Two Newly Discovered Exoplanets Are Probably the Result of a Catastrophic Collision



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How can two planets so similar in some ways have such different densities? According to a new study, a catastrophic collision could be the culprit.

In our Solar System, all the inner planets are small rocky worlds with similar densities, while the outer planets are gaseous giants with their own similar densities. But not all solar systems are like ours.

The Kepler mission discovered a wide variety of exoplanets during its nine years of operation. Thanks to this mission, we now know of only 2,000 confirmed exoplanets that have less than three Earth Rays. And although these 2,000 planets have a fairly small range of sizes, their densities can vary greatly.

The new paper was published in the journal Nature Astronomy by astronomers Aldo S. Bonomo and Mario Damasso of the National Institute of Astrophysics (INAF) and the Center for Astrophysics | Harvard and Smithsonian astrophysicist (CfA), Li Zeng. A large team of colleagues too numerous to list was also involved in the study.

Some of the 2,000 exoplanets mentioned earlier have lower densities than the Neptune gas giant, which consists of low-density volatiles, while some have higher densities than Earth, which consists mainly of rock (about 32% iron). A new study examined exoplanets the Kepler-107 system to try to understand how planets in the same system and with similar sizes can have such a wide range of densities.

NASA's Kepler Space Telescope, shown in this artist's concept, revealed that there are more planets than stars in the Milky Way. Credit: NASA
NASA's Kepler Space Telescope, shown in this artist's concept, revealed that there are more planets than stars in the Milky Way. Credit: NASA

The team focused on the Kepler-107 system because it contains four planets the size of sub-Neptune: Kepler-107b, c, d and e. The two innermost planets, 107b and 107c, have nearly identical radii of 1.5 and 1.6 earth rays, but 107c is more than twice as dense as 107b. How do these twins, who are part of a very compact system of planets, have such different compositions?

"This is one of the many interesting exoplanet systems that the Kepler Space Telescope has discovered and characterized."


Li Zeng, Department of Earth Sciences and Planetariums at Harvard University.

The short answer is that they formed under very different conditions, or that something dramatic happened after training to change their densities so drastically.

Before Kepler, astronomers had only our own solar system to pass through. And in our System, it seems that Jupiter, Saturn, Uranus, and Neptune formed at the outer boundaries of the protoplanetary disk, from the ice and cold gases that made up most of the outer Solar System material. In the inner confines of the young Solar System, the rocky planets formed from materials that survived the Sun's radiation, such as silicates and iron.

The concept of an exoplanet artist the size of Jupiter that orbits relatively close to its star (also known as
Artistic concept of exoplanet the size of Jupiter orbiting relatively close to its star (also known as "hot Jupiter"). Credit: NASA / JPL-Caltech)

But the Kepler mission has shown us that what we think is the norm, that is, our own Solar System, is only a path that solar systems can follow. Kepler discovered numerous so-called "Hot Jupiteres," great gaseous worlds that orbit very close to their own stars. These giant gaseous giants could not have formed so close to their stars, because the gases they formed would not have survived so close to their star. They must have graduated further away then migrated inward.

There is evidence that Jupiter formed in the confines of our Solar System, then migrated closer to the Sun, before finding the way to its current orbit. But as far as we know, the inner rocky planets did not migrate: they formed in the inner Solar System and stayed here.

Jupiter probably formed in the outer Solar System, then migrated closer to the Sun, before ending in its current orbit. South Pole of Jupiter, taken during a passage from Juno, December 16, 2017. Photo credit: NASA / JPL-Caltech / SwRI / MSSS / David Marriott
Jupiter probably formed in the outer Solar System, then migrated closer to the Sun, before ending in its current orbit. South Pole of Jupiter, taken during a passage from Juno, December 16, 2017. Photo credit: NASA / JPL-Caltech / SwRI / MSSS / David Marriott

The Kepler system 107 also shows us that solar systems can form differently from ours, and that a catastrophic collision between two worlds can change their densities.

Kepler 107b and 107c have radii of 1.53 and 1.59 earth rays, orbital periods of 3.18 and 4.9 days, but densities of 5.3 and 12.65 grams per cubic centimeter, respectively. What can explain the huge disparity in densities? If solar radiation were responsible, by boiling the volatiles, were not the two planets subjected to it? In addition, the outer planet has the greater density, not the inner one.

The team of astronomers argues that it was a catastrophic collision that is responsible for disparate densities.

What they think happened is that Kepler 107c, the outermost and densest planet, suffered a catastrophic collision that ripped off its silicate mantle, leaving only the iron core.

"This is one of the many interesting exoplanet systems that the Kepler Space Telescope has discovered and characterized," said Li Zeng of Harvard. "This finding has confirmed earlier theoretical work suggesting that the giant impact between the planets played a role during the formation of the planet. The TESS mission is expected to find more of these examples. "

Planetary collisions are not a new idea. Evidence shows that the Earth's Moon was created as a result of a catastrophic collision between Earth and another body called Theia. This new research suggests that they can be much more common than thought.

If catastrophic disruptions occur frequently in planetary systems, astronomers expect to find many other examples like Kepler-107, as an increasing number of densities of exoplanets is determined more accurately.

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