IT IS not often that bullies are a good thing for youngsters in a nursery. But in the case of stellar nurseries, violent stars could actually aid the formation of planets rather than prevent it. It now seems as if ultraviolet light from massive stars might help dust around nearby ordinary stars condense into planetary seeds.
Most stars form in dense clusters such as the stellar nursery in the Orion nebula, 1500 light years away along the same spiral arm of the Milky Way as our sun. Many young stars in the nebula are surrounded by thick discs of gas and dust that might go on to form planets. But there are also a few stars so bright that their light is blowing away the gas around the other stars, in a process known as photoevaporation. It was thought that the gas would take the dust along with it, preventing the formation of planets.
Now Henry Throop of the Southwest Research Institute and John Bally of the University of Colorado, both based in Boulder, suggest otherwise. Their calculations show that in many cases the dust in the discs has time to settle out, falling towards the mid-plane of the thick disc. So even when the gas is blasted from the surface of the disc, the dust remains. And a lack of gas might be just what is needed.
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鈥淎 few stars are so bright that they blow away the gas around other stars. This might be just what is needed to form planets鈥�
Many astrophysicists believe the next stage in planet formation is for bits of dust to randomly collide and stick together, slowly building up into kilometre-sized bodies called planetesimals. During the early stages, loose balls of dust a few metres across would easily be broken up by collisions, making it hard for them to grow much bigger. But this roadblock could be circumvented if instead parts of the dust cloud are able to collapse under their own gravity, directly forming robust planetesimals. However, the pressure created by too much gas in the disc could stop the dust鈥檚 weak gravity from pulling the clumps together.
Throop and Bally say that the problem is solved if the radiation from neighbouring stars strips away most of the gas. 鈥淚t鈥檚 a surprising result,鈥� says Throop. 鈥淒estroying the discs may make planetesimals easier to form.鈥�
Although it has usually been assumed that our sun was born alone in a cool, dark environment, recent evidence hints at a more cataclysmic origin in a nursery not unlike the Orion nebula. And as Throop points out, that might explain why Neptune and Uranus are so much smaller than Jupiter and Saturn. Photoevaporation is stronger as you go further out in the disc, so it might have ripped away much more of the gas in the outer parts where Neptune and Uranus formed. So maybe our solar system was kick-started by a bully of the sun.