‘Hot Jupiters’ spotted forming close to their suns

Of the alien solar systems we’ve spotted, many seem to have one intriguing thing in common: giant gas planets like Jupiter and Saturn orbiting very close to their parent star. How did such “hot Jupiters” form? Did they coalesce farther out and migrate inward, or were they born in situ? Now, a team using the world’s largest radio telescope array has found evidence for a close-in formation. The Atacama Large Millimeter/Submillimeter Array (ALMA), high up in the deserts of northern Chile, is sensitive to light from cooler objects of the cosmos: clouds of gas and dust rather than burning stars. The team used it to look at the disks of material around young stars from which planets form. In particular they looked at four “transitional disks,” which appear to have no dust close in to the star. Was it blown outward by stellar wind and radiation, or swept up by a forming exoplanet? As the astronomers report today in Astronomy & Astrophysics, they used the extreme sensitivity of ALMA, whose 66 dishes act as one telescope and can be positioned up to 14 kilometers apart, to scrutinize those gaps around the stars. They found that the gap still contained a lot of gas (blue in the picture, and dust brown), but that gas also had a smaller gap close to the star. Such an arrangement can only be explained, they say, by a giant exoplanet (just left of the star)—which ALMA can’t see—sweeping up all the material close to the star but pushing dust farther out still. Theorists will have to refine their models of planet formation, but will still have to explain how systems like our own ended up with giant planets farther out and small planets in closer orbits.