A small space telescope concept has been proposed to settle one of the most stubborn arguments in exoplanet science: why the galaxy seems to skip over worlds that are about 1.8 times the size of Earth. The proposed mission, called the Early eVolution Explorer (EVE), would stare at young star clusters for 2.5 years and try to catch planets while they are still being assembled — before the physics that carves the so-called radius valley has finished its work.
The mission concept, detailed in a recent arXiv preprint led by George Zhou and collaborators, has been pitched as a NASA Small Explorers (SMEX) candidate. It remains unfunded.
The valley that shouldn’t be empty
Of the thousands of exoplanets confirmed since the early 1990s, a strange gap shows up in the size distribution. Planets a little larger than Earth are common. Planets a little smaller than Neptune are common. Worlds right in between — about 1.6 to 1.8 Earth radii — are oddly rare. Astronomers have called this the radius valley, and they have spent more than a decade arguing about what creates it.
Two explanations dominate. The first, the shrinking gas-dwarf hypothesis, says these planets all start out puffy, with thick hydrogen and helium envelopes. Intense radiation from their young host stars strips those envelopes away. Planets that lose their atmospheres shrink into the super-Earth pile. Planets that hang on stay as sub-Neptunes. The valley is what’s left in the middle.
The second is the water-world hypothesis. It argues that these are fundamentally two different kinds of planet from birth — small rocky worlds on one side, and dense, water-rich worlds on the other, with no migration between them. A 2022 study led by University of Chicago astronomer Rafael Luque, reported in Popular Science, found that density — not radius — separated planets around red dwarf stars into three families: rocky, gaseous, and water-rich worlds that may be roughly half H2O by mass.
Both hypotheses fit the data the field currently has. That’s the problem.
Why young planets are the answer
To distinguish the two, astronomers need to look at planets before atmospheric stripping has run its course. If the gas-dwarf model is right, young planetary systems should be teeming with puffy sub-Neptunes that haven’t yet lost their envelopes. If the water-world model is right, those puffy planets shouldn’t be there at all — the population should look much like it does today, just younger.
The catch: young planets are scarce in the catalog. Young stars are noisy. They flare. They spot. They rotate fast. All of that makes it hard to pick out the tiny dip in starlight that signals a transiting planet.
EVE is designed around that noise problem.
Three sensors, one job
The telescope would carry three separate detectors covering near-ultraviolet, optical, and near-infrared wavelengths simultaneously. Stellar flares tend to brighten dramatically in the UV while barely registering in the infrared. A real transiting planet does the opposite — it produces a roughly wavelength-independent dimming. By comparing all three channels in real time, EVE’s pipeline can flag stellar activity and isolate the planetary signal.
The observing plan calls for monitoring 30 fields of young star clusters, each for 30 days, capturing light from about 20,000 newly formed stars across the 2.5-year mission. That cadence is long enough to catch multiple transits of close-in planets and short enough to fit dozens of clusters into one mission lifetime.
The numbers that will settle it
The Zhou team’s modeling produces two starkly different predictions, which is what makes the mission valuable. If the gas-dwarf hypothesis is correct and young sub-Neptunes really are puffy and abundant, EVE should find roughly 100 small young planets. If the water-world hypothesis is correct, EVE should find only about five.
That’s a 20-to-1 ratio. Few exoplanet experiments offer a discriminator that clean.
A null-ish result — five planets — would not be a failure. It would be a strong constraint, and a hint that the planets we see around mature stars were largely built the way they look, rather than carved by radiation over hundreds of millions of years. A planet-rich result would close the door on water worlds as the dominant explanation for the valley.
The funding question
EVE has been submitted into NASA’s Small Explorers line, the same program that has produced compact astrophysics missions like IXPE and SPHEREx. NASA’s 2026 Astrophysics Small Explorer Announcement of Opportunity set the framework for the current competition, with selected concepts moving through phased study before any flight decision.
The budget environment is tight. A House appropriations subcommittee in April advanced a fiscal year 2027 spending bill that held NASA’s overall funding flat at $24.438 billion, rejecting a White House proposal to cut the agency by 23%. Within that flat top line, science took a $1.25 billion reduction from 2026 levels, though the figure was well above the administration’s $3.9 billion request.
Flat funding is a softer landing than the proposed cuts, but it still squeezes the science portfolio. SMEX missions are relatively cheap by NASA standards — typically capped around $150 million plus launch — which is part of what makes them politically survivable in lean years. EVE’s modest scope and sharp scientific question are the kind of pitch the Explorer program was designed to reward.
Why this matters beyond one valley
The radius valley is a small feature on a histogram. The argument behind it is not. Whether sub-Neptunes are stripped gas dwarfs or born water-rich changes the inventory of potentially habitable real estate in the galaxy. It changes how astronomers interpret atmospheric measurements from JWST and future observatories. It changes which target stars are worth pointing at for biosignature searches.
It also changes the assumed history of every planet we’ve already found. A mature super-Earth that was once a puffy sub-Neptune is a very different object than one that has always been rocky — different interior chemistry, different volatile inventory, different odds of holding onto an atmosphere long enough for chemistry to get interesting.
Recent work on individual planets has hinted at how varied these worlds can be. JWST thermal-emission spectroscopy of the ultra-hot super-Earth 55 Cancri e, for instance, suggests the planet may have a volatile atmosphere sustained by a magma ocean — a configuration that doesn’t fit neatly into either side of the radius valley debate.
The mission’s logic
EVE is small, narrow, and built around a single question with a binary answer. That focus is a feature. Big flagship telescopes get justified by their flexibility — they can do many things passably. Explorer-class missions earn their slots by doing one thing better than anything else can.
The Zhou team’s bet is that catching planets young, in UV through near-infrared, around the noisiest stars in the sky, is something only a dedicated instrument can do well. If NASA agrees, the next several years could finally produce data that ends a debate the field has been having since the Kepler era.
If not, the radius valley keeps its secret a little longer.
