Furthermore, the densities measured for some of the transiting planets were too low for them to be purely rocky bodies. However, only the minimum masses of the Doppler planets were known, thus the terrestrial nature of these planets couldn't be definitively established. This naming was somewhat aspirational, reflecting the hopes that exoplanet detection had crossed a divide from gaseous to terrestrial, and that addressing questions of extrasolar habitability were just around the corner. The first examples of these objects that were discovered by Doppler (i.e., radial velocity) and transit surveys were nearly universally dubbed “super‐Earths” (Charbonneau et al., 2009 Dawson & Fabrycky, 2010 B. Although this class of planet is essentially defined as being unlike previously known objects, it was natural to try to identify the best solar system analog and associated formation history for comparison. We summarize the results of atmospheric studies of these objects in Section 5, and we conclude with a look at the future directions for research in this topic in Section 6.Ģ. Historical Perspective on the Characterization of Sub‐Neptune Size ExoplanetsĪ major hindrance to understanding the origins of sub‐Neptune exoplanets has been the persistent question of their basic nature.
We discuss the formation pathways implied by these results in Section 4. The most important insights to the nature of sub‐Neptune planets have come from the statistical distribution of their radii and densities, which we review in Section 3. We discuss the early history of the discovery and nomenclature of these objects in Section 2. In this article we review the current state of knowledge of close‐in, sub‐Neptune size planets. See Winn ( 2018) for more on selection biases of exoplanet surveys. The area in the bottom right of the figure is mostly empty due to selection effects. Most of the planet candidates that Kepler discovered are smaller than Neptune and are likely to be real planets. The color of the points (the “Score”) is related to the estimated likelihood of being a true planet, where larger values indicate a higher likelihood. Radii and orbital period for transiting planet candidates detected by Kepler from Thompson et al. ( 2018). Observations with upcoming facilities are expected to finally reveal the atmospheric compositions of these worlds, which are arguably the first fundamentally new type of planetary object identified from the study of exoplanets. However, most atmospheric characterization efforts have been confounded by aerosols. Atmospheric studies have the potential to break degeneracies in interior structure models and place additional constraints on the origins of these planets.
As with the mass‐loss mechanism, there are two contenders for the origins of the solids in sub‐Neptune planets: the migration model involves the growth and migration of embryos from beyond the ice line, while the drift model involves inward‐drifting pebbles that coagulate to form planets close‐in. The mechanism that drives atmospheric loss for these planets remains an outstanding question, with photoevaporation and core‐powered mass loss being the prime candidates. Planets above the radius gap were able to retain their atmospheres (“gas‐rich super‐Earths”), while planets below the radius gap lost their atmospheres and are stripped cores (“true super‐Earths”). This bimodality suggests that sub‐Neptunes are mostly rocky planets that were born with primary atmospheres a few percent by mass accreted from the protoplanetary nebula. Results from NASA's Kepler mission have revealed a bimodality in the radius distribution of these objects, with a relative underabundance of planets between 1.5 and 2.0 R ⊕. Planets intermediate in size between the Earth and Neptune, and orbiting closer to their host stars than Mercury does the Sun, are the most common type of planet revealed by exoplanet surveys over the last quarter century.
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