Free Astronomy Magazine September-October 2019

39 SEPTEMBER-OCTOBER 2019 SPACE CHRONICLES rally emitted by hydro- gen gas accreting onto the planets. The new ALMA observations in- stead image the faint radio waves given off by the tiny (about one- tenth of a millimeter across) particles of dust around the star. The ALMA data, com- bined with the earlier optical and infrared VLT observations, pro- vide compelling evi- dence that a dusty disk capable of forming multiple moons sur- rounds the outermost known planet in the system. “For the first time, we can conclusively see the telltale signs of a circumplanetary disk, which helps to support many of the current theories of planet for- mation,” said Andrea Isella, an astronomer at Rice University in Houston, Texas, and lead author on a pa- per published in The Astrophysical Journal Letters . “By comparing our observations to the high-resolution infrared and op- tical images, we can see that an oth- erwise enigmatic concentration of tiny dust particles is a planet-girding disk of dust, the first such feature ever conclusively observed,” Isella said. According to the researchers, this is the first time that a planet has been seen in these three distinct bands of light (optical, infrared, and radio). Unlike the icy rings of Saturn, which likely formed by the crashing to- gether of comets and rocky bodies relatively recently in the history of our Solar System, a circumplanetary this planet is shining so brightly in the infrared and hydrogen bands of light, the astron- omers can convincingly say that a fully formed planet is already in orbit there and that nearby gas continues to be siphoned onto the planet’s surface, finishing its adoles- cent growth spurt. This outer planet is located approximately 5.3 bil- lion kilometers from the host star, about the same distance as Neptune from our Sun. Astronomers estimate that this planet is ap- proximately 1 to 10 times the mass of Jupiter. “If the planet is on the larger end of that estimate, it’s quite possible there might be planet-size moons forming around it,” noted Isella. The ALMA observa- tions also add another important element to these conclusions. Optical studies of planetary systems are notoriously challenging. Since the star is so much brighter than the planets, it is dif- ficult to filter out the glare, much like trying to spot a firefly next to a searchlight. ALMA observations, however, don’t have that limitation since stars emit comparatively little light at millimeter and submillimeter wavelengths. “This means we’ll be able to come back to this system at different periods and more easily map the orbit of the planets and the concentration of dust in the system,” concluded Isella. “This will give us unique insights into the orbital prop- erties of solar systems in their very earliest stages of development.” C omposite image of PDS 70. Comparing new ALMA data to earlier VLT observations, astronomers determined that the young planet designated PDS 70 c has a circumplanetary disk, a feature that is strongly theorized to be the birthplace of moons. [ALMA (ESO/ NOAJ/NRAO) A. Isella; ESO] disk is the lingering remains of the planet-formation process. The ALMA data also revealed two distinct differences between the two newly discovered planets. The closer in of the two, PDS 70 b, which is about the same distance from its star as Uranus is from the Sun, has a trailing mass of dust behind it re- sembling a tail. “What this is and what it means for this planetary sys- tem is not yet known,” said Isella. “The only conclusive thing we can say is that the tail is far enough from the planet to be an independent feature.” The second planet, PDS 70 c, resides in the same location as a clear knot of dust seen in the ALMA data. Since !