Demolition derby: Planet crashes explain different densities


Research sheds light on the violence of exoplanet formation. Lauren Fuge reports.


An artist's impression of one planet smashing into another -- a likely mechanism that explains different densities in adjacent planets of roughly the same size.

An artist's impression of one planet smashing into another -- a likely mechanism that explains different densities in adjacent planets of roughly the same size.

MARK GARLICK/Getty Images

Colossal cosmic crashes may influence how exoplanets form, according to research by an international team of astronomers.

“Giant impacts are thought to have occurred during the early history of our own solar system,” explains team leader Aldo Stefano Bonomo, an astronomer at INAF-Osservatorio Astrofisico di Torino, Italy.

“The Earth-Moon system likely formed in this way, after the impact of our planet with a Mars-sized protoplanet, and the origin of the high obliquity of Uranus’ orbit and of the iron-rich composition Mercury can also be explained through giant impact events.”

The new study, published in the journal Nature Astronomy, now connects the properties of planets in our own solar system with a very different planetary system.

The team studied Kepler-107, a compact system of four planets about 1700 light-years from Earth.

The researchers specifically looked at Kepler-107b and Kepler-107c. These two “super-Earths” are nearly identical in size, at about one and a half times the size of our planet. However, they are exceptionally different from each other in density and composition. The outermost planet, Kepler-107c, is twice as dense and thus must have a more iron-rich core.

So how did these otherwise twin planets become so different?

Astronomers know that the strong irradiation of a young, hot star can strip off the atmospheres of planets close to it, contributing to planetary diversity. But if this were the case, it would be Kelper-107b – closer to the star – that would be denser.

Instead, Bonomo and colleagues hypothesise that early in its life, Kepler-107c underwent a high-speed, head-on collision with another protoplanet, or multiple collisions with smaller impactors. These encounters would have ripped off part of the planet’s rocky, silicate mantle and therefore increased its density.

Jonti Horner, an astronomer at the University of Southern Queensland in Australia, who was not involved in the study, thinks the impact hypothesis is an elegant way of explaining the compositional difference between the two planets.

“It also really shows the value of being able to put together transit observations – which give planetary diameters – and radial velocity observations – which give masses,” he says.

“By getting both, you can figure out planetary densities and hence infer their compositions and internal structure, and start inferring their formation histories.”

But although interesting, it is not an overly surprising result.

“This kind of paper is a natural evolution of our increasing knowledge of exoplanetary systems,” Horner adds. “It is also a really nice illustration of the importance of our solar system in the exoplanet sense.”

According to Bonomo, there are many more fascinating questions left to answer about how planetary systems form and evolve, especially those containing super-Earths.

For instance, why are super-Earths the most common planets around other stars and yet they did not form in our solar system? How do super-Earths form and migrate towards their star, and does this affect the formation of habitable planets? Can super-Earths host life? Do collisions affect the habitability of an exoplanet?

“The discovery and characterisation of an increasing number of exoplanets, and statistical studies of their properties, will help us understand more and more the formation and the evolution of planetary systems and of our solar system,” Bonomo concludes.

  1. https://doi.org/10.1038/s41550-018-0684-9
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