray emission decreased to zero in the space of three hours. The researchers estimate the exoplanet candidate to be about the size of Saturn at about twice Saturn’s distance from the Sun. So, a confirming observation is about 70 years away.