The First Candidate Planet Outside of the Milky Way

The location of the potential planet discovered in galaxy M51.

This image marks the general location of the potential planet discovered in M51.
Credit: NASA/CXC/SAO/R. Di Stefano et al.

NASA’s Kepler mission and other efforts have shown that planets are ubiquitous in our galaxy. However, finding planets outside the Milky Way is a different challenge. Because the light from many stars in an external galaxy is packed into a tiny area of the sky, it is very difficult to detect the signature of a planet orbiting an individual star. An alternative approach is to use X-ray emissions from a black hole or neutron star in an external galaxy because a planet passing in front of a compact X-ray emitter can produce an eclipse. A short-duration dip in X-ray flux could therefore indicate the presence of a planet in much the same way that a transiting exoplanet blocks the visible light from a star in our galaxy.

A team led by Rosanne Di Stefano of the Center for Astrophysics, Harvard & Smithsonian found a short-duration dip in the emission from an X-ray source in the Whirlpool Galaxy (M51) using NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton space telescopes. The team first examined the possibility that this X-ray dip was caused by a transiting object other than a planet, for example, a cloud of gas and dust or a brown dwarf. However, these possibilities were eliminated because a typical brown dwarf is too large to explain the features of the X-ray dip, and clouds of gas and dust should produce changes in the X-ray color (total energy at different frequencies), but no change was observed in this case. The data are better fit by a transit model corresponding to a planet roughly the size of Saturn. This work suggests the discovery of the first exoplanet outside of the Milky Way and shows the potential for discovering exoplanets with X-ray space telescopes. The team now plans to search the archives of both Chandra and XMM-Newton to look for additional exoplanet candidates in external galaxies. READ MORE