Last updated November 15, 2017 at 4:11 pm
The Hubble Space Telescope has been used to discover a blistering-hot giant planet outside our solar system where the atmosphere “snows” sunscreen – or at least its active ingredient titanium dioxide.
The giant planet, named Kepler 13Ab, was studied because it is one of the hottest of the known exoplanets, with a daytime temperature of nearly 2800 °C. However, Kepler-13Ab is so close to its parent star that it is tidally locked, meaning one side always faces the star while the other side is in permanent darkness. Temperatures on the night-time side of the planet are far lower, which is where the researchers discovered the sunscreen snowfall.
The astronomers didn’t go looking for titanium oxide specifically. While studying the atmosphere of the planet they found that the giant planet’s atmosphere is cooler at higher altitudes – which was surprising because it is the opposite of what happens on other hot giant planets (called hot Jupiters). Titanium oxide in the upper atmospheres of other hot Jupiters absorbs light and re-radiates it as heat, making the atmosphere grow warmer at higher altitudes. This effect is also found on our own solar system’s gas giants, even though they have much lower temperatures.
Following this unexpected discovery, the researchers concluded that the titanium oxide had been removed from the atmosphere of the dayside of the planet. Without the titanium oxide gas to absorb incoming starlight on the daytime side, the temperature there grows colder the higher you go.
What the researchers think causes this loss of titanium oxide are powerful winds which whip across Kepler 13Ab. These winds carry the titanium oxide around to the night-time side of the planet where it crystallises in the far lower temperatures. Due to Kepler 13Ab’s immense gravity (six times stronger than Jupiter), this crystallised form is pulled down into the lower levels of the atmosphere to form clouds. This low-level precipitated titanium oxide becomes trapped on the night-time side, a process called a “cold-trap.”
A similar system is thought to be a major influencer of atmospheric conditions of most hot Jupiters, however only Kepler 13Ab’s strong gravity traps the titanium oxide precipitate. “Presumably, this precipitation process is happening on most of the observed hot Jupiters, but those gas giants all have lower surface gravities than Kepler-13Ab. The titanium oxide snow doesn’t fall far enough in those atmospheres, and then it gets swept back to the hotter dayside, revaporizes, and returns to a gaseous state,” explained Thomas Beatty who led the research. “These observations 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.”
The findings provide a new perspective of the complex nature of weather and atmospheres on exoplanets. Understanding how these factors all influence each other will be useful for gauging the habitability of Earth-size planets in the future.
The research was published in The Astronomical Journal
Images courtesy of NASA, ESA, A. Field and G. Bacon (STScI)