Replies

Brown dwarfs may stretch out exoplanets' orbits
by David Shiga
New Scientist
20 September 2006
Many of the planets discovered orbiting other stars have very elongated orbits, unlike the relatively circular orbits of planets in our own solar system. That is puzzling because planets are thought to initially form in circular orbits, and astronomers are not sure what causes them to stretch out over time. One possibility is that two planets orbiting the same star may have close encounters that change both their orbits. Another possibility is that a planet's orbit could change early on because of gravitational interactions with the disc from which it formed. A third possibility involves a brown dwarf, often called a "failed star" because it is a ball of gas like regular stars but is not massive enough to sustain nuclear fusion. The brown dwarf would orbit the star at a much greater distance than the planet, pulling on the planet and gradually stretching its orbit over millions of years... A planet about two-thirds the mass of Saturn had previously been discovered in an oval-shaped orbit much closer to the star. Its orbit would fit inside that of Mercury in our own solar system.

Fit inside that of Mercury? Also a catastrophism topic.