Super-Earth exoplanets—worlds slightly larger than Earth—are common in the outer regions of planetary systems, where gas giants like Jupiter are typically found. A new international study, published today in Science, adds an important piece to the puzzle of planetary system formation by highlighting a previously underexplored population of cold, rocky planets orbiting far from their stars.
“We found a ‘super Earth’—meaning it’s bigger than our home planet but smaller than Neptune—in a place where only planets thousands or hundreds of times more massive than Earth were found before,” said Dr. Weicheng Zang, lead author of the study, awarded the Tsinghua University Special Prize in 2021, and now a CfA Fellow at the Center for Astrophysics (CfA), Harvard & Smithsonian. Much of the research was completed during his time at Tsinghua, which is the first institutional affiliation on the paper.
Using data from the Korea Microlensing Telescope Network (KMTNet), the team analyzed a total of 63 planets detected via microlensing, a sample which is a factor of three larger than the previous one. This study reveals a comprehensive picture of planetary populations in the cold outer regions of planetary systems. The team found a bimodal mass ratio distribution—evidence of two distinct populations of planets: one consisting of rocky super-Earths and another of massive gas giants. This research provides new clues for our understanding of the planet formation process.
Periods and mass ratios for planets detected with different methods. The magenta star indicates the super-Earth discovered in this study, located beyond Saturn’s orbit. Solid magenta circles represent the statistical sample from this work, consisting of 63 microlensing planets detected by the KMTNet survey. Open magenta circles are other planets detected by microlensing. This sample reveals two distinct populations of planets (super-Earth vs. gas giant) on Jupiter-like obits.
The new work was published in the journal Science on April 25, 2025. Weicheng Zang is the first author. Weicheng Zang, Youn-Kil Jung, Shude Mao (former chair of DOA) and Chung-Uk Lee from KASI are the co-corresponding authors. Other Tsinghua contributors to this work include Prof. Wei Zhu, former undergraduate Xiangyu Zhang, former doctoral student Hongjing Yang and Renkun Kuang, former summer undergraduate researcher Hanyue Wang, PhD student Jiyuan Zhang and Zhecheng Hu. In addition to KMTNet, the Optical Gravitational Lens Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA) survey groups contributed data for the planet characterization.
The work of the Tsinghua team was supported by a key program of the National Natural Science Foundation of China.
Paper Link: http://www.science.org/doi/10.1126/science.adn6088
Writer and photo credit: Zang Weicheng
Editor: Guo Lili
