For the first time, astronomers were able to catch the formation of a planetary system. The paper, published in Nature, suggests that the observed objects are infant planets, currently being pieced together by extremely hot gas and dust.
Using images of the system taken between 2009 to 2015, the team was able to identify two protoplanets – small objects that go on to form planets – alongside a potential third, moving around the star LkCa 15 in an elliptical orbit, as it is expected of planets.
Detecting planets forming is a complex task. Newborn stellar systems are usually enshrouded in a cloud of dust that muddies our view, making normal detection methods unsuitable. The team therefore had to develop a different approach to study the system.
Young stars produce large disks of material from which planets form. As growing planets move through this protoplanetary disk, they create gaps in the swirling debris, which astronomers can detect using infrared light. Several protoplanet candidates were discovered this way.
Planets undergoing formation increase in size thanks to the smaller disks of material, similar to the ones around stars, that form around them. As material from this circumplanetary disk falls on the planet, it gets hotter which makes the planet appear redder than it actually is, and visible to us in infrared.
The planet's magnetic field might also affect the disk of dust and gas surrounding it. A strong magnetic field forces the material in the disk to follow specific orbits. The energy generated within the disk heats up the magnetosphere of the planet to a temperature higher than the surface of the Sun, and the heated material produces a specific emission called hydrogen-alpha.
The gap around LkCa 15 was first observed in 2011, which indicated the potential for the star to host at least one exoplanet. For this latest study, the team was able to identify hot gas (9,700°C [17,500 °F]) falling onto the closer planet LkCa 15b. Looking at the data from the system, scientists realized there were other emissions in the gap: One signal was recognized as a second planet after being observed several times. A third emission is thought to be another planet but is yet to be confirmed.
"Observing emission lines such as H-alpha is a powerful tool for tracing accretion onto protoplanets directly. I think this technique will be very useful for finding protoplanets around other stars as well." Steph Sallum, lead author of the study, told IFL Science
This discovery and the success of this technique provides new opportunities to study how planetary systems form and how young planets interact with the disk of material around the star.