TESS Finds Its First Microlensing Exoplanet: A Super-Jupiter Named Gaia23bra b

NASA's Transiting Exoplanet Survey Satellite (TESS) has notched its first discovery of an exoplanet via microlensing — a technique the mission was never designed to use. The planet, a super-Jupiter known as Gaia23bra b, was found when its host star system acted as a gravitational lens, magnifying light from a more distant background star, as Physics World reported Tuesday.

A New Trick for an Old Telescope

TESS was built for the transit method, which has produced roughly 75 percent of all exoplanet detections by watching planets cross in front of their stars. Astronomers first flagged an unusual brightening in the Gaia23bra system in 2023 using the European Space Agency's now-retired Gaia telescope, but Gaia's sparse observations could not identify the cause. University of New Mexico astronomer Mallory Harris and colleagues then scoured TESS data from the same period — observations taken every 200 seconds for nearly 60 days — and found extra features in the brightness curve indicating a planet.

The Planet

Gaia23bra b is about 1.63 times the size of Jupiter and orbits an orange dwarf star 20 percent smaller than our Sun at a distance similar to Jupiter's from the Sun. Microlensing is most sensitive to planets orbiting at Earth-like distances or farther, making it complementary to transit surveys that favor close-in worlds — and it is currently the only method capable of detecting Earth-mass planets at Earth-like orbital distances, Harris noted.

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 analogue with microlensing and then wave at it as it goes by. The team plans to target Gaia23bra with the Nancy Grace Roman Space Telescope, and Harris says archived TESS data may hold more undiscovered microlensing planets.