NASA’s TESS spacecraft has spotted its first planet using gravitational microlensing, the warping of spacetime predicted by Einstein’s theory of general relativity. The world, named Gaia23bra b, is a super-Jupiter about 40,000 light-years from Earth, far beyond the close-in worlds TESS was built to hunt. The discovery was published July 1 in The Astrophysical Journal Letters, with a parallel peer-reviewed paper describing the microlensing event.
The find came after researchers traced a 2023 alert from ESA’s now-retired Gaia spacecraft back through TESS’s archived observations, a result detailed in the mission’s full writeup of the discovery. They say the result hints at a hidden population of worlds waiting in those files, and serves as a trial run for NASA’s next planet-hunter, the Nancy Grace Roman Space Telescope.
How TESS Caught a Planet It Wasn’t Built to Find
TESS was built for transits. The spacecraft, launched in April 2018, watches stars for the periodic dimming that occurs when an orbiting planet crosses in front of them, a method that favors close-in worlds. Unlike the star-hugging transiting planets TESS regularly reveals, the newfound Gaia23bra b is a super-Jupiter that orbits far from its host star, the kind of world the spacecraft was never expected to find. When TESS launched, no one expected it to ever be capable of finding this kind of planet, said Diana Dragomir, a professor at the University of New Mexico and a co-author of the study.
Microlensing works by a different mechanism. When two stars line up nearly along our line of sight, the nearer star’s gravity bends light from the more distant star, briefly magnifying it. A planet orbiting the foreground star adds its own smaller lensing signature to the brightening pattern, a flicker inside the larger flare. The transit method, by contrast, is best at finding large planets orbiting very close to their host stars, where the geometry of a planet crossing in front of its star is most likely. Microlensing is best at finding planets at Earth-like distances or farther from their stars.
ESA’s Gaia spacecraft first flagged the brightening, an event called Gaia23bra, in 2023 as part of its broader survey of the Milky Way. Gaia’s observations were too sparse to pick up on the planet, said Mallory Harris, a Ph.D. candidate at the University of New Mexico who led the study. TESS happened to be monitoring the same patch of sky during the event, and its denser time coverage showed extra features in the light curve caused by a planet. TESS captured the event during Sectors 63 and 64, from March 10 to May 4, 2023, at a cadence of 200 seconds, recording a finer-grained signal than Gaia’s sparser visits. Gaia gave the team the long-baseline alert, and TESS supplied the timing that mattered. The technique was a first for the spacecraft, and one the team had been quietly preparing to try.
| Transit method | Microlensing | |
|---|---|---|
| How it works | Planet dims starlight as it passes in front of its star | Star and planet bend and magnify a background star’s light |
| Best at finding | Large planets orbiting very close to their host star | Planets at Earth-like distances or farther from their star |
| Coverage | Within TESS’s typical search radius of about 150 light-years | Detected Gaia23bra b at about 40,000 light-years from Earth |
| Repeats? | Yes, transits can be observed again | No, the alignment geometry never returns |
- March 10 to May 4, 2023: TESS Sectors 63 and 64 captured the same patch of sky at a 200-second cadence.
- April 27, 2023: The Gaia alerts system flagged the Gaia23bra brightening event at its peak, a 20-sigma deviation from the baseline.
- 2023 to 2026: Researchers mined the archived TESS data and identified the extra features in the light curve that revealed the planet.
- July 1, 2026: The team published its analysis in The Astrophysical Journal Letters.
- July 2, 2026: NASA’s Goddard Space Flight Center issued the press release announcing the discovery.
A Super-Jupiter Far Outside TESS’s Usual Range
Gaia23bra b is a hefty world. The team measured its mass at 1.63 times that of Jupiter, the largest planet in our own solar system. It orbits an orange dwarf star about 80 percent as massive as the Sun at roughly the same distance Jupiter orbits the Sun.
What makes the find unusual is how far the system sits from Earth. Gaia23bra b is about 40,000 light-years away, far exceeding TESS’s usual search radius of about 150 light-years. That puts the system deep in the Milky Way’s disk, near the galactic plane rather than in TESS’s typical close-in stellar neighborhood. Reaching out to it required a technique TESS was never designed to use.
The technique itself is rare among confirmed exoplanet finds. Out of more than 6,000 known exoplanets, about three-fourths have been discovered via the transit method, and microlensing has revealed less than 5% of known exoplanets. The handful microlensing has turned up so far include a small number discovered with space-based photometry, including some detected by NASA’s Kepler spacecraft during its K2 mission. Gaia23bra b is the first confirmed bound planet caught using TESS data, and one of only a small number of microlensing planets discovered from space. The detection matters because microlensing is currently the only method capable of detecting Earth-mass planets at Earth-like orbital distances, a population our earlier coverage of how space telescopes hunt for life beyond Earth explored in more depth.
The find also illustrates how the two methods answer different questions. Transits give us the size of a planet, and in concert with other methods we can determine its mass and density, Dragomir said. Microlensing gives us masses and orbital distances for planets we’d otherwise never see. Each method sees a category of planet the other may not be able to detect. The new result sits in the gap that microlensing is built to fill, including planets in the habitable zone of their star and worlds with orbits resembling the outer planets of our own solar system. Gaia23bra b’s discovery matters because the upcoming Roman Space Telescope will use the same microlensing technique at much larger scale.
A One-Shot Glimpse of a World Passing By
Microlensing events are fleeting by nature. They happen when two stars cross paths in the sky, and once they pass, they never repeat, because the geometry that caused the lensing never returns. That makes microlensing planets hard to revisit, even with the largest follow-up instruments. The brightness spike they produce is the only signal astronomers ever get from the event.
Harris put the frustration in plain terms. Each microlensing event gives astronomers something transits cannot: a measurement of mass and orbital distance for a world that would otherwise stay invisible. That makes every confirmed microlensing planet a demographic data point, even when the world itself can never be studied again. The Roman Space Telescope, designed for microlensing, is expected to multiply those data points. The mission’s forecast calls for roughly 1,000 microlensing planets in its dedicated bulge survey alone.
I like to joke that we’ll probably find the first Earth analog with microlensing, and then wave at it as it goes by because we’ll never see it again.
Harris said this in NASA’s Goddard Space Flight Center release on July 2, 2026.
Eight Years of Archived Data, Newly Useful
The find also raises a question about what else is hiding in TESS’s files. TESS has been observing the sky for nearly eight years and has repeatedly monitored regions along the galactic plane, where this system is located, Harris said. Despite this extensive coverage, Gaia23bra b represents the first definitive microlensing planet discovered using TESS data. Researchers believe the mission may have captured other similar events that have yet to be recognized, events that have shown up in the light curves as small bumps without ever being followed up. The discovery implies that there are probably other so-called microlensing planets hiding in TESS’s data that we hadn’t previously thought to look for, Dragomir added.
Mining those archives will not be straightforward. TESS’s relatively coarse spatial resolution, 21 arcseconds per pixel, compared with Gaia’s roughly 0.059 arcseconds per pixel, means a microlensing signal often sits inside a single crowded TESS pixel. Researchers had to use difference imaging, subtracting reference frames to isolate the variable light, to recover the planet’s signature. The approach worked once, and the team expects it to work again.
The new finding gives the team a tested method for going back through the archive. Every Gaia alert that overlaps with a TESS sector becomes a candidate to mine. The TESS-Gaia pairing now has a confirmed discovery to anchor it, with full study details in the university release on the discovery.
A Preview of What the Roman Telescope Will See
Gaia23bra b also doubles as a test case for NASA’s next planet-hunter. The Nancy Grace Roman Space Telescope is on track for launch in 2026, with one of its core surveys dedicated to microlensing toward the galactic bulge. Astronomers expect Roman to reveal an estimated 1,000 microlensing planets and around 100,000 transiting planets over the course of its mission. This is a bit like a preview of the microlensing NASA’s Nancy Grace Roman Space Telescope will do, said Michael Fausnaugh, a professor at Texas Tech University in Lubbock and a co-author.
The two missions will complement each other, with Roman focusing on the galactic bulge and TESS on the wider galactic plane. Roman will specifically target the heart of the galaxy, where stars are packed so tightly together that microlensing events are common, but those same stars would blur together in TESS’s larger pixels. TESS, by contrast, scans nearly the whole sky, including other stretches of the galactic plane where stars are more spread out. Since TESS looks elsewhere in the galactic plane, it can naturally find microlensing planets in other parts of the galaxy, Dragomir said. Pairing the two opens up prospects for understanding planet formation in a diverse population of stars, Fausnaugh added, across regions of the galaxy with conditions that differ from the radiation-soaked galactic center.
Frequently Asked Questions
What is gravitational microlensing?
Gravitational microlensing is one prediction of general relativity. When two stars line up almost perfectly from Earth’s point of view, the gravity of the nearer star bends and briefly magnifies the light of the more distant star. If the foreground star has a planet, the planet adds its own short-lived signature to the brightening pattern, giving astronomers a way to detect worlds they could not otherwise see.
How is this different from how TESS usually finds planets?
TESS finds most of its planets using the transit method, spotting the brief dimming of a star as a planet crosses in front of it. That technique favors large planets orbiting close to their host stars, where the geometry of a transit is most likely. Microlensing picks up a different population, planets at Earth-like distances or farther, including worlds with wider orbits that the transit method rarely catches.
Will TESS find more microlensing planets?
That is the team’s working assumption. The team plans to revisit TESS sectors that overlapped with Gaia alerts, using the same difference-imaging approach that recovered Gaia23bra b. Researchers believe the mission may have captured other similar events in its archived data that have yet to be recognized.
When does the Nancy Grace Roman Space Telescope launch?
Roman is on track for launch in 2026. The mission’s core microlensing survey will target the galactic bulge, where the dense stellar population makes alignments relatively common. Astronomers expect the mission to reveal an estimated 1,000 microlensing planets and around 100,000 transiting planets over the course of its operations.
Why are microlensing events so rare in the confirmed planet count?
Because the geometry is so precise. Most confirmed exoplanets have been found by the transit method or by measuring the wobble a planet induces in its host star’s motion. Microlensing requires a near-perfect alignment between two unrelated stars, an alignment that happens once and never repeats, which is why less than 5% of known exoplanets have been discovered this way.
