For the first time, NASA's TESS (Transiting Exoplanet Survey Satellite) mission has identified a planet orbiting a distant star thanks to its warping of space-time. Unlike the star-hugging transiting planets TESS regularly reveals, the newfound microlensing world is a super-Jupiter orbiting far from its host star.

"When TESS launched, no one expected it to ever be capable of finding this kind of planet," said University of New Mexico Professor Diana Dragomir. "The discovery implies that there are probably other microlensing planets hiding in TESS's data that we hadn't previously thought to look for."

Astronomers first became aware of the alerting microlensing event, called Gaia23bra b in 2023, using ESA's (European Space Agency) now-retired Gaia space telescope. Gaia23bra b is fundamentally different from the transiting planets normally found by TESS. Instead of causing a dimming, the star-planet system magnified the light of a more distant background star (the "source"). This occurred when the mass of the foreground star (the "lens") and its planet bent the background star's light as the two systems briefly aligned on the sky, an effect known as gravitational microlensing. The time-dependent shape of this brightening is what revealed the presence of a planet and allowed researchers to measure the mass ratio between the planet and its host star.

Researchers later looked back through archived TESS data and found TESS had caught it too.

"Gaia's observations were too sparse to pick up on the planet. TESS happened to be monitoring the same area of the sky during the event, and its denser time coverage showed extra features in the light curve caused by a planet."

- Mallory Harris, UNM Ph.D. candidate

The team's analysis, which was published July 1 in the Astrophysical Journal Letters, revealed that Gaia23bra b is about 1.63 times as massive as Jupiter. It orbits an orange dwarf star that's about 80 percent of the Sun's mass at a similar orbital distance Jupiter's orbit around the Sun. Such a world would be impossible to detect using the primary transit method TESS was designed to employ.

The discovery also suggests that additional microlensing planets may be hidden within the past eight years of archived TESS observations. Although Gaia23bra b is the first confirmed planet/star system found using TESS data, researchers believe the mission may have captured other similar events that have yet to be recognized.

Microlensing 101

Out of more than 6,000 known exoplanets, about three-fourths were discovered via the transit method, TESS's typical planet-searching technique. Astronomers monitor hordes of stars, watching for ones that periodically dim because orbiting planets cross in front of them - an event called a transit. Large planets block out the most starlight regardless of their proximity to the host star. The reason the technique is particularly sensitive to close-in planets is because they have the highest probability to transit.

Microlensing, however, is most sensitive to planets orbiting at Earth-like distances or farther from their stars, making it an important tool for studying planetary systems more like our own solar system. Microlensing has revealed less than 5% of known exoplanets. This light-bending phenomenon occurs when two stars align closely from our vantage point. Light from the more distant star curves as it travels through the warped space-time caused by the nearer star's mass. If the alignment is especially close, the nearer star acts like a cosmic lens, focusing and magnifying light from the background star. Planets orbiting the foreground star may also modify the distant star's light, acting as their own tiny lenses. Astronomers often observe this effect as a spike in the star's brightness.

"The main advantage of microlensing lies in the kinds of planets it is sensitive to. Planets that orbit very close to their host stars effectively blend with the star's mass and do not produce a distinct microlensing signal. With microlensing, we can find smaller planets with greater orbital distances, including worlds in the habitable zone of their star and even farther away."

- Mallory Harris, UNM Ph.D. candidate

"Transits and microlensing are very complementary because they each reveal a category of planet the other may not be able to detect," Dragomir said. "And they offer different details. Transits give us the size of a planet, and in concert with other methods we can determine its mass and density. Microlensing gives us masses and orbital distances for planets we'd otherwise never see."

But microlensing observations are limited-time opportunities.

"Microlensing events happen once and they're gone - they don't repeat," Harris said. "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."

That makes detailed observations of microlensing planets difficult. However, as the sample of microlensing planets grows, it becomes possible to study how common wide-orbit, planets are throughout the galaxy and how planetary systems form and evolve over time. This information helps fill an important gap left by transit and radial-velocity surveys, which are strongly biased toward planets orbiting very close to their host stars.

"TESS has been observing the sky for nearly eight years and has repeatedly monitored regions along the Galactic Plane, where this system is located," said Harris. "Despite this extensive coverage, Gaia23bra b represents the first definitive microlensing planet discovered using TESS data."

The discovery also highlights the power of combining different kinds of space-based observations. Gaia supplied long-term monitoring that identified the event, while TESS observed the field every 200 seconds for nearly 60 days. Those rapid observations allowed researchers to detect subtle features in the microlensing light curve that are often missed by traditional surveys.

"Gaia23bra b is also one of only a very small number of microlensing planets discovered using space-based data, making it an important case study for the upcoming Nancy Grace Roman Space Telescope," said Harris. "Microlensing is currently the only method capable of detecting Earth-mass planets at Earth-like orbital distances, so demonstrating that these techniques work in real datasets is particularly valuable for future searches for potentially habitable worlds.

On track for launch in fall 2026, Roman will observe the center of the galaxy for one of its core surveys, revealing an estimated 1,000 microlensing planets and around 100,000 transiting planets. Because Roman will observe with a similarly continuous cadence, Gaia23bra b serves as an important case study demonstrating what high-cadence, space-based microlensing observations can reveal.

TESS looks at nearly the whole sky, and is only now beginning to look towards the center of the galaxy which was previously a difficult target due to stray light from the Earth and the Moon contamination. The high density of stars towards the Galactic Bulge increases Roman's odds of seeing microlensing events, but the stars would blend together in TESS's large pixels.

"Since TESS looks elsewhere in the Galactic Plane, it can naturally find microlensing planets in other parts of the galaxy, as demonstrated by this first microlensing planetary system," Dragomir said. "That means it could help us study planets in regions with different conditions."

That could have implications for the search for habitable worlds. Microlensing is currently the only planet-detection technique capable of routinely finding Earth-mass planets at Earth-like orbital distances, making it a critical tool for future studies of potentially habitable planetary systems. Most microlensing events are typically observed once per night or less frequently, especially outside the Galactic Bulge.

To learn more, visit the TESS mission.

Top image:
This artist's concept visualizes Gaia23bra b, the first microlensing planet orbiting a distant star found by NASA's TESS (Transiting Exoplanet Survey Satellite). This super-Jupiter orbits an orange dwarf star at a distance similar to Jupiter's distance from the Sun. Credit: NASA's Goddard Space Flight Center