[A]stronomers have found a brown dwarf that has a dusty disc encircling it, just like the discs encircling regular, young stars. It contains millimeter-sized solid grains, and around other newborn stars, these discs of cosmic dust are where planets form. Astronomers say this surprising find challenges theories of how rocky, Earth-scale planets form, and suggests that rocky planets may be even more common in the Universe than expected.
Rocky planets are thought to form through the random collision and sticking together of what are initially microscopic particles in the disc of material around a star. These tiny grains are similar to very fine soot or sand. However, in the outer regions around a brown dwarf, astronomers expected that grains could not grow because the discs were too sparse, and particles would be moving too fast to stick together after colliding. Also, prevailing theories say that any grains that manage to form should move quickly towards the central brown dwarf, disappearing from the outer parts of the disc where they could be detected.
“We were completely surprised to find millimeter-sized grains in this thin little disc,” said Luca Ricci of the California Institute of Technology, USA, who led a team of astronomers based in the United States, Europe and Chile. “Solid grains of that size shouldn’t be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do. We can’t be sure if a whole rocky planet could develop there, or already has, but we’re seeing the first steps, so we’re going to have to change our assumptions about conditions required for solids to grow,” he said.
This is the fulfillment of earlier studies that I noted back in 2005.
What about potentially Earth-like planets, though? As noted briefly in a press release from the University of Washington and at length over at Centauri Dreams, the steadily contracting habitable zones of slowly dimming brown dwarfs--and white dwarfs, too, post-sequence degenerate stars--means that worlds which might be in the habitable zone at a certain point would have been superheated before.
[B]oth brown and white dwarfs could support a habitable zone, but what sets them apart from red dwarfs is that they cool slowly and continuously, meaning their habitable zones shrink inward toward the star. Imagine, Barnes and Heller say, a planet that starts out as a Venus-like world beset with a runaway greenhouse effect. Eventually the habitable zone contracts enough to create the needed temperatures for liquid water to exist, but by now the planet’s surface water is gone and so is the chance for life.
Two possibilities come to my mind.
1. Young worlds orbiting young, bright brown dwarfs could still be habitable. Life would have to become present at a very early stage, however, and unless these brown dwarfs themselves orbited in the habitable zone of an actual stars these young worlds would be doomed to freeze over.
2. Desert worlds, worlds lacking water oceans, are apparently less likely to overheat Venus-style than worlds with oceans. Perhaps worlds located in closer orbits to brown dwarfs, initially inside the habitable zones of bright young brown dwarfs, might, if they don't host much water, avoid being sterilized Venus-style.