For the first time, the presence of planet-sized objects has been confirmed beyond the Milky Way. Astonishingly, the suspects are billions of light-years away, immensely abundant, and thought to be floating through space without stars.
In the 26 years since we first located planets outside the Solar System, we've found some using the “Doppler wobble” method around nearby stars, including the nearest. By studying dips in a star's brightness as planets pass across its face we have expanded the range.
To go beyond the galaxy, however, Professor Xinyu Dai and Dr Eduardo Guerras of the University of Oklahoma used “microlensing”, based on gravity's effect on light. A massive object in the right position focuses the light from something further away, acting like a lens.
We use the gravitational lens formed by giant galaxies to study objects that would otherwise be beyond the reach of our telescopes. Microlenses, as their name suggests, are smaller, and occur when a star and planets pass in front to a more distant source. The lens created by the star itself creates a bump in the light that is large enough for us to notice, drawing attention to the much smaller blips produced by planets before or after.
Microlensing has allowed us to find a planet with the mass of Uranus 25,000 light-years away, a quarter way across the galaxy. In Astrophysical Journal Letters, Dai and Guerras announce they have left that achievement in the shade by finding a population of planets in a galaxy 3.8 billion light-years away.
The galaxy lies almost half-way between us and the quasar RXJ 1331-1231. The galaxy's gravity creates a lens, whose effects vary as the objects within it move. Examination of the spectrum of light from the quasar makes clear the lens is made up of millions of individual objects, all shifting relative to each other, rather than a single gravitational well.
When Dai and Guerres created a model of how the lens should behave if the galaxy was purely made up of stars and brown dwarfs, the outcome didn't resemble what they saw. At this distance, planets near stars effectively add their mass to that of the star for lensing purposes. Adding billions of objects with masses that range from that of the Moon to Jupiter, located in the galactic halo unbound to any star, created a much better match.
The combined mass of these planet-sized objects in the galaxy appears to be at least 0.001 of the mass of stars, suggesting either thousands for every star, or hundreds of larger objects.
"There is not the slightest chance of observing these planets directly," Guerras said in a statement. "However, we are able to study them, unveil their presence and even have an idea of their masses. This is very cool science."