'Super-Earth' planets are more common than previously thought

Astronomers have discovered a “super-Earth” – a planet about twice the size of Earth – orbiting its star at a distance greater than where Saturn sits in our solar system.

The discovery, led by researchers at the Center for Astrophysics | Harvard & Smithsonian (CfA), upends the long-held assumption that only hulking gas giants form so far from their hosts.

“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 and the study’s first author.

Microlensing: The briefest flash of light

The newfound world emerged from the largest microlensing survey yet conducted. Microlensing exploits a quirk of general relativity: when a foreground planet and its star drift in front of a more distant background star, their gravity can bend and momentarily amplify the background star’s light.

The resulting brightening, though fleeting and subtle, carries within it the fingerprints of the intervening planet. Because microlensing does not depend on the planet’s own light, it is uniquely suited to detecting worlds in wide, cold orbits that other techniques – such as the transit or radial-velocity methods – rarely reach.

The research team analyzed observations from the Korea Microlensing Telescope Network (KMTNet), a trio of identical 1.6-meter telescopes stationed in Chile, South Africa, and Australia. Together they keep near-continuous watch on the densely packed star fields of the Milky Way’s bulge, where microlensing events occur most often.

“The current data provided a hint of how cold planets form,” said Professor Shude Mao of Tsinghua University and Westlake University, who co-supervised the project. “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.”

A census of far-flung worlds

This latest catalog triples the number of microlensing-detected planets and lowers the minimum planet mass by a factor of eight compared with previous samples.

By measuring planet-to-star mass ratios across the survey, the team constructed a statistical map of planetary populations occupying the chilly realm between Earth’s orbit and that of Saturn.

“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 from the Smithsonian Astrophysical Observatory.

The findings suggest that super-Earths – planets one to several times Earth’s mass – populate the outer zones of many systems. These planets populate just as frequently as they do the inner zones, shattering the notion that our own arrangement of small inner worlds and large outer giants is a cosmic norm.

Why this Super-Earth matters

The recently found planet orbits its parent star so distantly that a single year there might span more than a decade in Earth time.

Previous surveys had turned up only massive, Jupiter-like bodies in such orbits, reinforcing theoretical models in which abundant, icy material at those distances rapidly coalesces to form large gas envelopes.

The presence of a comparatively lightweight super-Earth hints that planetary formation can follow additional pathways, perhaps involving slower accretion rates or later-stage migration.

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

In other words, having a small rocky world – or one with a modest gaseous envelope – well beyond the snow line may be the rule rather than the exception.

A collaborative effort spanning the globe

Along with Zang, the research roster includes several other CfA scientists: postdoctoral fellow In-Gu Shin; former Harvard undergraduate Hangyue Wang, now a graduate student at Stanford University; and KASI scientist Sun-Ju Chung, who spent a sabbatical at the CfA from 2022 to 2023.

Data from the Optical Gravitational Lens Experiment (OGLE) and the Microlensing Observations in Astrophysics (MOA) surveys added crucial coverage, demonstrating how microlensing science thrives on international cooperation.

As KMTNet and companion programs continue to monitor our galaxy and beyond, astronomers expect the census of distant super-Earths to swell. Each new data point will tighten estimates of how frequently such planets occur and refine theories of how icy, outer regions of planetary systems assemble their worlds.

Ultimately, these results underscore that the Milky Way is teeming with planetary diversity. Our solar system – with its tidy inner terrestrials and hulking outer giants – may be only one of many possible configurations.

As microlensing surveys push to detect ever smaller planets and combine their statistics with those from transit and radial-velocity missions, a fuller portrait of planetary architecture throughout the galaxy is finally coming into view.

The study is published in the journal Science.

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