In other solar systems, stars eat their planets. Watching planets as big as Jupiter in other solar systems spiral toward their doom, astronomers wondered what force was strong enough to pull giant planets out of their orbits and toward their hungry parent stars.
"My feeling was that people had not thought sufficiently about the tides," said astrophysicist Mario Livio of Hopkins's Space Telescope Science Institute.
Livio theorizes that the gas tides that surround stars, analogous to ocean tides on our own planet, in other solar systems are responsible for throwing off the balance of these systems. When gravity draws a planet closer to the center of its parent star, the gas in the star's outermost layers is pulled toward the planet. This creates a bulge of gas around the star, throwing off the whole system's balance.
"The gas surrounding the star is being pulled by the planet at a different force than the star's center," Livio said. Though astrophysicists knew about the gas tides for a long time, few thought that this little bulge was powerful enough to throw off the planet's orbit, sending it spiraling towards its star.
The tidal pull doesn't always draw the planet to its doom, however. The planet's fate depends on the size of the star and the distance of the planet to the star. The planet may not be close enough to the star for the tides to pull it in.
As a star gets older, it expands and loses mass. If the planet is sufficiently distant, the planet won't be pulled as strongly toward the star. In this case, the planet actually moves away from the star, not toward it.
It comes down to the difference between star's mass loss (sending the planet away from the star) and its gaseous tides (pulling the planet toward the star). "The question is: Who wins?" Livio said.
Livio's theory makes sense to University of Arizona astronomer Brian Jackson, who also studies tides. "My initial reaction to his work is that it's probably right," Jackson said.
Livio's work (done in collaboration with Space Telescope Science Institute astrophysicist Eva Villaver) is one of the first attempts to describe this phenomenon in detail. Not only are Villaver and Livio some of the first astrophysicists to try and figure out why these planets are being eaten by their parents, but they're also among the few astronomers who took the idea seriously and put in the hard work to search for such planets, according to University of Florida astronomer Eric Ford.
However, Livio is quick to point out that there may be other factors lurking behind the phenomenon. He mentioned that the friction caused by the planet movements could play a role, though so far he doesn't think it is significant. "Even electromagnetic currents might affect the structure and evolution of such planets," Ford said.
Livio's not sure how his theory would play out in our solar system, whose star is still relatively young. "In the case of the Earth, whether or not it will get engulfed by the Sun depends so sensitively on a few rather uncertain factors," Livio said. "We cannot say with confidence which way it will go."
