An article published in “The Astronomical Journal” reports a study on the scarcity of exoplanets with a radius between 1.5 and 2 times the Earth’s. A team of researchers led by Trevor David of the Flatiron Institute in New York studied what is commonly referred to as a radius gap using data collected by NASA’s Kepler Space Telescope. By dividing the exoplanets studied between those older than 2 billion years and those younger, the exam offers new information confirming the theory that sub-Neptunes can lose most of their atmosphere and transform into super-Earths even after billions of years.
The mission of the Kepler space telescope ended at the end of October 2018 but over the course of years of observations, it made it possible to discover a few thousand exoplanets. This made greatly improved the statistics regarding the various types of existing planets, which turned out to be very diverse. However, planets with a radius between 1.5 and 2 times that of Earth are a rarity that came to be called a radius gap, which has become a topic of study.
Over time, various explanations have been proposed, among which there’s the possibility that a sub-Neptunian planet, or mini-Neptune, can lose most of its atmosphere transforming into a super-Earth. Trevor David’s team addressed the issue by examining exoplanets discovered thanks to the Kepler space telescope for which precise data is available. The 732 selected planets were split between the ones older than 2 billion years and the younger ones. The result of this division is that among the young planets the rarest ones have a radius of 1.6 times the Earth’s while among the ancient planets the rarest ones have a radius of 1.8 times the Earth’s.
The analysis of the size of exoplanets suggests that the smallest sub-Neptunes can’t retain their whole atmosphere, even though the transformation into super-Earths can take a few billion years. This process is longer for the larger Sub-Neptunes. More massive planets such as Neptunes and even more so the big gas giants seem to have sufficient gravity to maintain their atmosphere.
The results of this study also confirm two hypotheses already proposed over the years regarding the mechanisms that cause the loss of atmosphere of sub-Neptunian planets. Planetary formation leaves a residual heat and the emissions of a star provide more energy to its planets, phenomena that heat the atmosphere of a planet favoring the escape of gases into space. That’s why hot Neptunes, very close to their star, are prime candidates for transformation into super-Earths.
The infographic (Courtesy Simons Foundation. All rights reserved) illustrates the statistics of exoplanets with the radius gap and the possible mechanisms that make mini-Neptunes shrink until they turn into super-Earths.
Trevor David pointed out that planets are not static spheres of rock and gas, as we sometimes tend to think, adding that some of the planets examined by applying some atmospheric loss models were ten times larger at the start of their life.
Research on the evolution of sub-Neptunian planets is far from over. This study offers new confirmation of their transformation into super-Earths but there’s still much to understand about the influence of the various processes of warming of the atmosphere on their loss. Other mechanisms could also contribute, so more complete data on candidates is needed. New exoplanets are discovered every day, so we can expect new studies and new discoveries over the next few years.
