Interstellar Crashes Could Toss Out Habitable Planets
Our solar system, where planets have a range of sizes and move in near-circular paths, may be rather unusual, according to a German-British team led by Professor Pavel Kroupa of the University of Bonn. The astronomers, who publish their model in the journal Monthly Notices of the Royal Astronomical Society, find that forming planetary systems may be knocked around by crashes with nearby clumps of material, leading to systems where planets have highly inclined orbits and where the smaller (and potentially habitable) worlds are thrown out completely.
The new work suggests that oddly shaped orbits may result from a rather less smooth process. The team think that if the protoplanetary disk enters another cloud of material, it can draw off up to about 30 times the mass of Jupiter from the cloud. Adding this extra gas and dust tilts the disk and hence the angle of the final orbits. Most planetary systems are thought to form in clusters of stars, where the member stars are fairly close together, so these encounters may be very common.
Team member Dr. Ingo Thies, also of the University of Bonn, has carried out computer simulations to test the new idea. He finds that as well as tilting over, loading the protoplanetary disk with material can even reverse its spin, so that it turns in a ''retrograde" sense, where it rotates in the opposite sense to its parent star. At the same time, the encounter compresses the inner region of the disk, possibly speeding up the planetary formation process.
In those circumstances, the simulation suggests that any planets that form will then be in highly inclined or even retrograde orbits. In some cases the orbits may even be tilted with respect to each other, leading to a highly unstable system. One by one, the least massive planets will be ejected completely, leaving behind a small number of 'hot Jupiters', massive worlds that move in orbits extremely close to their star.
Dr. Thies believes the Sun and planets are amongst the more orderly systems. "Like most stars, the Sun formed in a cluster, so probably did encounter another cloud of gas and dust soon after it formed. Fortunately for us, this was a gentle collision, so the effect on the disk that eventually became the planets was relatively benign. If things had been different, an unstable planetary system may have formed around the Sun, the Earth might have been ejected from the Solar System and none of us would be here to talk about it."
Professor Kroupa sees the model as a big step forward. "We may be on the cusp of solving the mystery of why some planetary systems are tilted so much and lack places where life could thrive. The model helps to explain why our solar system looks the way it does, with the Earth in a stable orbit and larger planets further out. Our work should help other scientists refine their search for life elsewhere in the Universe."