When stars align, magical outcomes become possible. Microlensing, an astronomical technique based on trippy relativistic effects, is taking this lyrical sentiment to new and brilliant extremes by exposing chance alignments in the universe—and the unexplored frontiers that they reveal. 

The sheer variety of weird cosmic phenomena that can be captured by microlensing—and often only through microlensing—is positively head-spinning, to say nothing of what may be coming down the pike now that the tight-knit subfield is poised to hit its prime. Microlensing events are fortuitously situated right in the blind spot of other established methods of finding exoplanets, black holes, and other relatively small lurkers that wander far from the illuminating rays of any star. 

The technique has exposed worlds in wide orbits or free-floating through interstellar space and it remains the only way to find planets at extreme distances, including beyond our galaxy, the Milky Way. Isolated black holes, long hidden from view, are now observable through microlensing. The method offers unique insights about dark matter, an unidentified material that accounts for most of the universe’s mass, and is the main line of information on mysterious entities called MAssive Compact Halo Objects (MACHOs). Microlensing also has the potential to spot exotic structures, like wormholes or primordial black holes, that have never been seen before.

“It's really phenomenal that one probe, one type of technique, can actually be used to constrain so much of our field,” said Djuna Croon, a theoretical physicist and assistant professor at Durham University who uses microlensing in her research into dark matter.

Microlensing is a form of gravitational lensing, a phenomenon that occurs when one object in space, known as the “lens,” passes in front of a more distant object, known as the “source,” from our perspective on Earth. As light from the source travels through the gravitational field of the lens, it becomes warped and magnified by the curvature of spacetime, an Einsteinian interaction that can unlock a trove of insights about both the source and the lens. 

In this way, gargantuan structures, such as galaxy clusters, can project mesmerizing visions of background stars and galaxies that would be out of view without the aid of such handy “natural telescopes.” Gravitational lensing typically refers to events on these massive scales, in which the immense gravitational fields of galaxies or clusters produce images of extremely distant sources, like stars or quasars, as they appeared much earlier in the universe; for instance, the James Webb Space Telescope has captured a lensed image of a galaxy a whopping 21 billion light years from Earth. 

Microlensing, in contrast, occurs with smaller objects, distances, and timescales. Microlenses are typically in the mass range of planets or stellar-mass black holes, and sources are usually background stars within our own galaxy, or nearby galaxies. Alignments between microlenses and their sources are much more short-lived—typically lasting days, weeks, or months—compared to larger lenses that endure far beyond human timescales. 

The prefix “micro” doesn’t explicitly refer to these smaller scales, but rather means that microlensing events do not resolve an image of the background source like larger lenses. Instead, microlensing events simply cause a source object to brighten as it passes behind the lens.

“Microlensing is the same gravitational effect [as larger lenses] on a different scale,” explained Macy Huston, a postdoctoral researcher who specializes in microlensing at the University of California, Berkeley. “Typically this means a star in our own galaxy getting lensed by another star, black hole, or planet. On this scale, you can’t actually make out the individual warped images of the source, but instead you’ll see that the source star appears to temporarily get brighter and have its position changed very slightly as it passes behind the lens.”

The search for these events is fundamentally a game of chance that requires long-term observations of huge swaths of sky to capture fleeting conjunctions that may be visible for just hours or days in some cases. 

“Most modern telescopes peer in great detail at tiny specks in the heavens,” said the late astronomer Bohdan Paczyński, who pioneered the technique and coined the term “microlensing.” “They can’t keep track of what varies, pulsates, or flares up and disappears forever.”

The basic principles behind microlensing date back centuries to Isaac Newton, who ruminated about whether light could be bent by gravitational fields. But it only became a bonafide observational technique during the late 1980s and 1990s, as Paczyński and his colleagues recognized its potential to constrain models of dark matter. These early trailblazers established long-term surveys like the Optical Gravitational Lensing Experiment (OGLE), a project that has been running at the University of Warsaw since 1992 and which rapidly demonstrated that microlensing could uncover a dazzling range of elusive objects in the cosmic dark.

“Microlensing is most useful for detecting objects that cannot be discovered using other techniques,” said Przemek Mróz, a microlensing expert and assistant professor at the University of Warsaw. “I think it's fair to say that no one predicted that we would discover so many interesting things.”

For example, from its inception, microlensing has been used to study dim objects that fall under the catchall term MACHOs, a variety of dark compact objects, such as black holes, neutron stars, or brown dwarfs, which are a kind of failed star. MACHOs were once a leading candidate for dark matter, but microlensing helped to cast doubt on this hypothesis and propel the field toward newer dark matter models. Meanwhile, microlensing’s potential to spot planets was first realized in 2003 with the landmark discovery of a gas giant planet about 19,000 light years from Earth, while its ability to identify isolated stellar-mass black holes has only come into focus within the last few years. In addition to discovering singular objects, microlensing observations can be used to make and refine broader maps of large-scale structures, like the disk of the Milky Way.

But though it has been around for about 30 years, microlensing as a field is now experiencing a step-change in sensitivity that will have major implications for astronomy, cosmology, and the search for extraterrestrial life. In addition to surveys like OGLE or the Microlensing Observations in Astrophysics (MOA) project, the field is about to be supercharged by the introduction of next-generation approaches and instruments, such as NASA’s Nancy Grace Roman Space Telescope, due for launch in 2027.

“The field is still small and I know most of the people who are working on the topic,” said Mróz. “But in a few years from now, we will have Roman launched, so we need people.”

Exoplanets continue to be a major focus for microlensing surveys, especially in anticipation of Roman. Up until this point, most worlds beyond our system have been discovered through two techniques: the transit and radial velocity (RV) methods. Transits are the small dips in a star’s light that occur when a planet passes in front of it, and RV are the observable gravitational tugs that planets exert on their stars. Both methods indirectly detect planets through effects on their host stars, and are thus biased toward larger planets and smaller orbits, as those factors produce the most easily observed impacts.

Microlensing, in contrast, offers an ephemeral glimpse of all kinds of different planets that show up in all sorts of different places. For instance, scientists led by Takahiro Sumi, a microlensing expert and professor at Osaka University, published the first survey of microlensing events that detected planets with masses similar to Earth that are free-floating in interstellar space. Even smaller worlds, on the scale of Mars or Mercury, are expected to be spotted by new instruments. 

“Knowing only about bound planets is not enough,” said Sumi, referring to planets that orbit stars. “Only microlensing (especially Roman) can complete the statistical census of exoplanets as it is sensitive to low-mass planets at wide orbits or that are free-floating. No other technique can.”

In other words, exoplanets detected through microlensing will, for the first time, provide a sense of the broader abundance and variety of exoplanets that exist in our galaxy—not just those that are most detectable around stars. These include free-floating worlds that were ejected from their home systems through a variety of disruptions, including the passing migrations of large planets to outright collisions with other bodies. Some large planets, including brown dwarfs, may actually form in interstellar space by condensing from gasses in a scaled-down version of the birth of stars. 

Learning about the distribution and characteristics of these rogue worlds is critical to understanding planetary formation and therefore has implications in the search for extraterrestrial life, in part because microlensing enables astronomers to find a lot more terrestrial worlds that, like Earth, occupy orbits that overlap with stellar habitable zones. 

“Earth-mass exoplanets may be very common in the wider orbit where we don’t have any sensitivity yet,” Sumi said. He also speculated about whether microlensing could expose moons orbiting exoplanets, which would fill further gaps in our understanding of planetary evolution and habitability.

Microlensing’s abilities as an exoplanet detector are so unique that Huston, the Berkeley expert, has occasionally been inspired to beef a bit with their fellow researchers about it. Last year at Penn State’s Astrofest program, they tweeted a picture of a whiteboard with a scrawled message—“Microlensing is cooler than RVs and transits”—as a friendly flex to members of the radial velocity and transit groups.  

“There definitely are friendly (and occasionally not so friendly) rivalries among different subfields like exoplanet detection methods,” Huston said. “I’ve worked with a lot of folks who focus on transits or radial velocities in the past, and I find every opportunity I can to inject microlensing into exoplanet conversations.”

While exoplanet research attracts the most popular attention, it is far from the only field that stands to be revolutionized as microlensing hits its heyday. In 2022, scientists detected the first isolated black hole—the lonely remains of a massive star—using microlensing. There are an estimated 100 million of these solo travelers skulking through the Milky Way and microlensing is currently the only way to observe them.  

“For stellar mass black holes, the end products of the evolution of high mass stars, most detection methods require a binary companion,” Huston said. “We can detect merging black holes through gravitational waves, or a black hole with a companion star through the star’s orbital motion. But, so far, only one isolated stellar mass black hole has been found, OB110462, and that was through microlensing. So there’s a lot of exciting work to try to find more of these and use them to learn more about the processes of stellar evolution that create them.” 

Microlensing also continues to shed new light on the nature of dark matter. For instance, Mróz and his colleagues just published a study in Nature, based on microlensing data, that strongly disfavors the notion that dark matter is made up of primordial black holes, a hypothetical class of objects that originated in the early universe. Meanwhile, Croon, the physicist at Durham University, is developing new microlensing approaches to assess the viability of other dark matter candidates, including boson stars or axion mini-clusters.

“We know dark matter substructure exists at a lot of different scales, and especially these really small-scale objects are just very hard to probe otherwise,” Croon said.

We’re now on the cusp of spying an enormous population of celestial bodies and interactions that will inform our understanding of what—and perhaps who—is out there beyond Earth. And that’s just what we know we’ll find out there. Microlensing will unveil entities that we have only imagined, and some that we have not. 

“I would say that the next several years will be a golden age for microlensing,” concluded Mróz.