Gravitational microlensing can find planets where the transit and radial-velocity methods cannot reach. A new study describes four such planets, with masses of 0.2 to 2.5 Jupiter masses and projected separations of 0.7 to 6 au from their host stars. All of them lie at or beyond the snow line. The paper appeared in The Astronomical Journal (volume 172, issue 2) and is open access.
The planets were found in the events OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030 and KMT-2025-BLG-2272. The first of these also appears in the text under the designation KMT-2016-BLG-1679.
How a planet shows up in a distant star's light
Microlensing works like this. When one object passes between us and a distant star, its gravity magnifies the star's light. The light curve rises and falls smoothly. If the foreground star has a planet, a brief anomaly appears on that curve. All four events showed exactly this kind of signal.
The anomalies were modeled as a system of two lenses and one source of light. The model has seven principal parameters. Three describe the simple event: the time of closest approach, the minimum lens–source separation and the Einstein timescale, which is the event's duration. The other four describe the binary lens and the size of the source star. These are the projected separation of the lens components, their mass ratio, the angle of the source trajectory and the angular radius of the source. In all four systems the mass ratio is about 1:1000.
The team first ran a grid search over separation, mass ratio and angle, since these determine the shape of the caustics, the curves where magnification rises sharply. This revealed different local solutions, including those arising from degeneracies. The candidate solutions were then refined.
Which telescopes provided the data
Three surveys took part. KMTNet has three identical 1.6 m telescopes in Chile, South Africa and Australia. The network monitors the Galactic bulge almost continuously, at intervals of 15 minutes to 5 hours depending on the field. OGLE uses the 1.3 m Warsaw Telescope at Las Campanas Observatory in Chile. PRIME observes in the near-infrared with a 1.8 m telescope in South Africa.
Three of the events were observed by several groups. KMT-2025-BLG-0030 was recorded by KMTNet, OGLE and PRIME. KMT-2025-BLG-2272 was seen only by KMTNet.
What was measured
In three of the four events, finite-source effects were detected. When the source star passes near a caustic, its disk smooths out the signal. The shape of that smoothing yields the angular Einstein radius, which ties the event to physical scales.
The team then ran a Bayesian analysis, a statistical estimate that takes into account the event duration and the angular Einstein radius. The results: host masses of 0.07–0.6 solar masses and planet masses of 0.2–2.5 Jupiter masses. The systems lie 6.6–7.9 kpc away, or about 21,500–25,800 light-years. All are consistent with lenses in the Galactic bulge, the dense central region of the Milky Way.
Why it matters
The distribution of planet masses between sub-Saturns and super-Jupiters is still only partly characterized. Earlier microlensing work already hinted at a break in the mass-ratio function. Additional planets from homogeneous samples help test this.
Low-mass stars raise another question. The standard core accretion model predicts that giants form less efficiently around them. The hosts in this sample range from 0.07 to 0.6 solar masses, so they suit such a test. Another open question is how planet occurrence depends on location in the Galaxy, since metallicity and stellar populations differ between the disk and the bulge.
How solid is the result
The angular Einstein radius was measured in three of the four events. Masses and distances are statistical estimates rather than direct measurements, so the ranges are fairly wide. The conclusion that all the companions are giant planets takes these estimates into account.
Four planets do not make a demographic study on their own. But they add to a sample selected in a uniform way, and such samples are what statistics requires. Their position beyond the snow line makes them a useful sample of the cold planet population.
