Population study: Exploring the transition from Super-Earth to Sub-Neptune with a Hubble transmission survey
Résumé
Exoplanets with size between the Earth and Neptune (1-4R⊕) do not have any equivalent in our Solar System and remain challenging to characterize. Yet, there are ubiquitous in the Galaxy and their distribution -number of planets per star vs radius- is bimodal highlighting a gap in the number of planets around 1.7R⊕, also known as the radius valley. Planets with a radius below 1.7R⊕ are thought to be rocky planets, and called Super-Earth, above this limit planets are most likely made of gas and called Sub-Neptune. We made use of the available data from the Hubble Space Telescope in Near-Infrared -using the Wide Field Camera 3 Grism 141- and gathered 25 transmission spectra of planets with size below 6 R⊕ to study the transition between rocky and gaseous planets. We retrieved each spectrum homogeneously with an atmospheric Bayesian code, TauREx 3.0. While it is still difficult to differentiate between a primary cloudy and a secondary atmosphere, we proved that a primary clear atmosphere dominated by hydrogen and helium is rejected with high confidence for a large majority of planets in the sample. The detectability of intermediate-size planets in the Near infrared is linked to the amplitude of the water feature around 1.4 microns and, thus, we build a new metric to assess the size of this absorption. Using this metric, we studied the cloudiness of warm Sub-Neptune and compared observational values to simulated ones. We explored the correlation between the 1.4 micron's feature amplitude and the temperature by constructing a grid in irradiation, metallicity and cloudiness using Exo-REM, a self-consistent radiative, convective model. We showed that photochemical hazes created by the photodissociation of methane in the high atmosphere of warm Sub-Neptune are likely required to explain the observations of flat transmission spectra in the Near infrared.