Building Planets Into Rocky and Gassy Worlds

by | Aug 13, 2021 | Daily Space, Exoplanets | 0 comments

IMAGE: This diagram illustrates how planets are assembled and sorted into two distinct size classes. First, the rocky cores of planets are formed from smaller pieces. Then, the gravity of the planets attracts hydrogen and helium gas. Finally, the planets are “baked” by the starlight and lose some gas. The Magellan-TESS Survey aims to understand in more detail how the formation pathways for super-Earths and mini-Neptunes may differ. CREDIT: NASA/Kepler/Caltech (R. Hurt).

As frustrating as it may be that we can’t send missions to planets in our solar system more often (and it’s oh so very frustrating), it’s even more frustrating for folks studying planets around other worlds who can’t imagine ever sending spacecraft to the worlds they discover. Not only that, but the things we are most interested in studying – the evolution of planets over time – takes place at such a long scale that we can’t even dream of seeing one system evolve from start to finish on human timescales.

So instead, we look at as many systems as possible and try to build a consistent story.

One of our great frustrations is we keep seeing systems with a ton of rocky worlds way bigger than Earth and a myriad of tiny Neptunes smaller than the ice giants we have here. We actually see so many of these worlds that it’s weird we don’t have either in our solar system. And we don’t see any worlds in between these two types in size.

New work using TESS data adds evidence to the idea that this division is due to the ability of worlds to hold onto their atmospheres. It is thought that planets start with a continuous range of sizes, but there is a point at which a planet can either keep its extended atmosphere like a Neptune or that it is forced to lose that atmosphere to forces like stellar winds. That gap we’re seeing in mass is the difference in mass between a rocky super-Earth with and without its extended atmosphere.

It is going to be interesting to see how this trend works with stars of different types and such, but it feels like we are finally on the right path to understanding at least one part of planetary formation.

More Information

Carnegie Science press release

“The Magellan-TESS Survey I: Survey Description and Mid-Survey Results,” Johanna Teske et al., to be published in The Astrophysical Journal Supplement Series (preprint on


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