Observing planets being born is a difficult task. But in recent years, radio telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA) have captured images of glowing disks of material around young stars. The image above shows such a circumstellar disk around the star AS 209, about 395 light-years away in the constellation Ophiuchus. Holes in the disk are where fledgling planets are born, sweeping gas and dust onto their surfaces as they orbit.
It is even rarer to observe the much smaller and fainter luminous disc of material swirling around a forming planet itself, from which the moons could begin to accrete. Until recently, only two of these circumplanetary discs (CPD) had never been spotted and confirmed. These studies observed hot dust in CPDs.
Now astronomers have found what could be a third CPD – and for the first time they believe they have detected not its dust, but the much fainter emission from the gas within the disk. While the dust shines like a light bulb through a spectrum of wavelengths, the radiation emitted by the gas only emits at specific wavelengths. But using ALMA, the team was able to identify carbon monoxide light emitted from an otherwise empty space in AS 209’s disk. The work was published July 27 in Letters from the Astrophysical Journal.
What made the gas around this fledgling planet visible was the sheer size of the forming planet and its distance from its host star, says Jaehan Bae, an astronomer at the University of Florida and lead author of the ‘study. Previous CPDs were only about one astronomical unit (AU) in diameter, or the average Earth-Sun distance. The CPD in AS 209 is much larger, perhaps as large as 14 AU in diameter, and orbits 200 AU from its host star – more than 5 times Pluto’s average distance from the Sun.