More than 60 years ago, Albert Einstein calculated that the gravity of a celestial body could act as a giant magnifying glass by bending the light of a more distant object behind it.
But he dismissed the idea as a theoretical exercise, saying there was "no hope of observing such a phenomenon directly," since the probability of observing such an effect within our Milky Way Galaxy is generally less than one in a million.
What a difference a few decades make. With the advent of powerful telescopes, scientists in the late 1980's began capitalizing on this phenomenon, known as gravitational microlensing. Astronomers employ the microlensing detection technique to collect clues on things they can't observe directly, using this natural phenomenon to hunt for a whole range of unseen celestial material and objects, from dark matter, to extrasolar planets, to wandering stellar-mass black holes.
Now, for the first time, a telescope has penetrated the jam-packed core of a cluster of 10 million stars to search for microlensing events. Astronomers used the Hubble Space Telescope to search for unseen lightweight bodies - planets or "failed stars" called brown dwarfs - in the core of the globular cluster M22 by looking for their gravitational effects on the light from distant stars behind them. (A brown dwarf is 80 times more massive than Jupiter. It's called a failed star because it doesn't have enough hydrogen in its core to shine as a star.)
Here's how microlensing works: As an unseen body floats across the face of a background star, it acts like a powerful lens by gravitationally bending the starlight and thus creating two separate images of the faraway star. Even Hubble can't resolve these images, because the bending angle is about 100 times smaller than the telescope's angular resolution. But the object's gravity also amplifies the starlight, causing it to brighten as the body passes in front of the star.
In the Hubble telescope observation, most of the background stars reside in our Milky Way's central bulge. Hubble monitored 83,000 stars every three days for nearly four months. The thousands of stars near the cluster's core are so tightly packed together that only Hubble's sharp eyes can resolve them. Monitoring the stars in the central bulge, the orbiting observatory detected six "mystery objects" in the cluster that eclipsed the light from those background stars. In each case, a background star jumped in brightness for less than 20 hours before dropping back to normal brightness. These short eclipses mean that the objects must be much smaller than a normal star, perhaps as small as 80 times Earth's mass. Objects this small have never been detected through microlensing observations.
Astronomers determined that the mystery objects are adrift in the cluster, meaning that they're not orbiting stars. Otherwise, these dark denizens would have been buried in the glow of their parent stars and wouldn't have been found.
Hubble also discovered another microlensing event in which a dwarf star in the cluster transited the face of a background star in 18 days.
Through these microlensing events, astronomers can estimate a mass for an unseen body based on the duration of the eclipse and the amount of brightening of background starlight. For example, a brown dwarf 80 times heftier than Jupiter would transit a background star in about 15 days; a Jupiter-mass object, about 1.5 days.