Astronomers have identified a rare and distant planet, leveraging a remarkable space-time effect first proposed by Albert Einstein. This discovery marks only the third time a planet has been found so far from the dense center of our galaxy, using a highly uncommon detection technique known as microlensing.
Named AT2021uey b, the newly found world is a gas giant similar in size to Jupiter and is located approximately 3,200 light-years away from Earth. What makes this exoplanet truly unique is its distant orbit around a small, cool M dwarf star, taking nearly 4,170 days to complete a single revolution. Such a position, lying on the fringes of the Milky Way, makes this one of the most remote planetary discoveries to date.
The most intriguing aspect of this finding lies in how the planet was detected. Rather than the usual methods involving light dimming or star wobble, astronomers employed the phenomenon of microlensing — a space-time warping event that temporarily magnifies the light from a star when another massive object passes in front of it. In this case, the gravity of the exoplanet created a brief, sharp increase in the brightness of its host star, allowing scientists to infer the planet’s presence.
Microlensing is based on Einstein’s theory of general relativity, which posits that massive objects distort the very fabric of space and time around them. Instead of viewing gravity as an invisible force, Einstein described it as a natural consequence of the curvature in space-time caused by mass and energy. This curvature affects the path of everything — including light. Thus, when a planet or other massive body aligns precisely with a background star, the light from that star bends and magnifies as it passes through the curved space-time, acting like a lens.
“What fascinates me about this method is that it can detect those invisible bodies,” said Marius Maskoliūnas, co-author of the study and an astronomer at Vilnius University in Lithuania. “Imagine a bird flying past you. You don’t see the bird itself and don’t know what color it is — only its shadow. But from it, you can, with some level of probability, determine whether it was a sparrow or a swan and at what distance from us. It’s an incredibly intriguing process.”
The research findings were published on May 7 in the journal Astronomy & Astrophysics. The planet’s brief shadow, caused by the microlensing event, was first noticed in 2021 through data collected by the European Space Agency’s Gaia telescope. That temporary increase in brightness was a telltale signature of microlensing, prompting astronomers to delve deeper.
The team followed up with observations from the Molėtai Astronomical Observatory in Lithuania. By analyzing these detailed readings, they calculated that the object causing the brightness spike was a planet about 1.3 times the mass of Jupiter. The gas giant orbits a relatively cool host star, which emits heat at roughly half the temperature of our sun. The planet itself lies at a distance four times greater than that between Earth and the sun, indicating a long, cold orbit in a remote part of the galaxy.
“This kind of work requires a lot of expertise, patience, and, frankly, a bit of luck,” Maskoliūnas noted. “You have to wait for a long time for the source star and the lensing object to align and then check an enormous amount of data. Ninety percent of observed stars pulsate for various other reasons, and only a minority of cases show the microlensing effect.”
Microlensing is significantly less common than other exoplanet detection methods. Since the first exoplanet was confirmed in 1992, astronomers have discovered nearly 6,000 alien worlds beyond our solar system. The most widely used techniques — transit photometry and radial velocity — identify planets by detecting either a star’s dimming as a planet crosses in front of it or the wobble in a star’s movement caused by a planet’s gravitational pull. These methods have proven fruitful, especially for planets closer to their stars. However, they fall short when it comes to identifying distant planets in obscure regions of the galaxy.
Microlensing, in contrast, excels at spotting planets in the galaxy’s outer zones. These regions, often sparse in the heavier elements required for planet formation, present a challenge to traditional detection methods. The fact that AT2021uey b was found in such a location suggests that gas giants can indeed form and persist even in less chemically rich parts of the Milky Way.
According to the researchers, this unexpected find challenges existing ideas about where and how planets form. “When the first planet around a sun-like star was discovered, there was a great surprise that this Jupiter-type planet was so close to its star,” said Edita Stonkutė, the lead researcher on the microlensing project at Vilnius University. “As data accumulated, we learned that many types of planetary systems are completely unlike ours — the solar system. We’ve had to rethink planetary formation models more than once.”
Stonkutė’s remarks reflect a broader shift in the astronomical community. Initially, scientists believed our solar system’s architecture — rocky planets close to the sun, gas giants farther away — was the universal standard. But as discoveries of exoplanets have piled up, researchers now recognize that planetary systems exhibit an astonishing variety. From hot Jupiters hugging their stars tightly to planets orbiting dead stars, the diversity has forced a re-evaluation of how planets come into being and evolve.
The detection of AT2021uey b through microlensing is yet another reminder of the unpredictable and vast nature of our universe. Each new technique opens a fresh window into the cosmos, expanding our understanding of planetary systems beyond our own. This particular discovery not only underscores the potential of microlensing in revealing hidden corners of the galaxy but also hints at a broader cosmic truth — that planets, including massive gas giants, may be far more widespread than previously imagined, even in the galactic outskirts.
In the end, it’s the convergence of Einstein’s century-old theory, state-of-the-art space telescopes, and a bit of serendipity that made this discovery possible. As Maskoliūnas put it, “It’s an incredibly intriguing process.”