First Evidence of a Giant Exoplanet Collision

By Jake Parks | February 18, 2019 3:45 pm
A planetary collision is exactly as bad as you would imagine. Unlike an asteroid impact, there's not just a crater left behind. Instead, such a massive crash causes the surviving world to be stripped of much of its lighter elements, leaving behind an overly dense core. (Credit: NASA/JPL-Caltech)

A planetary collision is exactly as bad as you would imagine. Unlike an asteroid impact, there’s not just a crater left behind. Instead, such a massive crash causes the surviving world to be stripped of much of its lighter elements, leaving behind an overly dense core. (Credit: NASA/JPL-Caltech)

For the first time ever, astronomers think they’ve discovered an exoplanet that survived a catastrophic collision with another planet. And according to the new research, which was published Feb. 4, in the journal Nature Astronomy, the evidence for the impact comes from two twin exoplanets that seem to be more fraternal than identical.

Mass Matters

The pair of planets in question orbit a Sun-like star (along with two other planets) in the Kepler-107 system, which is located roughly 1,700 light-years away in the constellation Cygnus the Swan.

Known as Kepler-107b and Kepler-107c, these planets have nearly identical sizes (both have a radius of roughly 1.5 times that of Earth), yet one planet is nearly three times as massive as the other. The innermost planet, Kepler-107b, is about 3.5 times as massive as Earth, while Kepler-107c, which sits farther out, is a whopping 9.4 times as massive as Earth.

This means the inner planet, Kepler-107b, has an Earth-like density of around 5.3 grams per cubic centimeter, while the more distant Kepler-107c has a density of around 12.6 grams per cubic centimeter — which is extremely dense, even for an alien world. (For reference, water has a density of 1 gram per cubic centimeter.)

This perplexing density discrepancy left researchers scratching their heads. How could two equally sized exoplanets in the same system (and at nearly the same orbital distance) have such different compositions?

The Cause

To determine exactly why Kepler-107c is so dense, first the researchers considered what they already knew. Previous research has shown that intense stellar radiation can strip the atmosphere from a planet that sits too near its host star. But if the innermost planet lost its lighter atmospheric elements, it should be more dense than its twin, not less. According to the study, this would “make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c,” which is clearly not the case.

However, there is another way that a planet can lose a lot of mass: by getting smacked with another planet. And this is exactly what the researchers think happened to Kepler-107c.

The researchers argue that the denser planet, Kepler-107c, likely experienced a massive collision with a third, unknown planet at some point in its past. Such a gigantic impact, the study says, would have stripped the lighter silicate mantle from Kepler-107c, leaving behind an extremely dense, iron-rich core. According to the study, Kepler-107c could be as much as 70 percent iron.

Because the mass and radius of Kepler-107c matches what would be expected from a giant planetary impact, the researchers are fairly confident that the collisional scenario they’ve outlined in their paper is accurate; however, they still need to confirm their hypothesis. If proven correct, this new find would become the first-ever evidence of a planetary collision outside our solar system.

Closer to Home

Though astronomers have never confirmed a collision between exoplanets in another star system before, there is evidence that a similar cosmic crash occurred in our own solar system. In fact, a leading theory about the formation of the Moon is that it formed when a small protoplanet rammed into early Earth.

By analyzing lunar samples returned by the Apollo missions, scientists learned that the composition of Moon rocks is very similar to that of Earth’s mantle. Furthermore, the Moon is severely lacking in volatile elements, which boil away at high temperatures. Taken together, along with a few other lines of evidence, this indicates the Moon may have formed when a very large object (roughly the size of Mars) struck Earth with a glancing blow early in the solar system’s history, some 4.6 billion years ago.

This mash-up melted and tore off some of the outer layers of Earth, which may have temporarily formed Saturn-like rings around our planet. Over time, much of this ejected material drifted back to Earth’s surface, but there was still enough debris left in orbit that it eventually coagulated and formed the Moon.

With the discovery of Kepler-107c, it seems planet-shattering impacts are not just a sci-fi trope, but instead may occur much more frequently than we once thought. And with the long-anticipated launch of the James Webb Space Telescope coming up in March 2021, it may only be a few more years until they start to reveal themselves en masse, so be sure to stay tuned.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: exoplanets, stars
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  • Mike Richardson

    One of the biggest arguments I’ve seen from the “rare earth” proponents has been that life on Earth is dependent on the influence of our Moon, in terms of slowing our planet’s rotation, stabilizing it’s rotation to keep this world from tipping too far over and causing severe climate shifts, and raising tides which may have allowed life to arise in tidal pools. However, this finding suggests that perhaps large moons of terrestrial type planets might not be that uncommon, as collisions during planetary system formation could occur at a greater frequency than previously thought, with apparently some survivors of these collisions. There’s a great deal of variety in extrasolar planetary systems, but combinations like the Earth and the Moon could exist in greater abundance than we imagined.

    • Darth Malicus

      im really interested in what life would evolve like on planets where conditions differ greatly from ours, like what adaptation mechanisims would occur if a planet was tidaly locked, and had wind speeds of 300mph plus! or an axis that spun horizontaly

      • Mike Richardson

        Tidally locked worlds appear to be pretty common, because we’ve found many orbiting red dwarf stars in orbits close enough for this to be the case. There’s speculation that life could arise and flourish in the temperate twilight zones between the eternal day and night hemispheres. Plants would likely evolve leaves oriented in a more vertical alignment, always facing the sun low on the horizon. High winds would be a lot more problematic, at least for life on land. Maybe creatures low to the ground, with shells like a turtle, but more streamlined could cope with such high winds. Plants wouldn’t have difficulty dispersing seeds far with 300 mph winds, but would be challenged to evolve forms resilient to the buffeting. A horizontal spin axis, like Uranus has, would cause seasonal extremes like we see in the polar regions — at least part of the year, months long night and months long day. These are extremes we’ve been spared on Earth, but that doesn’t necessarily mean life couldn’t adapt to conditions like you’ve described.

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