Earths emerge from cosmic snowstorm

Hefty planets that weigh up to 15 times more than Earth – known as “super-Earths” – may actually be giant ice balls that formed during a cosmic snowstorm that lasted millions of years, according to research by a team of astronomers that includes Grant Kennedy from The Australian National University.

Mr Kennedy, a PhD scholar at the ANU Research School of Astronomy and Astrophysics at  Mount Stromlo Observatory and his colleagues Scott Kenyon from the Smithsonian Astrophysical Observatory and Benjamin Bromley from the University of Utah, calculated that some super-Earths build up rapidly when local temperatures drop and ices condense out of the surrounding gas. 

There is great variety among the 200 known planets that orbit other stars, ranging from the recently discovered icy “super-Earth” planets to low-mass gas giants. Scientists have been able to explain how planets form around Sun-like stars, but have been surprised by the discovery of planets around the relatively smaller and cooler red dwarf stars.

In a planetary system such as our Solar System, planets form within a disk of gas and dust surrounding a newborn star. Rocky planets like Venus, Earth and Mars form close to the star, where it is warm, while icy and gaseous planets like Jupiter, Saturn and Uranus form farther out. When it was young, the Sun was relatively stable, leading to a natural progression of small, rocky worlds in the hot inner Solar System and large, gaseous worlds in the cold outer solar system.

The planetary systems around newborn red dwarf stars undergo more dramatic changes. As the young star evolves, it dims and the warm inner disk begins to freeze, creating conditions where water and other volatile gases condense into snowflakes and ice pellets.

"It's like a massive cold front that sweeps inward toward the star," Kennedy said. "The ices add mass to a growing planet, and also make it easier for particles to stick together. The two effects combine to produce a planet several times the size of Earth.

“The disks that surround red dwarf stars tend to contain less material than the one that formed our Solar System. Without the "snowstorms" in these smaller disks, there isn’t enough material to make super-Earths.” 

The astronomers presented their calculations in a paper published in The Astrophysical Journal Letters. The paper is available at

This research provides an explanation for the formation of the small icy super-Earth found by the PLANET team that included RSAA Director Professor Penny Sackett earlier this year

Editor's Note: Original news release can be found here.