Astronomers measure Saturn-mass object drifting through Milky Way without a star
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Astronomers have directly measured the mass of a Saturn-sized object drifting through the Milky Way without a detectable host star. The object, designated KMT-2024-BLG-0792/OGLE-2024-BLG-0516, has a mass of about 0.22 Jupiter masses, or roughly 70 Earth masses, and lies about 3.05 kiloparsecs from Earth. The measurement, reported in a January 2026 Science paper, used gravitational microlensing data from ground-based telescopes and the Gaia spacecraft.
Key Facts
- The lensing event KMT-2024-BLG-0792/OGLE-2024-BLG-0516 was independently detected on 3 May 2024 by the Korea Microlensing Telescope Network and the Optical Gravitational Lensing Experiment.
- The fitted Einstein crossing time for the event was 0.842 days, indicating a short-duration microlensing event.
- The angular Einstein radius was measured at 18.6 microarcseconds using the finite-source effect.
- Gaia scanned the same sky position six times over a 16-hour period, providing a second viewpoint that yielded a microlensing parallax detection at about seven-sigma significance.
- The resulting lens mass was 0.219 Jupiter masses with uncertainty of plus 0.075 and minus 0.046 Jupiter masses.
Microlensing Detection
The Korea Microlensing Telescope Network and the Optical Gravitational Lensing Experiment independently detected the event on 3 May 2024. Telescopes in Chile, South Africa and Australia watched a background red giant brighten as the gravity of an unseen foreground object bent and magnified its light. The fitted Einstein crossing time was 0.842 days, and short events tend to indicate low-mass lenses because duration scales with the square root of lens mass. The event had a rounded peak caused by a finite-source effect, as the lens passed across the apparent disc of the background star rather than treating it as a point.
Gaia Parallax Measurement
Gaia happened to scan the same position six times over a 16-hour period from its orbit near the Sun-Earth L2 point, an unusually favourable coincidence for a survey spacecraft that could not be redirected towards an alert. The event peaked about 1.9 hours later in Gaia’s data than it did in the ground-based light curves, a delay arising because Gaia and Earth viewed the alignment from separated positions. Fitting that offset produced a microlensing parallax detection at about seven-sigma significance, and combining the parallax with the angular Einstein radius broke the usual mass-distance degeneracy. The resulting lens mass was 0.219 Jupiter masses, with uncertainty of plus 0.075 and minus 0.046 Jupiter masses, and its inferred distance was 3.05 kiloparsecs, with uncertainty of plus 0.58 and minus 0.43 kiloparsecs.
Interpretation and Caveats
The event showed no lensing signature from a host star, although the data cannot exclude a star on an orbit so wide that it made no detectable contribution. The measurement is a gravitational mass estimate, not a photograph of the object, and the phrase “apparently drifting without a star” is deliberate wording. The claim that such unseen worlds may outnumber stars by about 20 to one comes from a different paper and a different kind of evidence, and is a rounded population-model centre, not a count and not an upper limit.
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Astronomers measure Saturn-mass object drifting through Milky Way without a star
