We were miscalculating the size of the planets

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Technological Innovation Website Editorial Team - July 17, 2025

If not accounted for, the additional light from background stars reduces the estimated size of exoplanets. The square grid represents individual TESS pixels. [Image: Nikolai Berman/UC Irvine]

Size of exoplanets

By scanning more than 200 known exoplanets, astronomers have discovered an error in observations that indicates that many of the planets outside the Solar System that we know about are much larger than scientists had calculated.

It's a discovery that promises to change scientists' estimates of which distant worlds are potentially habitable, capable of accommodating extraterrestrial life.

"We've discovered that hundreds of exoplanets are larger than they appear, and this changes our understanding of exoplanets on a large scale," said Te Han of the University of California, Irvine. "This means we may have found fewer Earth-like planets so far than we thought."

Although the James Webb Telescope recently directly imaged an exoplanet, this is an exception—typically, astronomers discover planets by waiting for the planet to pass in front of its host star and then measuring the very subtle dip in the star's light—this is the planetary transit technique . "We're essentially measuring the planet's shadow," said Professor Paul Robertson.

The team was just reviewing observations of hundreds of exoplanets made by the TESS observatory (acronym for Transiting Exoplanet Survey Satellite), when they realized that the light from neighboring stars "contaminates" the light of any star being studied - in this case, to try to find its planets.

This can make any planet passing in front of a star appear smaller than it actually is, because smaller planets block less light than larger planets. However, some of the light captured isn't from the star itself, but from neighboring stars.

Data contamination

Using their calculations on this observational "noise," the team reanalyzed hundreds of exoplanet observations and classified these planets according to how different teams of astronomers measured their diameters, in order to estimate the degree of distortion in these measurements due to light contamination generated by neighboring stars.

The team also used observations from the GAIA telescope , which maps our galaxy in 3D, to help estimate how much light contamination is affecting TESS observations – Gaia has already collected a treasure trove of data about the Milky Way .

Te Han then built a model to correct for contamination in the TESS data, and the model confirmed that exoplanets are systematically larger than the original calculations had indicated.

Exoterras

The discovery of data contamination touches on a question that is central to the astronomical community: How common are Earth-sized exoplanets?

The number of exoplanets considered similar in size to Earth was already small, and now we need to start counting again. "Of the planetary systems discovered by TESS so far, only three were considered similar in composition to Earth," Han said. "With this new discovery, all of them are actually larger than we thought."

This means that rather than rocky planets like Earth, the planets are likely to be so-called Hyceans , planets covered by a giant ocean that tend to be larger than Earth, or even larger gas planets like Uranus or Neptune.

This could impact the search for life on distant planets, because while water worlds could potentially harbor life, they also may not have the same types of characteristics that contribute to the flourishing of life on planets like Earth.

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