Unlike in old B movies, real scientists don’t scream, “Fools! I’ll destroy them all!” before throwing the switch on their doomsday device. At least, most of the them don’t. However, the August 10 issue of the Astrophysical Journal reports that a team of scientists are working on destroying the world – not once, but repeatedly. Fortunately, the world they’re destroying exists only in a computer simulation and its destruction is in the service of learning more about planets revolving around other stars.
Bruce Fegley, professor of earth and planetary sciences at Washington University in St. Louis, and his team are dedicated world wreckers. They run computer simulations of what would happen to the Earth if it was subjected to temperatures hot enough to boil it away. Funded by the National Science Foundation and NASA, the team’s repeated destruction of the Earth is not entirely gratuitous – or so they claim. Its purpose is to answer one of the more vexing questions of modern astronomy – how to find out the composition of the recently detected super-earths outside the Solar System.
Super-earths are the most earthlike extrasolar planets yet detected. The term is a bit misleading because “earthlike” in this case doesn’t mean a planet that is necessarily habitable. A super-earth is merely a way of describing an exoplanet that is larger than the Earth, smaller than Neptune and isn’t a made up mainly of hydrogen and methane. Though they are called “earths,” the term only refers to a planet’s mass and the fact that they aren’t gas giants. Other than that, these super-earths can be very unearthlike in every other respect.
Because, like other exoplanets, super-earths don’t give off any light, they’re detected indirectly. Astronomers look for a star that has a slight wobble as if it’s being pulled off center by some large, dark companion orbiting it. Think of it as being similar to a washing machine with an off-balance load that’s bouncing around the laundry room. The exoplanet is the comforter bunched up in the drum causing all the trouble. This technique has helped astronomers detect hundreds of planets, but it does tend to favor detecting super-earths that are very close to their stars. Close enough, in fact to melt rock.
The atmosphere of the these super hot super-earths would be of steam and carbon dioxide along with trace elements. What scientists want is a way of learning the composition of these planets through these trace elements. Detecting these elements is relatively simple. In studying the planets of our own Solar System, astronomers for over a century have used spectroscopes to determine the make up of their atmospheres, if any. A more advanced version of this technique could be used with super-earths by waiting until the planet in question passed between its star and the Earth. The light transmitted through the planet’s atmosphere could then be run through a spectroscope and analyzed.
"It's not crazy that astronomers can do this and more people are looking at the atmospheres of these transiting exoplanets," Fegley said. "Right now, there are eight transiting exoplanets where astronomers have done some atmospheric measurements and more will probably be reported in the near future."
The only fly in the custard is that the scientists have data, but nothing to compare it to. There are no super-earths in the Solar System and the composition of one super-earth can vary greatly from another. It becomes the problem of learning about something you know little about by comparing it to something else you know little about. That’s not very productive.
How to bake a planet
One solution is to run simulations of what happens to super-earths when they are subjected to world-blasting temperatures. The technique used to detect super-earths also reveals their density and from that scientists can calculate which combinations of elements are compatible with a planet of a particular size and density. That still leaves a lot of variations, but the simulations allow scientists to calculate what trace elements will be present in the super hot atmosphere for a given composition. From this, Dr. Mark Marley's research group at the NASA Ames Research Center was able to create simulated spectra for the various atmospheres.
"We modeled the atmospheres of hot super-earths because that's what astronomers are finding and we wanted to predict what they should be looking for when they look at the atmospheres to decipher the nature of the planet," Fegley said.
The simulations used were based on two model Earths. One is like our world, with plenty of water. On such a planet, the water causes a granite crust to form that’s rich in silicon and oxygen. The second is like Venus. Without abundant water, the crust is of basalt and rich in iron and magnesium.
These virtual planets were then blasted with simulated heat in a computer to temperatures ranging from 270° C (518° F) to 1,700° C (3,092° F). By applying thermodynamic equilibrium calculation, the type of gases that would boil out of the crust into the atmosphere could then be determined.
In both models, the atmosphere was loaded with steam and carbon dioxide, but the earthlike planet also held methane and ammonia while the venuslike planet had sulfur dioxide. This means that the simulations have significance closer to home. The watery earth appears very similar to the primitive Earth with an atmosphere capable of producing the building blocks of life, while the dry one resembles the real Venus.
One very curious effect was that when the temperature was cranked to 1,430° C (2,606° F), silicon monoxide vented into the atmosphere, resulting in a very nasty form of rain consisting of pebbles. When Fegly was asked what happened to the simulated planets when the temperature was really pushed to limits he said, "you're left with a big ball of steaming gas that's knocking you on the head with pebbles and droplets of liquid iron, but we didn't put that into the paper because the exoplanets the astronomers are finding are only partially vaporized."
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