Science published an article on cold super-Earths which are common, low-mass exoplanets orbiting their host stars at large distances, written by a team of astronomers, including scientists from the Optical Gravitational Lensing Experiment (OGLE) led by Prof. Andrzej Udalski. Polish team’s research is co-funded by the National Science Centre.
In a recent paper published in the prestigious journal Science, an international team of astronomers – including scientists from the Optical Gravitational Lensing Experiment (OGLE) led by Prof. Andrzej Udalski at the Astronomical Observatory of the University of Warsaw – presents a detailed study of low-mass exoplanets orbiting their host stars at large distances. The study shows that so-called "cold super-Earths" are a common component of planetary systems: on average, they are found in every third system around stars in the Milky Way.
Artist’s impression on how common planets are around the stars in the Milky Way planetary systems in the Milky Way.
Just 30 years ago, no planets had yet been discovered orbiting stars similar to our Sun. Since then, the field of exoplanet research has undergone a revolution. From the first discoveries in the 1990s, astronomers have progressed to routinely identifying distant planetary systems, thanks to the development of new detection techniques such as the transit and gravitational microlensing methods. OGLE, one of the pioneers of these methods, has been conducting one of the largest photometric sky surveys for over 33 years.
A major breakthrough came with the launch of planet-hunting space telescopes like Kepler and TESS, which dramatically increased the number of known exoplanets. With this vast dataset, astronomers can now study planetary systems using statistical methods. Distributions of planet masses and their distances from host stars carry crucial information about how such systems form and evolve.
However, all current planet detection techniques have limitations. Most are especially sensitive to planets orbiting close to their stars, meaning the majority of known exoplanets are those with orbital periods shorter than one year. Among them, "super-Earths" – planets with masses between that of Earth and Neptune (~17 Earth masses) – are particularly abundant and well-characterized.
Much less is known about distant regions of planetary systems, located beyond the so-called snow line (1–2 astronomical units from the star), where water is present as ice and giant gas planets like Jupiter or Saturn are believed to form. Only gravitational microlensing enables the detection of planets in these remote regions. However, such events are rare, and the number of planets discovered via microlensing is significantly lower than by other methods.
Microlensing has already revealed how often massive, Jupiter-like planets occur on wide orbits. But whether smaller, lower-mass planets – like super-Earths – are also present there, and how frequently, has remained unclear.
In the Science study, astronomers tackled this question by analyzing distant, low-mass planets discovered through microlensing. A key part of the work is the analysis of a planet observed during the microlensing event OGLE-2016-BLG-0007, discovered in February 2016 by the OGLE team. The data show that this planet has a mass similar to Earth and orbits a 0.6-solar-mass star at a distance of about 10 astronomical units.
This is a typical, very low-mass super-Earth on an orbit similar to Saturn’s in our Solar System. A new record-holder in this category, says Prof. Andrzej Udalski, OGLE's principal investigator.
Microlensing anomalies caused by such low-mass planets are extremely brief – often lasting only a day – and require continuous, round-the-clock observations. For a broader statistical analysis, the study includes data on 64 low-mass microlensing planets: the newly described one and 63 others discovered between 2016 and 2019, which were observed by the Korean KMTNet project, capable of 24-hour sky monitoring. Thirty-eight of these events were discovered by the OGLE team.
After correcting for the detection sensitivity, the team derived the distribution of planet frequency as a function of mass. The results show that low-mass, cold super-Earths are very common, typically found around one in every three stars in the Milky Way.
Interestingly, the frequency of planet occurrence does not follow a simple power-law function of planetary mass. Instead, the observed distribution is best described by a combination of two components resembling Gaussian curves. This may point to distinct processes for forming planets of different masses – or a single process in which gas giants form only above a certain core mass threshold, while lower-mass planets remain as super-Earths.
The study represents a major advance in our understanding of planetary system architecture. Ongoing and future microlensing surveys are expected to increase the number of known planets of this type and help refine the results reported in Science.
We have high hopes for NASA’s Roman Space Telescope mission, planned for launch in 2027. It could revolutionize the field of microlensing-based exoplanet discovery – much like Kepler did for transitbased searches, says Dr. Przemek Mróz, OGLE team member, co-author of the Science paper, and discoverer of many exoplanets.
The OGLE project is one of the largest and longest-running sky surveys in the world. For over 33 years, it has been conducting regular photometric monitoring of vast regions of the sky from Las Campanas Observatory in Chile. OGLE has contributed to many areas of modern astrophysics, including the search for exoplanets, studies of the structure and evolution of the Milky Way and nearby galaxies, variable stars, quasars, transient phenomena such as novae and supernovae and studies of dark matter.
The OGLE project is co-funded by Polish Ministry of Science and Higher Education, National Science Centre (MAESTRO, OPUS, HARMONIA, SONATA and SYMFONIA grants) and Foundation for Polish Science.
Paper presenting results of these studies appeared on April 25, 2025 in Science.
