VAPORIZING PLANETS IN THE NAME OF SCIENCE

One sure-fire way to grab an audience's attention is to vaporize a planet, amirite? We saw the destruction of Vulcan in Star Trek, the end of Krypton in the Christopher Reeves-era Superman, while the Death Star vaporized Alderaan in Star Wars: A New Hope. As for Hitchhikers Guide to the Galaxy, Douglas Adams went all in, vaporizing the Earth right off the bat, all because an alien race known as the Vogons want to make way for a hyperspatial express route, leaving poor Arthur to roam about the Milky Way in his bathrobe. But can you really vaporize a planet? According to the latest computer simulations by a couple of planetary scientists in St. Louis, you betcha! As outlined in their new paper in The Astrophysical Journal, Bruce Fegley and his colleagues (Katharina Lodders and Laura Schaefer) mathematically constructed a couple of model "Super-Earths" and put them through a series of atmospheric simulations. The object wasn't really to study how to destroy the Earth. Fegley et al were trying to learn more about the kinds of atmospheres most likely to be found on Super-Earths -- i.e., exoplanets with masses that are more than that of Earth but less than that of Neptune, while still being rocky in nature, instead of, say, a gas giant. Having detailed knowledge of likely chemical compositions could help astronomers who hunt for such planets find them. And one way of gaining that knowledge is to build computer models of Super-Earths and vaporize them. Most exoplanets within that size range that have been found are gaseous in nature, because they orbit so close to their host stars that any rocky stuff gets melted. (How Stuff Works has an excellent summary of the various techniques astronomers use to hunt for exoplanets.) For instance, using photometry, astronomers can detect an exoplanet as it transits the host star, because of predictable periodic dimming of a star's brightness as the planet momentarily blocks its light. Astronomers can also determine the chemical composition of said planet's atmosphere because the star's light gets filtered through that atmosphere -- think of it as stellar spectroscopy. This, in turn, provides clues as to the planet's density, because the gases in the atmosphere likely came about because of vaporized rock. So it would be nice to have tidy simulated models to compare with the measured spectra of actual exoplanets. One model Super-Earth had a continental crust just like our Earth, dominated by granite, while the other simulated Earth's composition before its crust formed, when it was mostly bulk silicate. (Water is the key ingredient in getting Earth today from that precursor Earth. Without it, our planet's crust would more closely resemble Venus.) Then they plugged in the likely surface temperatures of observed Super-Earths, ranging from between 270 to 1700 degrees Celsius, just to see what would happen to the atmosphere. "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," Fegley explained via press release. "Ultimately this puts all the constituents of rick into the atmosphere." In both models, the atmospheres would likely be mostly steam and carbon dioxide. Once the Super-Earths achieved temperatures above 760 degrees Celsius, there would also be sulfur dioxide. Think an especially steamy Venus. And at temperatures higher than 1430 degrees Celsius, the uber-heated rock would produce silicon monoxide vapor. Even exoplanetary atmospheres have "weather," so should a "storm front" move through at those extreme temperatures, the simulations showed that the silicon monoxide could condense and produce "pebble rain." Crank the temperature really, really, high, and you wouldn't just vaporize the Earth's crust and mantle. Theoretically, at least, you could destroy the entire planet. "You're left with a big ball of steaming gas that's knocking you on the head with pebbles and droplets of liquid iron," said Fegley. "But we didn't put that into the paper because the exoplanets the astronomers are finding are only partially vaporized." Or maybe they just didn't want to give the Vogons any bright ideas.