Cold Giants Beyond the Snow Line: Discovery of Four Planets by Microlensing
We report the discovery of four cold giants from high-cadence microlensing surveys: OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030, and KMT-2025-BLG-2272. Brief anomalies in the light curves correspond to a binary lens with a single source model at mass ratio q~10^{-3}. Finite-source effects measured in three events yielded the angular Einstein radius. A Bayesian analysis incorporating timescale and Einstein radius indicates host masses of 0.07–0.6 M⊙ and planet masses of 0.2–2.5 M_J, consistent with giant planets. Projected separations of 0.7–6 AU place them beyond the snow line. Lens distances of 6.6–7.9 kpc are consistent with bulge lenses. This result expands the sample of cold giants from uniform surveys, highlighting the effectiveness of microlensing for finding planets beyond the snow line and refining the properties of giants around low-mass stars.
Context
The search for planets beyond the snow line is key to understanding giant planet formation. The traditional transit method, which brought fame to William Borucki, poorly detects distant orbits. Gravitational microlensing, on the contrary, probes precisely cold worlds. It uses the bending of light by massive objects, where the constant speed of light enters the scale of the Einstein angular radius. Besides planets, this method is sensitive to dark matter — at one time MACHO objects were even searched for. But in our work, the focus is on giant planets around bulge stars, whose spectral types were determined by color, without direct spectroscopy.
Methods
Data from the KMTNet, OGLE, and PRIME telescopes with cadences up to 15 minutes were processed. Photometry in I, V, H bands underwent pixel reduction and was normalized. The search for solutions was carried out by exploring parameters of a binary lens: time scale t_E, separation in units of the Einstein angular radius, mass ratio q~10⁻³. For three events the finite source parameter ρ was reliably measured, allowing calculation of θ_E. Unfortunately, parallax was not detected due to insufficient precision, so distances were estimated via Bayesian method taking into account Galactic models. By the way, future observations with the Hubble Space Telescope (named after Edwin Hubble) will help resolve the remaining degeneracies.
Results
All four signals — brief anomalies on smooth light curves — are well described by models with a planetary mass ratio q from 10⁻³. Planet masses turned out to be in the range ~0.2–2.5 M_J, and hosts — from ~0.07 to ~0.63 M_⊙. One system is located near the brown dwarf boundary, where the temperature is insufficient for hydrogen burning — an element whose role in stars was first assessed by Cecilia Payne-Gaposchkin. Projected orbits from 0.7 to 6 AU mean the planets lie beyond the snow line. Distances of 6.6–7.9 kpc are typical for the bulge. Interestingly, such space-time distortions resemble gravitational waves from merging black holes, except here the lens is static.
Implications
The discovery expands the uniform sample of cold planets, providing statistics to test formation theory. Core accretion models predict difficulties in birthing giants around low-mass stars, but the data show the opposite. Differences in metallicity between disk and bulge populations may manifest in planet frequency. Interestingly, in the early Universe after the Big Bang there were no heavy elements, and planets could have formed differently.
Future development
The topic will advance with the launch of new telescopes (Roman, Euclid) and ground-based instruments. High-precision photometry will allow measurement of parallax and resolution of degeneracies. Infrared observations with Hubble and JWST will refine masses and orbits. Possibly, statistics will reveal a link with multiplanet systems.
Impact
The results will impact planet formation models, especially in the low-metallicity regime. They are important for planning direct exoplanet search missions and for understanding the distribution of matter in the Galaxy.
Next steps
Next steps include high-angular resolution to separate lenses and sources, as well as statistical analysis of the full microlensing planet sample to constrain the mass function and frequency.
Key open problems
The work is related to unsolved problems: why do giants form around low-mass stars? How does planet frequency depend on metallicity and position in the Galaxy? Microlensing also helps search for isolated black holes and investigate dark matter if it consists of compact objects.
🎯 During a microlensing event, the star's brightness can increase by hundreds of times, but the event itself is unrepeatable — it will not recur.
Key numbers
- Planet mass: 0.2–2.5 MJ
- Host star mass: 0.07–0.63 M☉
- Projected separation: 0.7–6 AU
- Distance to systems: 6.6–7.9 kpc
- Typical event time scale: 5–50 days