Scientists from Vilnius University’s Faculty of Physics, in collaboration with colleagues from Poland and other countries, have made a rare astronomical discovery: a distant gas giant planet identified using an unusual method called gravitational microlensing—a phenomenon first predicted by Albert Einstein.
This is only the third time in history such a planet has been found using this technique. The discovery has been published in the esteemed journal Astronomy & Astrophysics.
A Rare Cosmic Alignment
“This kind of research demands a great deal of expertise, patience—and, frankly, a bit of luck,” says Dr Marius Maskoliūnas, head of the Lithuanian research team.
Gravitational microlensing occurs when a massive object—such as a star or a dark, unseen body—passes in front of a more distant star. The gravity of the foreground object bends and magnifies the light from the background star, much like an invisible magnifying glass. This brief brightening is what astronomers look for as they sift through vast amounts of data.
“About 90% of stars we observe show brightness changes for entirely different reasons,” explains Dr Maskoliūnas. “Only a small number display the genuine microlensing effect.”
A Discovery Born from Collaboration
The project began during a visit to the Astronomical Observatory of the University of Warsaw, where microlensing specialist Prof Łukasz Wyrzykowski proposed a joint Polish–Lithuanian initiative. The idea was to analyse data from the European Space Agency’s Gaia telescope, cross-check it, and enhance it with ground-based observations using the telescopes at Molėtai Astronomical Observatory in Lithuania.
In 2021, the team detected the telltale signs of microlensing. After careful verification, they confirmed the presence of a planet—AT2021uey b—located approximately 3,262 light-years from Earth. This gas giant is about 1.3 times the mass of Jupiter and orbits a small, relatively cool M-dwarf star once every 4,170 days (just over 11 years).
The planet’s large size relative to its host star made it easier to detect. Identifying smaller, Earth-like planets using this method remains far more challenging.
A Planet in an Unusual Place
While most microlensing events are detected near the dense centre of the Milky Way, this planet was discovered in the galactic halo—a sparsely populated outer region of the galaxy.
“This is only the third planet ever discovered so far from the Galactic bulge,” says Associate Professor Edita Stonkutė, who leads the Lithuanian side of the collaboration. “Its location makes the discovery even more remarkable.”
Why It Matters
Since the first planet orbiting a Sun-like star was discovered in 1995, nearly 6,000 exoplanets have been confirmed. Yet exoplanet science is still evolving, often challenging our assumptions.
“When that first discovery was made, it was a shock to find a Jupiter-sized planet so close to its star,” explains Assoc. Prof. Stonkutė. “As more data came in, we realised that many planetary systems look nothing like our Solar System. We’ve had to rethink our models of how planets form.”
Microlensing offers an exciting frontier in astronomy because it can reveal objects that are otherwise invisible. According to Dr Maskoliūnas, “If we add up all the visible matter in the Milky Way, it accounts for perhaps only 10% of the galaxy’s total mass. The rest remains unseen—and microlensing gives us a glimpse into that dark, hidden part of the universe.”
He likens the process to watching shadows: “Imagine a bird flying past you. You don’t see the bird itself—just its shadow. Yet from that, you might infer whether it was a sparrow or a swan, and how far away it was. That’s what microlensing is like. It’s an incredibly intriguing way of studying the invisible.”
This article is based on a press release issued by Vilnius University.
