Cold Giants Beyond the Snow Line: Discovery of Four Planets by Microlensing ✛
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.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This work reports the discovery of four cold giants via gravitational microlensing, using data from the KMTNet, OGLE, and PRIME surveys. Analysis of light curve anomalies allowed the determination of mass ratios q ~ 10⁻³, planet masses of 0.2–2.5 M_J, and projected orbital separations of 0.7–6 AU, placing all planets beyond the snow line. Distances to the systems were estimated using a Bayesian method, ranging from 6.6 to 7.9 kpc.
Where its strength is
A homogeneous sample of cold planets with well-measured parameters was obtained, providing material for statistical studies of giant planet formation around low-mass stars.
Where it could go next
- It might be interesting to compare the obtained statistics of cold giants with predictions from formation models in low-mass systems, using approaches developed in the work on weighing planets by their shadow.based on: How to Weigh a Planet by Its Shadow arXiv:2607.10175
- Adjacent is the work on the unexpected atmospheric composition of a warm mini-Neptune; perhaps similar atmospheric analysis methods could be applied to cold giants in future high-resolution observations.based on: Unexpected Atmosphere Composition of a Warm Mini-Neptune arXiv:2605.02036
- To refine lens parameters and resolve degeneracies, one could consider the methodology used in the work on a black hole lens capturing light from distant nebulae, which discusses subtle microlensing effects.based on: Black Hole Lens Catches Light from Distant Nebulae arXiv:2608.00688v1
Worth trying
- Test how the addition of high-cadence PRIME observations affects the measurement of the finite source parameter ρ for events with short anomalies.
- Validate the Bayesian distance estimation scheme on synthetic events to assess systematics from the choice of Galactic model.
- Compare the mass ratio distribution q for the sample of cold giants with predictions from different formation mechanisms.
Why this may matter
Expanding the sample of cold planets beyond the snow line is important for testing theories of giant planet formation, especially around low-mass stars, where models predict a deficit of such objects. This could shed light on the dependence of planet frequency on metallicity and on processes in the early Universe.
Nearby works in our archive
- Black Hole Lens Catches Light from Distant Nebulaesimilarity 0.63
- A Lunar Telescope Will Reveal the Shadows of Distant Black Holessimilarity 0.62
- A Sun-Sized Lens Searches for Life Around Distant Starssimilarity 0.60
- The Wind That Snuffs Out Starssimilarity 0.60
- How to Weigh a Planet by Its Shadowsimilarity 0.60
- The Milky Way Speaks the Language of Neutrinossimilarity 0.60
- Unexpected Atmosphere Composition of a Warm Mini-Neptunesimilarity 0.59
The closest work in the archive sits at 0.63 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2607.04594v1
We report the discovery of four cold giant planets identified through the analysis of microlensing events detected by high-cadence surveys: OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030, and KMT-2025-BLG-2272. The planetary signals appear as short-duration anomalies in the light curves and are well described by binary-lens single-source models with mass ratios between the lens components of order $$q \sim 10^{-3}$$. Finite-source effects are securely measured in three out of four events, enabling determinations of the angular Einstein radius. A Bayesian analysis incorporating the measured event timescale and angular Einstein radius yields host masses of $$\sim 0.07$$--$$0.6~M_\odot$$ and companion masses of $$\sim 0.2$$--$$2.5~M_{\rm J}$$, confirming that all companions lie in the giant-planet regime. The projected separations are ~ 0.7--6 au, placing all planets at or beyond the snow lines of their host stars. The inferred lens distances span $$\sim 6.6$$--$$7.9$$ kpc, with all systems consistent with bulge lenses. These detections expand the sample of cold giant planets from homogeneous high-cadence surveys and highlight the sensitivity of microlensing to planetary systems beyond the snow line, providing further constraints on the occurrence and properties of giant planets around low-mass stars.
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