Researchers Look At Hot Jupiters And Planetary Migration
June 7, 2013

Stars Don’t Consume Their Hot Jupiters (Often)

April Flowers for — Your Universe Online

Stars have an alluring pull on the planets that surround them, especially a class of planets known as hot Jupiters. These planets are gas giants that form farther from their stars before migrating inward and heating up.

Hot Jupiters, despite their close-in orbits, are not regularly consumed by their stars, as a new study conducted with data from NASA´s Kepler Space Telescope reveals. These planets remain in fairly stable orbits for billions of years, until the day comes when they might eventually get eaten.

"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," said Peter Plavchan of NASA's Exoplanet Science Institute at the California Institute of Technology. "The planets mostly stabilize once their orbits become circular, whipping around their stars every few days."

The study is the first to demonstrate how the hot Jupiter planets halt their inward march on stars, revealing that the gravitational, or tidal, forces of a star circularize and stabilize a planet's orbit; when its orbit finally becomes circular, the migration ceases. The findings of this study were published in the Astrophysical Journal.

"When only a few hot Jupiters were known, several models could explain the observations," said Jack Lissauer, a Kepler scientist at NASA's Ames Research Center. "But finding trends in populations of these planets shows that tides, in combination with gravitational forces by often unseen planetary and stellar companions, can bring these giant planets close to their host stars."

Giant balls of gas that resemble Jupiter in mass and composition, hot Jupiters don´t begin life under the glare of a sun. Rather, they form in the chilly outer reaches, as Jupiter did in our solar system. The hot Jupiters eventually head in towards their stars. This process is relatively rare and poorly understood.

Before this study, there were a handful of theories to explain the end of the planets´ travels. One of those theories proposed that the star´s magnetic field prevented the planets from going any further. A planet forming disk of material surrounds a young star. In a process called accretion, the material falls inward toward the star until it hits the magnetosphere. The magnetosphere is basically a magnetic bubble around the star. The material travels up and around the bubble, landing on the star from the top and bottom. The theory proposed that this bubble could be halting migrating planets.

A second theory suggested that the planets stopped marching forward when they hit the end of the dusty portion of the planet-forming disk.

"This theory basically said that the dust road a planet travels on ends before the planet falls all the way into the star," said Chris Bilinski of the University of Arizona, Tucson. "A gap forms between the star and the inner edge of its dusty disk where the planets are thought to stop their migration."

Another theory held that a migrating planet stops once the star´s tidal forces have completed their job of circularizing its orbit. This is the theory that the current researchers found to be correct.

To test the prior theories, the researchers examined 126 confirmed planets and more than 2,300 candidates. Most of the candidates and some of the known planets were identified using data from NASA´s Kepler mission, which has found planets of all sizes and types, including rocky ones that orbit where temperatures are warm enough for liquid water.

The team examined how the distance of the planets from their stars varied depending on the mass of the star. The different theories for explaining what stops migrating planets vary in their predictions of how the mass of a star affects the orbit of the planet. On average, the hot Jupiters of more massive stars would orbit further out, according to the “tidal forces” theory.

The results of the current study matched the “tidal forces” theory, showing even more of a correlation between massive stars and farther-out orbits than predicted.

Though this study may explain the mechanism by which migrating planets halt, the journey itself is still a mystery of many questions. Scientists think that as gas giants voyage inward; they sometimes kick smaller, rocky planets out of the way — essentially ending any chance of life evolving. Further study will be needed to help explain the secrets of planetary migration.