Gas giants orbiting other stars at distances that would fall well inside of Mercury's orbit were the first extrasolar planets discovered. Because of their mass and their close-in orbit, hot Jupiters' effects on their parent stars are more pronounced than in other systems. Once researchers had identified these planets as gas giants, the chin-scratching began.
In our solar system, Jupiter and the other outer gas planets formed beyond what researchers have dubbed the solar system's frost line: a region in the early sun's disk of dust and gas where water, ammonia, methane, and other hydrogen-bearing compounds freeze into ice grains. Inside the frost line, the rocky planets formed.
Two competing scenarios emerged to explain how Jupiter-like gas giants migrated inward. The new report has led one team member to come to a definitive conclusion in the debate.
The earliest explanation suggested that a hot Jupiter forms beyond the frost line, but gravity from a passing star, or perhaps another massive companion planet, kicks the Jupiter into a highly elliptical orbit around its star. Each time the planet passes close to the star, its orbit is gradually reshaped until the orbit is far less elliptical orbit and so close that its “year” can be as fast as 19 hours.
According this explanation, the hot Jupiter would have destroyed or ejected any planetary munchkins within its orbit as the orbit evolved.
More recently, scientists have posited that Jupiter-class planets could simply migrate inward through the disk of dust and gas surrounding a young star. Under one scenario, it would get so close that it would induce tides on its star, similar to the tides the moon sets up on Earth's seas. In effect, that stellar “high tide” would transfer enough energy back to the planet's orbit to stabilize it and keep the planet from falling into the star.
According to this explanation, lower-mass planets could survive in the hot Jupiter's general region.
When the team hunted for evidence of additional planets in the 63 hot-Jupiter systems it studied, it found none.
“I was a bit surprised there were none in the sample,” says Steffen, who was the lead author of the PNAS paper reporting the results.