The quest to unravel the enigma of the exoplanet radius valley has led NASA to propose the Early eVolution Explorer (EVE) mission, a groundbreaking endeavor that could revolutionize our understanding of planetary formation. This mission aims to address a decade-long debate among planetary scientists regarding the scarcity of exoplanets with a radius of approximately 1.8 times that of Earth. The proposed mission, as outlined in a pre-print on arXiv, seeks to explore two contrasting hypotheses that attempt to explain this phenomenon.
The Shrinking Gas-Dwarf Hypothesis
One hypothesis suggests that protoplanets begin as rocky cores and gradually accumulate massive, gaseous atmospheres composed of hydrogen and helium. However, proximity to their host stars can lead to a dramatic transformation. The intense radiation and heat from young stars can cause these atmospheres to evaporate, leaving behind a rocky core, resulting in the formation of super Earths. In contrast, Sub-Neptunes are believed to be planets that have managed to retain their gaseous envelopes due to their distance from their active stars.
Dense Water Worlds Hypothesis
An alternative theory proposes a more fundamental distinction between the two types of planets from the outset. According to this scenario, Super Earths are formed from dry rocks, close to their host stars and within the 'snow line,' where water can freeze. On the other hand, Sub-Neptunes are considered water worlds that form beyond the snow line, resulting in a composition of approximately 50% rock and 50% water. The 'radius valley' in this hypothesis is attributed to the difference in size between the maximum physical size of a dry rock and the minimum physical size of a half-water, half-rock hybrid.
The Challenge of Detecting Young Exoplanets
Determining which hypothesis is correct requires capturing exoplanets during their early stages of formation. However, this presents a significant challenge. Out of the approximately 6,000 exoplanets discovered, only around 20 are younger than 50 million years old. The EVE mission aims to address this issue by monitoring 30 different fields of young star clusters for 30 days each, capturing light from roughly 20,000 newly formed stars over its 2.5-year lifespan.
EVE's Sensor Suite
To combat the difficulty of detecting planets around young, active stars, EVE will be equipped with a sophisticated sensor suite. It will feature three separate sensors: a near-ultraviolet (NUV), an optical, and a near-infrared (NIR) sensor. Solar flares, which are prominent in the ultraviolet band, can be subtracted from the data using the NUV instrument, allowing the detection of actual planets in the vicinity.
Implications and Future Prospects
The outcomes of the EVE mission are expected to vary depending on the correctness of the hypotheses. If the universe indeed produces puffy gas-dwarfs, as suggested by the first hypothesis, EVE could potentially uncover as many as 100 small, young planets, specifically Sub-Neptunes. However, if Sub-Neptunes are indeed dense water worlds, the mission is projected to identify only about 5 new planets, as the rest would be too small to distinguish from their host stars.
While the EVE mission is not yet funded, it is positioned as a NASA Small Explorers (SMEX) mission, which has been receiving increased attention and funding. If adopted by a major space agency, EVE could provide a crucial template for tracking planets through another stage of their evolutionary process, offering valuable insights into the long-standing question of how exoplanets form and evolve.
In conclusion, the EVE mission represents a significant step forward in our quest to understand the exoplanet radius valley mystery. By addressing the limitations of current exoplanet research, it has the potential to unlock new discoveries and deepen our understanding of planetary science.
