Hundreds of exoplanets are much larger than we thought

For years, astronomers scanning the sky with NASA’s Transiting Exoplanet Survey Satellite (TESS) have cheered each new detection of a small, rocky planet. These findings seemed to inch us closer to answering the age‑old question of whether life exists elsewhere in the cosmos.

However, a fresh look at the data suggests many of those celebrated worlds are not the Earth‑like bodies scientists hoped for.

According to a new study led by the University of California, Irvine, the radii of more than 200 TESS planets have been systematically underestimated – sometimes by enough to bump them out of the “potentially habitable” category altogether.

“We found that hundreds of exoplanets are larger than they appear, and that shifts our understanding of exoplanets on a large scale,” said Te Han, a UC Irvine doctoral student who led the research. “This means we may have actually found fewer Earth‑like planets so far than we thought.” 

Misjudging exoplanet size

Conventional planet-hunting relies on the transit method. Astronomers monitor a star’s brightness and watch for the slight dip that occurs when a planet crosses the star.

From that dimming, they infer the planet’s radius. But the technique works only if the starlight is clean – an increasingly tall order in crowded regions of the sky.

“We’re basically measuring the shadow of the planet,” explained co‑author Paul Robertson, a UC Irvine astronomy professor.

If the telescope’s pixels collect extra light from background stars, the shadow looks shallower than it really is, making the planet seem smaller. This dilution problem worried astronomers even during NASA’s Kepler mission.

However, TESS, with its wide‑field cameras and larger pixels, is even more vulnerable. Most teams assumed existing software pipelines handled the issue well enough. Han decided to test that assumption.

Calculations of planet density

Digging through hundreds of peer‑reviewed papers, Han compiled measurements for single‑planet systems announced by TESS teams through late 2024. He then applied a deblending code called TGLC, which models exactly how each pixel’s light gets smeared across the detector.

The code incorporates star positions and brightness data from the European Space Agency’s ultra‑precise Gaia catalog. When TGLC processed the light curves, the transit dips deepened, implying bigger planets.

“TESS data are contaminated, which Te’s custom model corrects better than anyone else in the field,” Robertson said.

Statistical tests showed the literature had, on average, shaved about six percent off planetary radii. That may sound minor, but a radius error compounds when scientists calculate density, leading to roughly 20 percent overestimates.

Furthermore, densities feed directly into guesses about a planet’s composition – whether it is rocky like Earth, watery, or rich in gas.

Earth twins reclassified

Before the reanalysis, astronomers believed TESS had spotted three solo exoplanets close enough to Earth’s size and composition to warrant detailed follow‑up. Han’s corrections knock all three off that list.

“Of the single‑planet systems discovered by TESS so far, only three were thought to be similar to Earth in their composition,” noted Han. “With this new finding, all of them are actually bigger than we thought.”

The revamped sizes place them in the realm of so‑called water worlds – planets cloaked in global oceans – or even miniature Neptunes, wrapped in thick envelopes of hydrogen and helium.

Both categories are intriguing, but neither offers the same surface conditions that make Earth habitable.

New targets for the Webb Telescope

The stakes go beyond tallying Earth analogues. Space- and ground‑based observatories have limited time, and astronomers rely on planet catalogs to decide which worlds merit a closer look.

“This has important implications for our understanding of exoplanets – including prioritization for follow‑up observations with the James Webb Space Telescope, and the controversial existence of a galactic population of water worlds,” Robertson said.

A planet now revealed to be larger – and therefore puffier – may actually be a better Webb target. Its atmosphere is easier to probe, making it more suitable for detailed study. Conversely, a world once hailed as a rocky twin could drop in priority.

More exoplanet size revisions ahead

Han’s team will revisit planets previously dismissed as too large or too low‑density to support life. Some may shrink back into the habitable zone once more precise corrections are applied; others may grow even farther from it.

Equally important, the researchers urge colleagues to treat raw TESS light curves with caution. The same lessons apply to upcoming missions, such as the European Space Agency’s PLATO, which will also rely on wide‑field, transit‑based searches.

The study highlights an ongoing theme in exoplanet science: indirect measurements of size and other properties can mislead, and small biases ripple outward into big conclusions about life in the universe.

But there is a bright side. Astronomers now have better tools to compensate for contamination. Thanks to Gaia, they also have a stellar map precise enough to support those tools.

Every correction sharpens our cosmic census, bringing the truly Earth‑like worlds into clearer focus – even if there turn out to be fewer of them than we hoped. In the hunt for extraterrestrial life, knowing where not to look can be just as valuable as knowing where to point the telescope next.

The study is published in The Astrophysical Journal Letters.

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