Astronomers initially noticed an unexpected brightening of a background star in 2023 using the European Space Agency’s retired Gaia space telescope. While Gaia detected the anomaly, its observations were too sparse to identify the source as a binary-star system or to isolate the planet itself. By scouring TESS data from the same period, researchers found the missing pieces of the puzzle. TESS observed the region every 200 seconds for nearly 60 days, providing dense data that revealed a planet about 1.63 times the size of Jupiter. This super-Jupiter orbits an orange dwarf star 20% smaller than our Sun at a distance similar to Jupiter’s orbit in our own solar system.
The Mechanics of Gravitational Lensing
The transit method accounts for roughly 75% of all exoplanet detections to date. It relies on observing the periodic dimming of a star’s light as a planet passes between the star and Earth. However, this method only works for planets with orbital planes almost exactly edge-on to our line of sight. If a planet does not cross directly in front of its host star from our vantage point, the transit method cannot detect it.
Photometric microlensing offers a more flexible alternative. It relies on the gravity of a star-planet system warping space and magnifying the light from a more distant background star. Albert Einstein predicted this phenomenon as part of his general theory of relativity. The gravitational lensing effect produces characteristic spikes in the background star’s brightness, betraying the presence of the hidden planet. Because it does not depend on the planet blocking light directly, it can find worlds with a wider range of orbital orientations.
Uncovering the Super-Jupiter Gaia23bra b
Mallory Harris, an astronomer at the University of New Mexico who led the TESS study, explained that Gaia picked up the brightening but lacked the resolution to identify the planet. The Gaia space telescope, which retired in January 2024, provided valuable long-baseline observations but missed the fine details. TESS, with its high-cadence observations, filled in the gaps. The synergy between high-cadence data and high-precision long-baseline observations allowed the team to confirm the microlensing exoplanet.
Harris noted that high-cadence observations, even with low spatial precision, can work together with high-precision long-baseline observations to find a microlensing planet. This specific pairing of TESS and another instrument opens new doors for planetary science. The team scoured the archived data and identified extra features in the brightness of the light curve that indicated the presence of a planet in the system. Subsequent analyses confirmed the size and orbit of Gaia23bra b, marking a significant milestone for the TESS mission.
Complementary Techniques for Planetary Detection
The transit technique excels at finding planets that orbit close to their host stars. These planets are more likely to transit and block a noticeable fraction of starlight. They also have shorter orbital periods, meaning they cross in front of their stars more frequently. In contrast, microlensing is most sensitive to planets orbiting at Earth-like distances or further from their stars. This makes it a better tool for studying planetary systems that resemble our own solar system.
Team member Diana Dragomir emphasized that the two techniques are complementary. Each reveals a category of planet that the other may miss. “With microlensing, we can find smaller planets with greater orbital distances, including worlds in the habitable zone of their star and even farther away,” Harris said. This capability makes microlensing a crucial tool for astronomers seeking to understand the full diversity of planetary systems in the Milky Way. By combining transit and microlensing data, scientists can build a more complete picture of how planets form and evolve.
The Fleeting Nature of Microlensing Events
Despite its advantages, microlensing presents a unique challenge: it is a limited-time opportunity. Unlike transits, which repeat with every orbit, a microlensing event happens only once. The precise alignment of the foreground star-planet system and the background star must occur for the event to be visible from Earth. Once the relative motion of the stars shifts the alignment, the magnification event ends.
“Microlensing events happen once and they’re gone – they don’t repeat,” Harris explained. “I like to joke that we’ll probably find the first Earth analogue with microlensing and then wave at it as it goes by because we’ll never see it again.” This fleeting nature requires astronomers to act quickly and coordinate across different telescopes to capture the full light curve before the alignment shifts. It also means that archival data, like the TESS observations used in this study, becomes a treasure trove for future discoveries.
What Happens Next
The discovery of Gaia23bra b sets the stage for future missions. Harris identified the Nancy Grace Roman Space Telescope as an important observing target for similar events. Microlensing remains the only method capable of detecting Earth-mass planets at Earth-like orbital distances. The Nancy Grace Roman Space Telescope, scheduled to launch in the coming years, will use this technique to search for potentially habitable worlds across the galaxy.
Although Gaia retired last January, making future Gaia-TESS combinations unlikely, researchers believe other microlensing planets might hide in eight years of archived TESS data. Harris expressed excitement about the potential for future surveys to work together with TESS to find more microlensing planets in different parts of the galaxy. This TESS exoplanet discovery proves that existing transit missions can contribute significantly to the search for distant worlds. By applying the synergy between long-time-scale surveys and high-cadence observations, astronomers can continue to uncover hidden planets and expand our understanding of the cosmos.
— Priya Nair, science desk, AXO News