Ever since Swiss astronomers astonished the world by finding a gas-giant planet orbiting close to its star, researchers have wondered how these so-called hot Jupiters arose. None exists in our solar system, where the planetary giants—Jupiter, Saturn, Uranus and Neptune—reside in the deep freeze beyond the asteroid belt. Now a surprising discovery is providing fresh insight, suggesting that the more iron a star was born with, the more likely it is that the star's hot Jupiter had a violent past.
"Hot Jupiters are sort of a red flag that our simple picture of how planetary systems form and evolve, which we used to have, is not sufficient," says Rebekah Dawson, a graduate student in astronomy at Harvard University. In that picture, planets arose from a disk of gas and dust around a star and stayed as far from their sun as where they originated.
But not hot Jupiters. Just as the fossil of a tropical plant in Antarctica suggests continental drift, so hot Jupiters reveal that planets can move toward their star. Gas giants form far from their stars, where the protoplanetary disk is so cold that ice condenses and conglomerates into ice-rock-iron cores roughly 10 times more massive than Earth. In our solar system, the gravity of two such cores attracted so much hydrogen and helium that they swelled into the gas giants Jupiter and Saturn, which are 318 and 95 times more massive than Earth. Two other cores failed to accrete much gas and remained smaller; they became Uranus and Neptune, the sun's "ice giants," and weigh in at only 14.5 and 17.2 Earth masses.
So a hot Jupiter starts life far from its star and must move inward until it gets hotter than any world in our solar system. But exactly how does the planet move inward? Astronomers once favored a gentle process, in which the protoplanetary disk slowly drags the planet sunward, leaving it on a circular orbit in the plane of the star's equator. In 2008, however, astronomers began finding hot Jupiters with tilted orbits, suggesting instead past violence: The gravity of other gas giants had kicked the Jupiters inward.
Now Dawson has discovered a startling correlation between a star's iron abundance and the planets' orbital shapes that reveals their origins. Because hot Jupiters are close to their stars, stellar tides tug on the planets and make their orbits circular. So she looked instead at gas giants somewhat farther out, where stellar tides are too weak to significantly alter orbital shapes, finding that stars with more iron than the sun tend to have gas giants on much more elliptical paths.
In work to be published in the April 20 issue of the Astrophysical Journal Letters, Dawson and her advisor, Ruth Murray-Clay, explain the finding as follows. If a star and its protoplanetary disk are born with lots of iron and other heavy elements, the cores grow fast, because they consist mostly of these elements; therefore, the cores attract hydrogen and helium and give rise to more gas giants. A multitude of gas giants increases the chance that one planet's gravity will fling another sunward, where it becomes a hot Jupiter. In contrast, stars born with less iron may host at most one gas giant, which can move inward only by inching through the disk.
"The result is pretty striking," says Daniel Fabrycky, an astronomer at the University of Chicago. He believes the message is clear: There is more than one way to make a hot Jupiter.
After Dawson posted her work online, Stuart Taylor, an astronomer in Hong Kong, emailed her to say he had discovered the same correlation and presented it last summer at a conference in Beijing. But Taylor thought a star's high iron content might be an effect rather than a cause, because giant planets on elliptical orbits kick other planets into their star, boosting its iron level. "I think that our explanations are really complementary," Taylor says, because both processes may operate: An iron-rich star has more gas giants, causing more planets to crash into their sun.
In any event, it's a good thing "our" Jupiter stayed put. "No matter how a gas giant moves in, it's probably bad news for any small, Earth-sized planets that are in its way," Dawson says. "They would most likely get scattered into the sun."
Indeed, if the sun had been born with more iron, Uranus and Neptune might have grown into gas giants—and catapulted Jupiter or Saturn into our corner of the solar system.