Evaporating Gas Giants

 

One of our scientific interests is the formation and evolution of extrasolar planets (exoplanets). In one of our research, we found that some extrasolar gas giants may completely evaporate due to the atmospheric escape of gas into space.

 

Background: discovery of evaporating gas giants

Thanks to the recent progress of exoplanet searches, more than one thousand of exoplanets have been discovered. Some of them are “Hot Jupiters”: gas giants located at close-in orbits around their host stars. The surface temperature of Hot Jupiters reach several thousand Kelvin. Observations with the Hubble Space Telescope found that Hot Jupiters are losing their gas into space (Figure 1). We studied the mechanisms of atmospheric escape and the impact on the evolution of Hot Jupiters.

 

説明: Macintosh HD:Users:kurokawahiroyuki:Desktop:HJ.png

Fig. 1: Image of evaporating Hot-Jupiter HD209458b (copyright: NASA)

 

Method: Numerical calculation of planetary evolution

A possible mechanism to induce atmospheric mass-loss is atmospheric heating due to stellar X-ray and extreme UV emission (XUV). The rate of mass-loss driven by XUV heating depends on the properties of planets such as mass and radius. We simulated the evolution of evaporating planets by calculating the planetary structures and the rate of mass-loss.

 

Result: Complete evaporation of Hot Jupiters

We found that some Hot Jupiters completely lose their gaseous envelopes due to mass-loss. Hot Jupiters having closer orbits and lower masses evaporate more dramatically. As a result, mass-loss creates a lack of Hot Jupiters in a population of exoplanets (Figure 2). The calculated zone where Hot Jupiters completely evaporate agrees with the observed distribution of exoplanets. This agreement suggests that populations of close-in exoplanets might change after their formation.

 

説明: Macintosh HD:Users:kurokawahiroyuki:Dropbox:web:mass-loss:sub_jupiter_desert_e.png

Figure 2: Population of exoplanets and zone of complete evaporation

 

Future: Toward a comprehensive understanding of formation and evolution

Solar system planets and exoplanets are the outcomes of complicated formation and evolution processes. Studies of evolution processes, such as mass-loss, are important to bridge the gap between observations of planets and theories of planetary formation. Our ultimate goal is to obtain a comprehensive understanding of the formation and evolution of solar system planets and exoplanets.

 

Dr. Hiroyuki Kurokawa, Postdoctoral researcher