TheMadChemist

Microlensing is a light-bending phenomenon that occurs because anything that has mass warps the fabric of space-time, sort of like the sag a bowling ball makes when set on a trampoline. The effect is extreme around the heftiest objects, like black holes and entire galaxies. But even stars and planets cause a detectable degree of warping.

This animation illustrates the concept of gravitational microlensing. When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the warped space-time around the foreground star. This star acts like a virtual magnifying glass, amplifying the brightness of the background source star. If the nearer star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source. When astronomers find planets this way, they can measure their mass and orbital distance from their host star.

Here’s how it works. Light travels in a straight line, but if space-time is bent — which happens near something massive, like a star — light follows the curve. Any time two stars align closely from our vantage point, light from the more distant star curves as it travels through the warped space-time around the nearer star.

If the alignment is especially close, the nearer star acts like a cosmic lens, focusing and amplifying light from the background star. Planets orbiting the foreground star may also modify the lensed light, acting as their own tiny magnifying glasses. Astronomers see the effect as a spike in the star’s brightness.

This animation illustrates how a star and orbiting planet can lens light from a farther star when they align from our vantage point.

Even for powerful telescopes, it’s extremely difficult to see these little surges of starlight. For one thing, they’re rare — they rely on chance alignments between two widely separated and unrelated stars drifting through space. And the signals often only last a few hours before disappearing forever, so there is some luck involved.

Telescopes on the ground also have to peer through Earth’s hazy atmosphere, which can make images too blurry to tell different stars apart. Put all that together, and it’s little surprise that less than 5% of known exoplanets have been found using this method so far.

A rogue planet drifts through the galaxy alone, untethered to a star.

But microlensing can reveal a wide variety of worlds, including ones too small or far from their host star for other methods to detect. While other techniques are biased toward bizarre planets, like lava worlds and hot Jupiters, microlensing is best suited to finding familiar ones — worlds in their star’s habitable zone and even farther out. 

That includes ice giants, like Uranus and Neptune in our solar system, and even rogue planets — worlds freely roaming the galaxy unbound to any stars.

Most microlensing exoplanets discovered so far were found by ground based observatories, though our retired Kepler and Spitzer space telescopes found a few too. Soon our Nancy Grace Roman Space Telescope will begin surveying the cosmos, and its wide, sharp view will take us from dozens of microlensing planets to more than a thousand! Who knows how many Earth-like planets will be among them?