Giant impacts caused by interplanetary collisions

“Astronomers have found fresh evidence for significant planetary diversity within a single exoplanet system, suggesting that giant high-speed collisions are partly responsible for planetary evolution.

An international team of scientists led by Italy’s National Institute for Astrophysics (INAF) and involving physicists from the University of Bristol spent three years observing the exoplanetary system Kepler-107 via the Telescopio Nazionale Galileo in La Palma.

They gathered more than a hundred spectroscopic measurements of all four sub-Neptune mass planets in Kepler-107 – named after the NASA Kepler space telescope that discovered the exoplanetary system five years ago. Unlike Earth’s relation to the sun, the planets in the Kelper-107 system are much closer to each other and their host star (their equivalent of our sun). All of the planets have an orbital period of days as opposed to years.

It is not uncommon for the planet that is closest to the host star to be the densest due to heating and interaction with the host star which can cause atmosphere loss. However, as reported in Nature Astronomy, in the case of Kepler-107, the second planet, 107c, is denser than the first, 107b. So much so that 107c contains in its core an iron mass fraction at least twice as large as that of 107b, indicating that at some point, 107c had a head-on high-speed giant collision with a protoplanet of the same mass or more collisions with multiple planets of a lower mass. These impacts would have ripped off part of the rock and silicate mantle of Kepler-107c, suggesting that it is denser now than it was originally.


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Image: “One frame from the middle of a hydrodynamical simulation of a high-speed head-on collision between two 10 Earth-mass planets. The temperature range of the material is represented by four colours grey, orange, yellow and red, where grey is the coolest and red is the hottest. Such collisions eject a large amount of the silicate mantle material leaving a high-iron content, high-density remnant planet similar to the observed characteristics of Kepler-107c. Credit: Zoe Leinhardt and Thomas Denman, University of Bristol”

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