For more than a decade, astronomers have puzzled over a cosmic mystery: why do exoplanets seem to cluster into two distinct sizes, with almost nothing in between? Planets tend to be either small and rocky—called super-Earths—or larger and puffier, called sub-Neptunes, with a conspicuous gap at around 1.8 times Earth's radius. Now NASA is proposing a new mission to crack the case.
The Early eVolution Explorer, or EVE, aims to solve what planetary scientists call the "radius valley" by doing something that has proved extraordinarily difficult: catching planets while they're still young enough to reveal how they form. Of the roughly 6,000 exoplanets discovered so far, only about 20 are younger than 50 million years old. EVE wants to change that dramatically, monitoring 30 different fields of young star clusters for 30 days each over its planned 2.5-year lifespan, capturing light from roughly 20,000 newly formed stars.
Two competing theories explain the radius valley's existence. The first, the "shrinking gas-dwarf" hypothesis, suggests all protoplanets start as rocky cores and accumulate massive clouds of hydrogen and helium. If a planet orbits too close to its young, scorching star, intense radiation boils away that gaseous envelope, leaving behind a stripped rocky super-Earth. Sub-Neptunes, by contrast, form just far enough away to retain their gas and maintain that puffy appearance.
The alternative theory paints a different picture: that super-Earths and sub-Neptunes are fundamentally different from birth. In this scenario, super-Earths are made of dry rock, forming close to their host star. Sub-Neptunes are actually water worlds, forming beyond the "snow line" where water freezes, with a composition roughly split between rock and water. The radius valley, then, simply reflects the difference between the maximum size dry rock can reach and the minimum size of a half-water, half-rock world.
To distinguish between these theories requires observing planets in their infancy, before billions of years of evolution blur the original story. But finding young planets is devilishly tricky because young stars are extraordinarily active, producing frequent flares that mimic planetary signals in telescope data. EVE solves this problem with elegant engineering: it will carry three separate sensors operating in different wavelengths—near-ultraviolet, optical, and near-infrared light. Since solar flares show up prominently in ultraviolet data, EVE's instruments can subtract that noise from the signal, allowing genuine planets to emerge clearly.
The results EVE produces will depend entirely on which theory is correct. If planets regularly form as puffy gas dwarfs, the mission could discover around 100 young sub-Neptunes. But if sub-Neptunes are actually dense water worlds, EVE would only find about five new planets, since the rest would be too small to spot against their bright host stars, even with the mission's sophisticated signal processing.
The project, led by researchers including George Zhou, is currently positioned as a NASA Small Explorers mission—a category that has recently received increased attention and funding. Though not yet funded, it represents a promising next step in understanding how planets evolve from their earliest days. If adopted, EVE could provide a template for tracking planetary development across another crucial stage of cosmic history, finally answering how our solar system's neighbors came to be.