A debate has been raging amongst planetary scientists for over a decade - why are there so few exoplanets with a radius of about 1.8 times that of the Earth? Exoplanets are currently largely grouped into two distinct groups - “super Earth” are below that size and have rocky interiors, whereas “Sub-Neptunes” are above that size limit and appear “puffier.” But we don’t really understand what about the path of planetary evolution forces this bifurcation. A new mission proposal, called the Early eVolution Explorer (EVE) wants to find out, and a draft of its concept can be found in pre-print form on arXiv.

Currently, the debate about where this “radius valley” exists in the exoplanet population focuses on two hypotheses. First is the “Shrinking Gas-Dwarf” hypothesis. In this scenario, all protoplanets start as rocky cores and sweep up massive, puffy clouds of lightweight hydrogen and helium for millions of years. But if the planet is too close to its host star, the intense radiation and heat from these relatively young stars boil away the atmosphere leaving behind a rocky core, becoming a super Earth. Sub-Neptunes, on the other hand, are the planets just far enough away from their young, active star to not be stripped of their gas envelopes.

The alternative hypothesis can be thought of as showcasing dense water worlds. In this scenario, the two types of planets are fundamentally different even at birth. Super Earths are formed from dry rocks, close to the host star and inside the “snow line” where water can freeze. Sub-Neptunes, in this theory, are actually water worlds that form beyond the snow line, resulting in a bulk composition of about 50% rock and 50% water. The “valley” in this theory is just 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.

Fraser discusses the gap in exoplanet populations that is the driving force of the EVE mission.

To tell which of these theories is correct, exoplanet researchers think we have to catch the planets early on in their formation to see if there is actually a “split” as detailed in the water world hypothesis or whether a planet’s early years are the same no matter its eventual fate, as in the Shrinking Gas-Dwarf hypothesis. The problem is finding “young” exoplanets. Of the 6,000 or so we’ve found so far, only around 20 are younger than 50 million years old.

EVE wants to drastically increase that number. Its intention is to monitor 30 different fields of young star clusters for 30 days each, capturing light from roughly 20,000 newly formed stars during its 2.5 year lifespan. That seems simple enough, but the devil is in the details - or in this case the sensors.

It's notoriously difficult to find planets around young stars, simply because those young stars are so active. They actively flare very often, resulting in spectral signals that look like planets, but aren’t. To combat these false positives, EVE will be equipped with three separate sensors. A near-ultraviolet (NUV), an optical, and a near-infrared (NIR). Solar flares show up prominently in the ultraviolet band, so the data from EVE’s NUV instrument can be used to subtract out the value of the flares from the data, allowing whatever planets might be in the vicinity to shine through.

Fraser discusses the future of exoplanet research.

The end results the project team expects from EVE vary depending on which hypothesis is correct. If the universe regularly makes puffy gas-dwarfs, as in the first hypothesis, EVE could find as many as 100 small, young planets (specifically sub-Neptunes). The mission focuses on small planets and explicitly excludes true hot Jupiters or warm Jupiters. But, if sub-Neptunes are actually dense water worlds, the team expects EVE would only find about 5 new planets, as the rest would be too small to spot against their host stars even with EVE’s noise-cancelling abilities.

To be clear, the project isn’t funded yet, but is positioned as a NASA Small Explorers (SMEX) mission, which have received increased attention and funding lately. If it is eventually adopted by one of the big space agencies, we might soon get a template for tracking planets through another stage in their evolutionary process - and answer some long-standing questions about exactly how they do that.

Learn More:

G. Zhou et al - Preparing for the Early eVolution Explorer: Detecting the Primordial, Transiting Exoplanet Population

UT - Closing The Exoplanet Radius Gap

UT - NASA Defines Gaps In Exoplanet Science

UT - A Statistical Analysis of Exoplanet Habitability Turns Up One Great Candidate - And Significant Observational Bias