In an unprecedented scientific breakthrough, astronomers have identified a rare exoplanet, AT2021uey b, located at the far reaches of the Milky Way galaxy. This discovery was made possible through a method rooted in Albert Einstein’s theory of general relativity, specifically using gravitational microlensing. The findings, published in the Astronomy & Astrophysics journal in May 2025, signify a major advancement in the exploration of distant exoplanets and their environments, reaching beyond the limitations of traditional detection methods. The exoplanet, approximately 3,200 light-years from Earth, orbits a small, dim star, underscoring the extraordinary nature of this discovery.
A Rare Discovery at the Edge of the Milky Way
AT2021uey b’s location at the galaxy’s periphery is noteworthy for several reasons. As a Jupiter-sized gas giant, it is only the third planet discovered in such a remote region of the Milky Way. This area is notably sparse in heavy elements, which are typically considered essential for planet formation. The presence of AT2021uey b in this region challenges existing models and suggests that planets might form in environments previously deemed unsuitable, offering new insights into planetary formation theories.
The planet completes an orbit around its M dwarf star every 4,170 days, or roughly 11 years. This star, significantly cooler and less luminous than our Sun, adds another layer of intrigue to the discovery. The existence of a gas giant like AT2021uey b in such a distant and cool part of the galaxy challenges traditional models of planetary formation, which often assume that such planets form closer to their stars.
The Role of Gravitational Microlensing in Planet Detection
What truly distinguishes this discovery is the technique used to detect AT2021uey b: gravitational microlensing. This method involves observing a temporary increase in a star’s brightness, which occurs when the gravitational field of a planet bends space-time, acting like a magnifying lens on the star’s light. The Gaia space telescope first observed this microlensing event in 2021, when the planet’s shadow caused a brief brightening of the star’s light.
This technique, derived from Einstein’s theory of general relativity, revolutionized our understanding of gravity by demonstrating that it is the warping of space-time caused by mass and energy, rather than a force. This insight allows astronomers to detect otherwise invisible planets, as their gravitational impact on light is the sole evidence of their existence. Despite its potential, microlensing is not widely used in exoplanet discovery due to the precise alignment required between the star and the planet, with only a small fraction of stars exhibiting the microlensing effect.
“This kind of work requires a lot of expertise, patience, and, frankly, a bit of luck,” remarked Marius Maskoliūnas, an astronomer at Vilnius University in Lithuania. “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.”
Challenging Existing Models of Planetary Formation
The discovery of AT2021uey b pushes the boundaries of current planetary formation models. Traditionally, scientists believed that gas giants like Jupiter could only form closer to their stars, where conditions are warmer and more conducive to the accumulation of gas. However, Edita Stonkutė, the leader of the microlensing project, highlighted the broader implications of this finding:
“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,” she noted. “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.”
This insight suggests that the formation of planets, particularly gas giants, may occur in ways vastly different from the patterns observed in our own solar system. The discovery of AT2021uey b challenges conventional understanding and opens new avenues for the study of distant worlds across the universe.
As astronomers continue to explore the vast expanse of the Milky Way, discoveries like AT2021uey b remind us of the universe’s complexity and the potential for finding planets in the most unexpected places. With each new finding, the boundaries of our knowledge expand, offering fresh perspectives on the formation and evolution of planetary systems.