Microlensing trick reveals a rare gas giant exoplanet

In 1936 Albert Einstein described how gravity could briefly magnify the light of a distant star, an effect now called gravitational microlensing.

A team of astronomers has now used that cosmic trick to reveal the presence of a gas giant planet more than 3,200 light-years from Earth. This adds a rare data point to the growing exoplanet catalog.

The discovery is special because only two other planets have ever been found using this method outside the crowded center of the Milky Way. The international project is led by Dr. Marius Maskoliūnas at Vilnius University’s Faculty of Physics.

Microlensing reveals distant exoplanets

Most exoplanets are detected when they dim or tug on their host stars – an approach that favors the discovery of hot new worlds hugging their suns.

On the other hand, gravitational microlensing measures a transient brightening when two stars align, allowing astronomers to probe cool planets in wide orbits.

Since the alignment is never repeated, each event provides a unique, one-time glimpse – free from the glare of the host star. This means that even planets orbiting dim red dwarfs accessible for study.

Simple concept, challenging practice

This capability is crucial for testing how planetary systems assemble beyond the region where volatiles freeze and giant cores can quickly accrete gas.

Detecting such events demands round-the-clock monitoring of millions of stars and rapid follow-up once a brightness surge appears. The logistical challenge keeps planets discovered using microlensing to a small fraction of all confirmed discoveries.

The principle is simple in concept but hard in practice. Astronomers must distinguish a genuine microlensing spike from thousands of variable stars, instrumental glitches, and asteroids that clutter automated surveys.

Custom software and worldwide alert networks now flag promising signals in minutes, allowing observers to swing telescopes onto the event before the brief peak fades.

Detecting an exoplanet with microlensing

In August 2021 the Gaia satellite spotted a subtle uptick in a star about four billion times dimmer than what the unaided eye can see. Telescopes in Lithuania and Poland pivoted within hours, capturing the distinctive U-shaped light curve that reveals a planetary companion.

“This kind of research demands a great deal of expertise, patience, and, frankly, a bit of luck,” said Dr. Maskoliūnas. He and his colleagues sifted through two years of photometry data before confirming the exoplanet’s existence.

Gaia’s real-time alert arrived 39 days before the main peak, giving the team enough time to coordinate observations on three continents.

Continuous coverage enabled them to model the light curve with unprecedented precision for a halo event, thus pinning down the planet’s mass to within about 20 percent.

A gas giant exoplanet

The newcomer, AT2021ueyL b, weighs about 1.3 times as much as Jupiter, sits roughly 3,262 light-years away, and appeared in data collected from both Gaia and ground-based surveys.

The planet circles a small M-dwarf star roughly once every 11 Earth years, staying well beyond its host’s snow line. Because the mass ratio between planet and star is large, the brightening was easier to catch, despite the faintness of the lens.

For comparison, the first microlensing planet detected outside the bulge surfaced only in 2018, highlighting how slowly such detections accumulate.

The host star glows at about half the Sun’s mass and temperature, placing the planet roughly four times farther out than Earth sits from the Sun.

At that distance, temperatures likely linger below minus 200°F (93°C), a zone where methane and nitrogen could condense onto any moons.

Exoplanet in a quiet part of the galaxy

Most microlensing events cluster toward the dense galactic center, yet this one lies in the sparse galactic halo, nearly 19 degrees below the plane. Its unusual location extends the range of environments where planets are known to form.

“This is only the third planet ever discovered so far from the Galactic bulge,” noted Professor Edita Stonkutė of Vilnius University. The halo setting raises fresh questions about planet formation in metal-poor regions.

Microlensing detects rare exoplanets

As of June 2025 astronomers have confirmed the existence of 5,921 exoplanets, using a variety of techniques.

Only about one percent of those worlds come from microlensing surveys, yet they provide vital statistics on orbits beyond about two astronomical units.

Cold planets are hard to study with transits because they block their stars infrequently, and their radial-velocity signals are slow.

Microlensing fills that gap by sampling systems that resemble the outer reaches of our own Solar System.

Microlensing and exoplanet formation

The presence of a Jupiter-mass planet in orbit around a half-solar-mass star challenges models that predict giant planets should be rare around low-mass hosts.

Planet counts from microlensing already hint that nature makes such pairings more often than theory once allowed.

Each new detection helps refine those statistics, especially when the host resides in a different galactic environment.

Comparing halo, disc, and bulge populations will reveal whether metal-poor stars build planets in the same way as their richer cousins.

Improving microlensing discoveries

The Nancy Grace Roman Space Telescope is slated to launch no later than May 2027 and will devote months to a high-cadence microlensing survey.

Simulations suggest that Roman could unveil more than a thousand cold planets, including Earth-mass bodies that are invisible using other techniques.

Until then, collaborations like the Lithuanian-Polish team are combing Gaia alerts and ground observations for the fleeting brightenings that betray hidden worlds.

Every success, including AT2021ueyL b, sharpens the strategies that Roman and future missions will apply at scale.

Research on this unlikely alignment expands the map of where planets can thrive, offering a reminder that even the quiet outskirts of the galaxy can host giant worlds.

The study is published in the journal Astronomy & Astrophysics.

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