Tour: Possible Evidence for First Planet in Another Galaxy

Astronomers have announced that they have found evidence for the first possible planet discovered outside of our Milky Way galaxy. To do this, they used a technique looking for the temporary dimming of X-rays from an object with NASA's Chandra X-ray Observatory. This intriguing result could open up a new window for exploring exoplanets — that is, planets outside our Solar System — at greater distances than ever before.

Until now, astronomers have found all known exoplanets — defined as planets outside of our Solar System — and exoplanet candidates in the Milky Way galaxy. Most of these have been less than about 3,000 light-years from Earth.

This new Chandra study looks at a system in the Messier 51, or M51, galaxy. This galaxy is nicknamed the "Whirlpool" galaxy because of its spiral shape. It is located about 28 million light years from Earth.

One system in M51 in particular caught the attention of a research team. This system, known as M51-ULS-1, belongs to a class of objects that astronomers call X-ray binaries. In these systems, a massive star is in orbit around either a neutron star or a black hole. Because the neutron star or black hole has such a strong gravitational force, it pulls material from the companion star into a disk swirling around the denser object. These disks glow brightly in X-rays because the material becomes superheated by the forces applied to it.

Because the region producing bright X-rays is relatively small, a planet passing in front of it could block most or all of the X-rays. This is known as a transit. Astronomers have been using transits to discover exoplanets within the Milky Way using optical telescopes for many years. However, these optical light transit searches need to be extremely sensitive because the planet blocks only a tiny fraction of the light from the star. In principle, the X-ray transit technique could allow astronomers to find exoplanets at much greater distances because the X-rays can completely temporarily disappear.

While this is a tantalizing result, it is not proof of the existence of an exoplanet in M51. Due to the nature of the suspected orbit, astronomers estimate they would need to wait at least 70 years for another transit to occur and confirm this current finding.

Regardless, this result provides motivation for researchers to look for more of these planet candidates in other galaxies and in our own, using X-rays.