There are few methods to determine the existence of distant planets. They are so far away that no telescope can observe them, Dai told The Washington Post. So Dai and postdoctoral researcher Eduardo Guerras relied on a scientific principle to make the discovery: Albert Einstein's theory of relativity.
Einstein's theory suggests light bends when tugged by the force of gravity. In this case, the light is coming from a quasar — the nucleus of a galaxy with a swirling black hole — that emits powerful radiation in the distance.
Between that quasar and the space-based laboratory is the galaxy of newly discovered planets. The gravitational force of the galaxy bends the light heading toward the Milky Way, illuminating the galaxy in an effect called microlensing. In that way, the galaxy acts as a magnifying glass of sorts, bringing a previously unseen celestial body into X-ray view.
The technique was first used to first identify planets outside of our solar system but inside the galaxy, known as exoplanets.
“Microlensing is probably the only way,” Dai said.
In a university news release, Guerras had a less formal way to describe the complicated process: “This is very cool science.”
The photo that emerged is a modest image of the extraordinary find that Dai said will advance the study of planetary science. The central elliptical object is the galaxy where the planets reside. The linked white dots at the top and solo white mark at the bottom are the lensed images of active radiation pulsating from a black hole.
Dai estimated the distant galaxies contain 2,000 planets for every star. That means trillions of planets probably reside there, he said, consistent with the ratio of free-floating planets found in the Milky Way, which contains billions of planets.
“This discovery, if the interpretation of the data holds up, looks very exciting indeed,” Priyamvada Natarajan, a theoretical astrophysicist at Yale University, told NBC.
But other experts stressed caution about the findings. David Bennett, a gravitational lensing expert at NASA, said that the research was “interesting” but that there “might be ways to interpret the data that would not imply a large population of free floating planets in the lens galaxy.
“In particular, it appears that the authors have not investigated the possibility of the large population of brown dwarfs that microlensing results in our own galaxy imply,” Bennett said.
Microlensing can reveal surprising things, such as the same star exploding four times, but objects in space are not static and will not always have the perfect quasar backdrop to bathe a given object with arcing light. That gives limited time for a discovery to be made, with nearly boundless galaxies in the sky.
That might make this discovery all the more remarkable — the fact that it might not have happened at all. Dai was simply looking to study the environment in and around a black hole. He may have found something more.
“It was serendipity,” he said.
Correction: A previous version of this article incorrectly stated that NASA's David Bennett had expressed skepticism that the objects were not “extragalactic.” In fact, Bennett said the data could be interpreted to conclude objects other than planets were found. The article has been updated.