Hot Jupiters - giant worlds of gas orbiting close to their parent stars - are not consumed by the star around which they orbit, according to new research - at least not most of the time.
Hot Jupiters resemble the planet Jupiter in that they are massive and are composed largely of gas. It is known that hot Jupiters begin their lives far from their local star and migrate inward, heating as the traveled inward. Jupiter did this long ago. The cause of this behavior is still unclear to astronomers. As the giant world draws closer to its sun, tidal forces shape the orbit into an increasingly-circular formation. When the orbit of the planet becomes nearly circular, the inward motion of the planet ceases, and motion of the world around its sun stabilizes.
"Eventually, all hot Jupiters get closer and closer to their stars, but in this study we are showing that this process stops before the stars get too close," Peter Plavchan of NASA's Exoplanet Science Institute at the California Institute of Technology, said.
There were two other main theories to explain this behavior. One involved the magnetic field of the star. When planetary systems are young, an accretion disk forms around the star, made up of tiny bits of rocks and dust. These particles are known to occasionally stop falling inward as they meet the stars magnetic field. The theory held that hot Jupiters exhibit the same behavior.
The other theory was that a dust-free zone existed around such stars, and these giant worlds halted the trek when they ran out of available dust to collect.
"When only a few hot Jupiters were known, several models could explain the observations. But... tides, in combination with gravitational forces... can bring these giant planets close to their host stars," Jack Lissauer, of NASA's Ames Research Center, said.
To determine which of the three theories match observations, astronomers carefully studied measurements made of 126 known planets outside our solar system, as well as 2,300 likely planetary systems. What they found was that the mass of a star determines how far away a hot Jupiter will stabilize into a circular orbit. More massive stars create orbits for the hot gas giants at a greater distance from their centers than do smaller stars. This behavior was predicted only by the theory that tidal forces ceased the inner spiral.
The majority of the exo-solar planet candidates studied to make this finding, along with many confirmed planets, were first detected by the Kepler spacecraft.
Because of their significant gravitational influence on their parent stars, hot Jupiters cause a wobbling in the star as it revolves around its Sun. This movement can be detected here on Earth, making hot Jupiters among the easiest types of extra-solar planets to find.
The research leading to this new finding is detailed in the Astrophysical Journal.
