Are We There Yet? Understanding How Often Earth-like Worlds Exist Around Other Stars

Rachel Fernandes and collaborators set out to answer one of astronomy’s most compelling questions: how common are planets like Earth that orbit within their star’s habitable zone ,  the region where liquid water could exist? This frequency is represented by the symbol η⊕ (Eta-Earth). Determining η⊕ is vital for understanding how many potentially habitable worlds might be close enough for future telescopes, such as NASA’s Habitable Worlds Observatory, to study in search of life.

Defining “Earth-Like” and the Habitable Zone

To measure η⊕, researchers must decide what counts as “Earth-like.” Fernandes explains that a planet is typically considered Earth-like if it’s rocky and similar in size to Earth, between about 0.5 and 1.5 times Earth’s radius. Larger planets often become “sub-Neptunes,” gas-rich and less likely to be habitable.

The habitable zone (HZ) is defined as the distance from a star where a planet could have liquid water on its surface. For the Sun, early models placed this region between 0.95 and 1.37 astronomical units (AU), later refined to roughly 0.99–1.70 AU. Because each star’s brightness differs, the HZ moves inward for dimmer stars and outward for brighter ones. These shifting definitions already make it difficult to measure η⊕ consistently.

From Kepler to Today: The Challenge of Counting Earths

The Kepler mission revolutionized our understanding of planets by revealing that small worlds are common. Yet even Kepler struggled to find true Earth analogs. Its limited observing time meant few planets were seen completing full orbits in the HZ, so scientists had to extrapolate from planets with shorter orbits or larger sizes.

Different studies using Kepler data have produced widely varying values for η⊕, from less than 0.2 to almost 1.0. Fernandes and colleagues trace this variation to factors such as differences in the definitions of “Earth-sized,” the data sources used, and how each study corrected for completeness (how many planets are detected) and reliability (how many detections are real). Even with improvements from Gaia, which refined stellar sizes, large uncertainties remain. The team emphasizes that the scarcity of detections in the true Earth-like regime makes current estimates unreliable.

Hidden Complications: Stars, Systems, and Chemistry

Several factors further complicate η⊕ estimates. Binary stars, which make up roughly half of Sun-like stars, can distort measurements. A hidden companion star can make a planet appear smaller or alter how easily it can be detected. Correcting for this “binary contamination” could increase η⊕ by up to 40%.

Similarly, many stars host multiple planets, but η⊕ calculations usually treat planets as independent. In reality, systems may have several Earth-sized planets within their habitable zones, meaning η⊕ might underestimate the true number of habitable worlds per star. Fernandes’s team also highlights stellar composition, a star’s metallicity and chemical makeup, as an emerging factor. Stars richer in heavy elements might form fewer watery, Earth-like planets because their disks contain less water ice.

The Role of Time and Evolution

A planet’s age and its star’s evolution also influence its habitability. Over time, stars grow brighter, shifting the habitable zone outward. Meanwhile, young planets can lose their atmospheres through intense radiation or internal heating, shrinking from mini-Neptunes to rocky worlds. These processes create “radius gaps” and complicate attempts to infer how many Earth-sized planets survive in habitable conditions billions of years after formation.

Looking Ahead: Toward a Sharper η⊕

Fernandes and her co-authors conclude that measuring η⊕ precisely will require new tools. Future missions like the Habitable Worlds Observatory, LIFE (Large Interferometer for Exoplanets), and next-generation telescopes will observe nearby systems directly rather than relying on statistical extrapolation. More complete surveys of binary stars, longer observation baselines, and better chemical and age data will also help refine η⊕.

Conclusion

In short, while astronomers have made great progress since Kepler, Fernandes reminds readers that the road to accurately counting “Earths” is still long. Understanding η⊕ remains one of the field’s grand challenges, essential for determining whether our planet is a cosmic rarity or just one of many havens for life among the stars.

Source: Fernandes