TESS discovers its first planet using gravitational microlensing

What happened?

NASA's Transiting Exoplanet Survey Satellite, or TESS, has identified a planet using gravitational microlensing for the first time. The planet, Gaia23bra b, is a gas giant about 1.6 times the mass of Jupiter orbiting an orange K-type dwarf star nearly 40,000 light-years from Earth.

The European Space Agency's Gaia mission first flagged the event in 2023 when a star temporarily became brighter. Researchers later found that TESS had observed the same patch of sky over two consecutive observing sectors. Its more frequent brightness measurements revealed extra features in the light curve that a single foreground star could not produce.

A combined model of the Gaia and TESS measurements indicates a host star with about 0.79 times the Sun's mass and a planet with about 1.63 Jupiter masses. The projected separation is at least about 4.8 astronomical units, similar to Jupiter's distance from the Sun. This makes Gaia23bra b the first gravitationally bound microlensing planet detected by TESS.

The simple version

TESS normally finds exoplanets using the transit method. If a planet crosses the face of its star, it blocks a small fraction of the light and the star appears slightly dimmer. That works especially well for large planets in close orbits that transit repeatedly.

Microlensing produces the opposite-looking signal. When a nearer star passes almost exactly in front of a more distant star, the nearer star's gravitational field bends the background light. The foreground system acts like a natural magnifying lens, so the background star appears brighter for a short time.

If the foreground star has a planet, the planet adds a smaller distortion to the lens. Astronomers do not see the planet as a resolved dot. They identify its effect in a graph of brightness against time and test whether a star-plus-planet model explains the shape better than a single-star model.

This particular alignment will not repeat. The two stars continue moving across the sky, so microlensing discoveries depend on telescopes collecting measurements while the brief event is happening.

Worked equations

Why it matters

TESS was designed mainly to search for repeating transits around comparatively nearby stars. Finding Gaia23bra b shows that the same archive also contains useful microlensing observations from much farther across the Milky Way. Researchers can now search existing TESS data for events that were not recognised as planets the first time around.

Different detection methods reveal different planet populations. Transits favour planets that pass in front of their stars and often find large, short-period worlds. Microlensing can reveal colder planets in wider orbits, including systems whose geometry never produces a transit from Earth.

This matters because a catalogue made using only one method is biased towards the planets that method can detect most easily. Combining transit and microlensing surveys gives a less distorted picture of how planet masses and orbital distances vary across the Galaxy.

The result also previews the work of NASA's Nancy Grace Roman Space Telescope, which is designed to monitor crowded star fields repeatedly and find large numbers of microlensing planets. TESS can add measurements in other regions of the Galactic plane.

Physics you already know

At A Level, a gravitational field is a region where a mass experiences a force. General relativity extends this picture: mass and energy curve spacetime, and light follows paths through that curved spacetime. A lensing star does not pull on photons as if they were ordinary massive projectiles.

The familiar inverse-square law still helps explain why gravitational effects weaken with distance, but gravitational lensing needs relativity for a quantitatively correct deflection angle. This is a useful example of a school model remaining helpful while a more complete theory is needed for precision.

The discovery is also an exercise in data analysis. TESS measured brightness at closely spaced times, producing a light curve. Astronomers compared the measured shape with mathematical models and used the fit to estimate the masses and projected separation.

The very large distance and planet mass are reported in standard form or in comparison units such as light-years, solar masses and Jupiter masses. Converting those units makes the physical scale easier to compare with familiar objects.

Science ideas to understand

A Level stretch

A microlensing light curve depends on the changing alignment between the observer, foreground lens and background source. When a planet is present, regions called caustics can produce sharp changes in magnification. TESS recorded caustic-crossing features that exposed the binary nature of the lens.

The measured signal does not provide one unique answer without modelling. The team combined the brightness data with estimates of the star's properties and reported uncertainty intervals for the star and planet masses. The central values are useful summaries, not exact known constants.

The quoted 4.8 astronomical units is a minimum projected separation on the plane of the sky, not necessarily the full three-dimensional orbital radius. Projection is a recurring limitation when distant systems are observed from one viewing direction.

Microlensing is sensitive to mass rather than the light emitted by the planet. That makes it capable of detecting faint or even free-floating planets, but the one-off alignment makes follow-up observations and atmospheric measurements difficult.

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