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.
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.
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