Why Stars Don't Eat Their Young: A Study

Despite their alluring pull on planets, especially those in a class called hot Jupiters, a new study by NASA scientists suggests that stars are not in the habit of consuming them.

As gas giants adopted by their host star from one farther away, hot Jupiters tend to have close-in orbits that leave them especially vulnerable.
Nevertheless, based on information derived from the space telescope Kepler, astronomers have shown that the planets seem to enjoy fairly stable for billions of years before they are ultimately devoured.

"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 in a news release. "The planets mostly stabilize once their orbits become circular, whipping around their stars every few days."

Published in Astrophysics Journal, the study is the first to demonstrate just how this happens, describing how the gravitational forces of a star circularize and stabilize a planet’s orbit. Thus, when its orbit finally becomes circular, the migration ceases.

"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, Moffet Field, Calif., not affiliated with the study. "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."

Previous theories covered a range of possibilities, including the ability of the star’s magnetic field to prevent the planets from colliding with it.

Another theory posited that the planets stopped their steady march forward when they come to the end of the dusty portion of the planet-forming disk characteristic of a star’s early years.

"This theory basically said that the dust road a planet travels on ends before the planet falls all the way into the star," said co-author 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."

And finally, a third theory, which stated that a migrating planet simply stops once the star’s digtal forces have completed their job of circularizing its orbit, was the one that, in the end, proved to be correct.

In order to test these theories and other scenarios, the scientists looked at 126 confirmed planets and more than 2,300 candidates, the majority of which and some of the known planets were identified via NASA’s Kepler mission.

They then looked at how the planets’ distance from their stars varied depending on the mass of the star, determining in the process that the results matched the “tidal forces” theory and even showed more of a correlation between massive stars and farther-out orbits than predicted.