The formation of planetary systems is a very complex mystery of the universe. Scientists found evidence of two planets hitting each other so hard that one of them lost its atmosphere.
Evidence of the collision exists in a very young system, 95 light-years from Earth. According to their analysis, the unusual dust surrounding the young star is 23 million years old. HD 172555 was the result of the impact of a planetary collision so violent that it partially stripped a part of its atmosphere.
"This is the first time we have detected this phenomenon, from a protoplanetary atmosphere stripped away in a giant impact," said astronomer Tajana Schneiderman of the Massachusetts Institute of Technology (MIT).
"Everyone is interested in observing giant impacts, but we don't have evidence in many systems for it. Now we have additional insight into these dynamics."
Planet formation
The process of planet formation is a complex one, and researchers must piece together what they know of the fully formed system, and other systems in the Milky Way galaxy that we have seen at various stages of development.
When a star forms from clumps of dust and gas in a molecular cloud, the vast disk of material that forms will "feed" the growing star.
This stellar disk then undergoes a transformation, possibly starting before the star has finished forming. When the spots and the fragments within them begin to stick together, first electrostatically, and then, as objects accumulate mass by gravity.
These larger and larger blobs collided and merged, eventually gaining enough mass for the disparate nuclei to settle at the center, and eventually becoming a planet.
However, not all 'baby' planets can survive. There is also one planet the size of Mars that did not enter the Solar System, colliding with Earth to create the Moon for example.
Astronomers think that most planets don't form in their final places either. Instead, they form elsewhere and migrate to their final position.
Hence, such collisions are considered a fairly common occurrence during the formation of planetary systems. Indeed, they seem to play an important role in the way planets grow, and the main architecture of that system.
The HD 172555 has long been considered a bit odd. The dust swirling around it has an unusual composition and grain size, namely unusual amounts of silica and solid silicon monoxide, as well as much smaller than average dust grains.
Previously, this condition was interpreted as the result of hypervelocity impact. So Schneiderman and his colleagues decided to take a closer look at the carbon monoxide surrounding the star.
According to the team's modeling, the most suitable for observations of HD 172555 is a giant impact. They can even narrow down when and how it happened.
At least 200,000 years ago (recently enough that carbon monoxide didn't have time to decompose), a rocky planet the size of Earth was hit by a smaller object at 10 kilometers per second.
The collision was so powerful that it blew up at least part of the rocky planet's atmosphere. This then explains the presence of carbon monoxide and silica-rich dust.
"We expected a giant impact. The time scale, age and morphological and compositional constraints match. The only plausible process that could produce carbon monoxide in this system, in this context, is a giant impact," said Schneiderman.
The results provide us with new data to identify when a major impact has occurred. If we find large amounts of carbon monoxide where it shouldn't be around a star, it could be a sign that things went awry during the formation of a planetary system.
We can then study this gas and other debris, not only to learn more about the aftermath of the collision, but to understand what planets other systems are made of, which could lead to a better understanding of how planets are born.
