The paper "Born Dry or Born Wet? A Palette of Water Growth Histories in TRAPPIST-1 Analogs and Compact Planetary Systems" was authored by Chen, an assistant professor of space sciences, and researchers from NASA, Johns Hopkins University and Harvard University, was published in The Astrophysical Journal Letters in September. It explores the likelihood that TRAPPIST-1's three innermost exoplanets contained no water when they formed, despite existing in a zone where water is viable.
TRAPPIST-1 is a red dwarf star located about 40 light-years away from us. (One light year is about 6 trillion miles.) It is thought to be about 7.6 billion years old, or 3 billion years older than our Sun.
Astronomers are captivated by the TRAPPIST-1 system because its seven known planets are rocky and Earth-like. They also fall within the star's habitable zone: the distance range from a star at which temperatures are not too hot or cold to support liquid water.
Researchers are searching for any evidence of water on these planets, but have yet to detect anything. Some think a lack of gas in the atmosphere is disrupting the light needed to pick up detailed visuals. Others predict water could have escaped the planets' atmospheres throughout their evolution.
Chen and his team, however, decided to research a different theory: that there was no water to begin with because there was no gas to contain it. He would test it not from an observational perspective, but with mathematical modeling of the planets' initial formation.
"You have astronomers who are using telescopes to see what's out there. I come from a different perspective," Chen said. "I'm both trying to explain what we're seeing while trying to make predictions about what we can't."
The researchers created models that examined the composition and growth of these planets starting when they were as small as one kilometer wide. They simulated how material aggregated during collisions with other celestial objects until they reached their final planetary formations.
There are several key factors in collision events that heavily influence a planet's final composition. Chen's models incorporated impact delivery, which is the transfer of materials like water and gases during a celestial collision; impact erosion, which refers to the removal of materials in a planet's atmosphere due to impact; and mantle-atmosphere exchange, which is the transfer of water and gases between a planet's atmosphere and mantle to maintain its conditions.
The team ran hundreds of collision simulations, which returned thousands of different possibilities for how TRAPPIST-1's planets might have formed. They varied several components, such as the amount of water available to the system, the profile of the initial planet formation environment, the planets' density profiles and the initial system conditions. For the inner worlds, specifically the first three planets, most of the simulations came back dry.
"Whatever we did, we couldn't get much water in these inner planets," Chen said.
He believes that the main reason the planets couldn't acquire water is due to the nature of the collision events. Compact planet collisions are higher velocity, so they are more aggressive and energetic, Chen said. This means that instead of acquiring material for a gaseous atmosphere, planets' atmospheres were completely cleared out by the power of the collisions. With no gas in the atmosphere to contain water, it's possible that any previously existing water escaped back into space during these collision events.
Understanding a planet's earliest characteristics, its water, air and carbon content, builds the foundation for how they evolve. That way, when researchers identify a planet that seems viable for life at the surface level, they can use Chen's model to simulate what these distant worlds might be like on the inside, on the surface and in the air.
Combining the theoretical context of a planet's formation with the state in which it was discovered can help researchers - and NASA - make informed, efficient decisions on which planets are worth investigating and when it's time to move on to the next.
Research Report:Born Dry or Born Wet? A Palette of Water Growth Histories in TRAPPIST-1 Analogs and Compact Planetary Systems
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