The quest to unravel the enigma of the radius valley in exoplanets has led NASA to propose the Early eVolution Explorer (EVE) mission, a bold endeavor that could revolutionize our understanding of planetary formation. This mission, currently in the pre-print phase, aims to address a decade-long debate among planetary scientists about the scarcity of exoplanets with a radius of approximately 1.8 times that of Earth. The proposed solution? A meticulous examination of young star clusters, seeking the elusive 'split' in planetary evolution that could validate either of two competing hypotheses.
The Radius Valley Mystery
The radius valley mystery revolves around the classification of exoplanets into two distinct groups: 'super Earths' with rocky interiors and 'sub-Neptunes' with 'puffy' compositions. The question is, what forces these planets to diverge in size? The two leading hypotheses offer contrasting explanations.
The 'Shrinking Gas-Dwarf' theory suggests that protoplanets initially gather massive, hydrogen-helium clouds, but proximity to their host stars can strip away their atmospheres, leaving behind rocky cores. Conversely, the 'Dense Water World' theory posits that sub-Neptunes are essentially water worlds, formed beyond the 'snow line' where water can freeze, resulting in a 50/50 rock-water composition. The radius valley, in this scenario, is the size difference between the maximum physical size of a dry rock and the minimum size of a water-rock hybrid.
The Challenge of Young Exoplanets
Determining which hypothesis is correct requires observing young exoplanets during their formative years. However, finding such young planets is akin to finding a needle in a haystack. Out of the 6,000 exoplanets discovered, only around 20 are younger than 50 million years old. This scarcity highlights the challenge of capturing the early stages of planetary evolution.
EVE's Innovative Approach
EVE's mission is to monitor 30 fields of young star clusters for 30 days each, capturing light from 20,000 newly formed stars over 2.5 years. The key to its success lies in its sensors: near-ultraviolet (NUV), optical, and near-infrared (NIR). These sensors are designed to differentiate between planetary signals and false positives caused by active young stars.
The NUV sensor is particularly crucial, as solar flares are prominent in the ultraviolet band. By subtracting the flare data, EVE can reveal the presence of planets, even if they are small and close to their host stars. This technique is a game-changer in exoplanet detection, especially for sub-Neptunes.
Unraveling the Hypotheses
The expected outcomes from EVE depend on the validity of the competing hypotheses. If the 'Shrinking Gas-Dwarf' theory prevails, EVE could discover up to 100 small, young planets, primarily sub-Neptunes. However, if the 'Dense Water World' theory is correct, the mission might only uncover around 5 new planets, as the rest would be too small to detect against their host stars.
The Future of Exoplanet Research
While EVE is not yet funded, its potential as a NASA Small Explorers (SMEX) mission is promising. If adopted, it could provide a template for tracking planets through different evolutionary stages, offering insights into the formation and diversity of exoplanets. This mission could answer long-standing questions and reshape our understanding of planetary evolution.
In my opinion, EVE represents a significant leap forward in exoplanet research. It combines innovative technology with a compelling scientific question, offering the potential to unlock the secrets of the radius valley. As an expert, I am eager to see the mission's progress and the insights it may bring to the field of planetary science.
