August 3, 2012
Vaporizing Super-Earths Provides Better Understanding Of Exoplanets
Lee Rannals for redOrbit.com - Your Universe Online
Scientists collaborated recently to produce simulations of Earth-like planets being vaporized in order to help astronomers have a better grasp of what to look for in the atmosphere of candidate super-Earths.Super-Earths are rocky exoplanets that are more massive than Earth, but less massive than Neptune. Most of those that have been found so far orbit very close to their stars.
Scientists writing in The Astrophysical Journal show that Earth-like planets as hot as those crowding their host star's space would have atmospheres composed mostly of steam and carbon dioxide, with smaller amounts of other gases that could be used to distinguish one planetary composition from another.
The team converted the gas abundances and have calculated them into synthetic spectra, so astronomers hunting planets would be able to compare spectra they measure.
Planet hunting techniques allow astronomers to both identify exoplanets, and measure their average density.
Combining the average density plus theoretical models allows astronomers to determine the bulk chemical composition of gas giants.
If astronomers can observe the light from the star filtered by the planet's atmosphere, then they can determine the composition of the planet's atmosphere, which allows them to distinguish alternative bulk planetary compositions.
The team modeled the atmospheres of hot super-Earths because they are what astronomers are finding. The researchers said they wanted to predict what astronomers should be looking for when they look at the atmospheres to decipher the nature of the planet.
The scientists ran calculations on two types of super-Earths, one with a composition like that of the Earth's continental crust, and the other with a composition like the Earth's before the continental crust formed.
The difference between the two models created by the team is water. The Earth's continental crust is dominated by granite, which has to have water in order for it to be made. Without water, planets end up with a basaltic crust like that found on Venus.
Both crusts are mostly silicon and oxygen, but a basaltic crust like Venus is richer in elements like iron and magnesium.
The super-Earths used as references by the team are thought to have surface temperatures ranging from 520 to 3,090 degrees Fahrenheit.
Using thermodynamic equilibrium calculations, the team determined which elements and compounds would be gaseous at these high temperatures.
“The vapor pressure of the liquid rock increases as you heat it, just as the vapor pressure of water increases as you bring a pot to boil,” Bruce Fegley, PhD, professor of earth and planetary sciences at Washington University in St. Louis, said in a prepared statement “Ultimately this puts all the constituents of the rock into the atmosphere.”
The continental crust melts at 1,720 degrees Fahrenheit, and the bulk silicate on Earth at about 3,145 degrees Fahrenheit.
According to the researchers' calculations, the atmospheres of both model Earths would be dominated over a wide temperature range by steam and carbon dioxide.
The major difference between the models is that the BSE atmospheres is more reducing, which means it contains gases that would oxidize if oxygen were present.
Fegley told redOrbit in an email he hopes the researchers' work has an influence on future scientific studies.
"Hopefully our work helps astronomers identify rocky extrasolar planets as they do more and more spectroscopic observations of transiting exoplanets," he told redOrbit.
While the simulations of vaporization Earth-like planets works well for helping scientists better understand the make-up of distant exoplanets, Fegley talked about what would need to play out for the Earth to actually vaporize. He implied this scenario is not just one of fantasy, but one that is inevitable.
"The Sun is a middle aged main sequence star with about 4.5 billion years left in its life," he told redOrbit. "At that point in the future it will balloon up in size (to beyond the orbit of Mars) and become a red giant. At that point the Earth will be vaporized as it is swallowed up in the expanding Sun."