Planets form out of disks of gas and dust around stars, but catching them doing so is hard. Although astronomers have often found signs of what might be planets partway through forming, we've never been quite sure. Now two teams have independently concluded we've found the real thing – a star with at least three planets being made before our eyes.
Observations of very young stars taken using millimeter-wavelength telescopes have revealed extensive surrounding disks. Slightly older stars are disk-free, leading to the conclusion that the missing material is making up planets. Some of these so-called protoplanetary disks even have gaps in them, suspected of having been carved out by partially formed planets.
Nevertheless, Dr Christophe Pinte of Australia's Monash University told IFLScience that alternative explanations for these gaps exist. Condensation fronts, where molecules freeze onto dust grains, could change the opacity of disks and create apparent gaps, and there are also theoretical ways magnetic fields could produce similar effects.
Pinte is first author of a study of the disk around the 4-million-year-old star HD 163296, published in the Astrophysical Journal Letters (preprint available on arXiv), confirming one of these gaps does indeed house a growing planet. By studying the movement of disk material, Pinte and his colleagues revealed the gravitational pull of a planet 1.9 times the mass of Jupiter.
Pinte thinks the planet is probably approaching its final mass, but simulations show it is still acquiring some extra material, and he acknowledges the final size is hard to predict.
The planet lies 260 astronomical units (AUs) from HD 163296, almost nine times further out than Neptune. Pinte told IFLScience; “There are two main theories about planetary formation and neither of them explain how you would get something forming this far out.”
Nevertheless, the finding is not entirely unexpected, since the disk around HD 163296 is exceptional, extending far further than most of the equivalents we know. In the same edition of the Astronomical Journal Letters, another paper describes the work of a different team studying HD 163296's disk. Using the direction of movement of carbon monoxide gas in the disk they report the existence of two other partially formed planets, at 80 and 140 AUs.
Pinte told IFLScience the other team's technique can measure differences in the velocity of gas, while his own observed the velocity itself, an approach more suited to the disk's outer edges. There is also evidence of texture within the disk hinting at additional planets closer to the star.
Both teams chose to study HD 163296 because the disk is so large and bright. Consequently, Pinte said, it remains to be seen whether it is an anomaly, or if planets form at these tremendous distances quite often. Only studies of disk stars will tell.