How massive stars steal planets

Our Sun has a rather lonely existence in the Milky Way galaxy. It sits on its own, four light years away from the nearest star, with only its planetary system for company. But it wasn’t always like this. We almost exclusively observe young stars in groups, so-called stellar nurseries, where they brush shoulders with stellar siblings.

These stellar nurseries are densely populated places, where hundreds of thousands of stars often reside in the same volume of space that the Sun inhabits on its own. Violent interactions, in which stars exchange energy, occur frequently, but not for long. After a few million years, the groups of stars dissipate, populating the Milky Way with more stars.

Our new paper, published in the Monthly Notices of the Royal Astronomical Society, shows how massive stars in such stellar nurseries can steal planets away from each other – and what the signs of such theft are.

Almost immediately after young stars are born, planetary systems begin to form around them. We have had indirect evidence of this for more than 30 years. Observations of the light from young stars display an unexpected excess of infrared radiation. This was (and still is) explained as originating from small dust particles (100th of a centimetre) orbiting the star in a disc of material. It is from these dust particles that planets are (eventually) formed.

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The field of star and planet formation underwent a revolution in late 2014 when the first images of planet-forming discs around stars were seen with the Atacama Large Millimetre Array (Alma) telescope in the Chilean desert. The first, and subsequent, images from Alma were nothing short of spectacular. Many of the discs had features and structures that can be attributed to the presence of fully formed, Jupiter-like planets.

Planet formation happens rapidly after the onset of star formation, and certainly while the star is still interacting with its siblings in the stellar nursery. Because planets form so quickly, they will be affected by the densely populated star-forming environment. Planets can have their orbits altered, which can manifest in several ways.

Wandering planets