Astronomers observe a „radius valley,“ a curious lack of exoplanets around 1.8 times Earth’s radius, with many smaller rocky super-Earths and larger sub-Neptunes. The proposed NASA Early eVolution Explorer (EVE) mission aims to solve this by studying young planets in newly formed star clusters.
Two main theories explain the gap. One suggests close-orbiting planets lose their thick hydrogen/helium envelopes due to stellar radiation, becoming rocky super-Earths, while others farther out retain atmospheres and become sub-Neptunes. The other theory proposes planets form distinctively from birth—super-Earths as dry worlds near stars, and potentially water-rich sub-Neptunes farther away. EVE will monitor thousands of young stars (under 50 million years old) using multi-band sensors to differentiate planet signals from stellar flares, hoping to determine if gas stripping or initial formation dictates planet types.
Mankind venturing into space ended up creating more questions than it answered, and one of the dilemmas is related to the planet sizes. Astronomers have found plenty of rocky super-Earths and plenty of puffier sub-Neptunes, but far fewer planets with a radius of about 1.8 times Earth’s.
That gap is known as the radius valley, and a proposed mission called the Early eVolution Explorer, or EVE, wants to figure out why it exists. NASA has a simple plan: look at planets while they are still young. The mission concept, detailed in a new arXiv preprint and covered by Phys.org, would focus on newly formed star clusters to see what small planets look like before billions of years of evolution.
NASA
The fight is gas dwarfs versus water worlds
There are two main explanations on the table. One says many small planets begin life with thick hydrogen and helium envelopes. If they orbit too close to young, active stars, stellar radiation strips that gas away, leaving rocky super-Earths behind. Planets farther out keep their atmospheres and become sub-Neptunes.
The other theory says the split is baked in from birth. In that version, super-Earths form as dry rocky worlds closer to their stars, while sub-Neptunes form farther out as dense, and possibly water-rich planets. So it is more of a cosmic sorting line rather than a dramatic stripping event. EVE would try to settle the argument by studying planets younger than 50 million years.
EVE would watch thousands of young stars
This artist’s concept shows what the hot sub-Neptune exoplanet TOI-421 b could look like. NASA, ESA, CSA, Dani Player (STScI)
The proposed mission would monitor 30 different fields of young star clusters for 30 days each, covering roughly 20,000 newly formed stars over a planned 2.5-year mission. To deal with the chaos of young, flare-happy stars, EVE would use three sensors, including near-ultraviolet, optical, and near-infrared.
Since stellar flares can mimic planet signals, the multi-band solution would bring better clarity. Researchers hope EVE could clean up the noise and reveal actual transiting planets by tracking flare activity in ultraviolet light.
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