Free Astronomy Magazine January-February 2018

21 JANUARY-FEBRUARY 2018 SPACE CHRONICLES These kinds of observations provide insight into the complexity of weather and atmospheric composi- tion on exoplanets, and may some- day be applicable to analyzing Earth-size planets for habitability. “In many ways, the atmospheric studies we’re doing on hot Jupiters now are testbeds for how we’re going to do atmospheric studies on terrestrial, Earth-like planets,” said lead researcher Thomas Beatty of Pennsylvania State University in University Park. “Hot Jupiters provide us with the best views of what climates on other worlds are like. Under- standing the atmo- spheres on these planets and how they work, which is not un- derstood in detail, will help us when we study these smaller planets that are harder to see and have more compli- cated features in their atmospheres.” Beatty’s team selected Ke- pler-13Ab because it is one of the hottest of the known exoplanets, with a dayside temperature of nearly 5,000 °F (near-ly 2,800 °C). Past observa- tions of other hot Jupiters have re- vealed that the upper atmospheres increase in temperature. Even at their much colder temperatures, most of our solar system’s gas gi- ants also exhibit this phenomenon. Kepler-13Ab is so close to its parent star that it is tidally locked. One side of the planet always faces the star; the other side is in permanent dark- ness. (Similarly, our Moon is tidally locked to Earth; only one hemi- sphere is permanently visible from Earth.) The observations confirm a theory from several years ago that this kind of precipitation could occur on massive, hot planets with pow- erful gravity. “Presumably, this pre- cipitation process is happening on most of the observed hot Jupiters, but those gas giants all have lower s u r f a c e gravi- ties than K e p l e r - 1 3 A b , ” Beatty explained. “The titanium oxide snow doesn’t fall far enough in those atmospheres, and then it gets swept back to the hotter day- side, revaporizes, and returns to a gaseous state.” The researchers used Hubble’s Wide Field Camera 3 to conduct spectroscopic observa- tions of the exoplanet’s atmo- sphere in near-infrared light. Hubble made the observations as the distant world traveled behind its star, an event called a second- ary eclipse. This type of eclipse yields information on the tem- perature of the con- stituents in the atmosphere of the ex- oplanet’s d a y s i d e . “These ob- serva- tions of Kepler-13Ab are telling us how condensates and clouds form in the atmospheres of very hot Jupiters, and how gravity will affect the composition of an atmosphere,” Beatty explained. “When looking at these planets, you need to know not only how hot they are but what their gravity is like.” The Kepler-13 system resides 1,730 light-years from Earth. !