Missing methane: a clue
The scientists detected water and carbon monoxide in the exoplanet's atmosphere, but not methane.
The lack of methane "tells us that there must be mixing between the different layers of the atmosphere, much like a lava lamp swirls material up and down," Konopacky said. "Since methane is a sensitive molecule, it can be destroyed when it gets mixed into the deeper, hotter parts of the atmosphere. This mixing tells us about the atmospheric conditions in young Jupiter-like planets."
In addition, although the researchers see a lot of water vapor in the atmosphere of HR 8799c, "we actually detect slightly less than we would have expected if the planet had the same composition as its host star," Konopacky said. "This tells us that the planet has a slightly elevated amount of carbon compared to oxygen." [Types of Alien Planets Explained (Infographic)]
This high ratio of carbon to oxygen is a clue regarding the exoplanet's formation. The researchers suggest that grains of water ice condensed in the disc of matter surrounding HR 8799 that gave rise to the planets orbiting the star. Oxygen inside the ice depleted any other oxygen for the formation of HR 8799c.
"These ice grains stuck together to make bigger ice chunks, a few kilometers across, that kept colliding and building up the planet's solid core," Konopacky said. "The atmosphere came later — from gas that the planet attracted after it got big enough. By the time that happened, some of the ice grains were gone and the gas didn't have as much water in it."
How planets are born
These findings imply that a planet-building mechanism known as core accretion led to the formation of HR 8799c, "much in the same way we think the planets in our own solar system formed," Konopacky said. The exoplanet's core arose first, and the atmosphere came afterward.
"These results represent a first step in finding direct evidence about how planets form, which in general, is a difficult thing to do observationally," Konopacky said. "It is really exciting that we have these tantalizing suggestions that this extrasolar system that looks like our own solar system in so many ways may have formed in the same way."
Researchers are now tinkering with existing models of core accretion to see how planets might form via the process at great distances from their stars. For instance, there may be more matter at the outer edges of the protoplanetary discs of matter around stars that give rise to planets than before thought, or perhaps solid matter could stick together and form planetary cores easier or faster than previously suspected.
"By further refining the core accretion model of formation to explain the HR 8799 planets, we may be able to learn more about the formation of planetary systems in general, including our own solar system," Konopacky said.
"We would also like to discover more planets through direct imaging that can be studied at this level of detail," Konopacky added. "We work on a new instrument called the Gemini Planet Imager that is designed to do just this. It will arrive at the Gemini South Telescope in Chile this year, and discover new planets that are both smaller than the HR 8799 planets and closer to their parent star."
Konopacky and her colleagues Travis Barman, Bruce Macintosh and Christian Marois detailed their findings online March 14 in the journal Science.
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