Super-Earths are common outside the solar system, new study shows

An international team including astronomers from the Center for Astrophysics | Harvard & Smithsonian (CfA) has announced the discovery of a planet about twice the size of Earth orbiting its star farther out than Saturn is to the sun.

These results are another example of how planetary systems can be different from our solar system.

“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 Weicheng Zang, a CfA Fellow. He is the lead author of a paper describing these results in the latest issue of the journal Science.

The discovery of this new, farther-out super Earth is even more significant because it is part of a larger study. By measuring the masses of many planets relative to the stars that host them, the team has discovered new information about the populations of planets across the Milky Way.

This study used microlensing, an effect where light from distant objects is amplified by an intervening body such as a planet. Microlensing is particularly effective at finding planets at large distances—approximately between the orbits of Earth and Saturn—from their host stars. The largest study of its kind, this work has about three times more planets and includes planets that are about eight times smaller than previous samples of planets found using the microlensing technique.

The researchers used data from the Korea Microlensing Telescope Network (KMTNet). This network consists of three telescopes in Chile, South Africa, and Australia, which allows for uninterrupted monitoring of the night sky.

“The current data provided a hint of how cold planets form,” said Professor Shude Mao of Tsinghua University and Westlake University, China. “In the next few years, the sample will be a factor of four larger, and thus we can constrain how these planets form and evolve even more stringently with KMTNet data.”

Our solar system consists of four small, rocky, inner planets (Mercury, Venus, Earth and Mars) and four large, gaseous, outer planets (Jupiter, Saturn, Uranus and Neptune). The searches for exoplanets to date using other techniques, i.e., transiting planet from telescopes like Kepler and TESS and radial velocity searches, have shown that other systems can contain a variety of small, medium, and large planets in orbits inside that of the Earth.

The latest work from the CfA-led team shows that such super-Earth planets are also common in the outer regions of other solar systems.

“This measurement of the planet population from planets somewhat larger than Earth all the way to the size of Jupiter and beyond shows us that planets, and especially super-Earths, in orbits outside the Earth’s orbit are abundant in the galaxy,” said co-author Jennifer Yee of the Smithsonian Astrophysical Observatory, which is part of the CfA.

“This result suggests that in Jupiter-like orbits, most planetary systems may not mirror our solar system,” said co-author Youn Kil Jung of the Korea Astronomy and Space Science Institute, which operates the KMTNet.

The researchers are also looking to determine how many super-Earths exist versus the number of Neptune-sized planets. This study shows that there are at least as many super-Earths as Neptune-size planets.

Other CfA contributors to this study include post-doctoral fellow In-Gu Shin, former Harvard undergraduate student Hangyue Wang (now at Stanford), and Sun-Ju Chung, a KASI scientist who visited CfA on sabbatical from 2022–2023.

In addition to KMTNet, the Optical Gravitational Lens Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA) survey groups contributed data for the planet characterization.