A brown dwarf has been discovered in a distant orbit around a star known to harbour a planet. Its discoverers say the brown dwarf’s gravitational influence may be responsible for stretching the planet’s orbit into an ellipse, an effect that may apply to other systems, as well.
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.
However, testing this hypothesis has proven difficult because these distant brown dwarfs are extremely dim and therefore difficult to observe.
Now, in a survey specifically designed to root out brown dwarfs, astronomers have found one orbiting a star known to have a planet in an elongated orbit.
Kevin Luhman of Pennsylvania State University in University Park, US, led the discovery team, which used NASA’s infrared Spitzer Space Telescope. The brown dwarf was found orbiting a Sun-like star called HD 3651, which is 36 light years from Earth.
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.
The brown dwarf is much farther away from the star, more than 10 times Pluto’s maximum distance from the Sun. Although it is far from the planet, the brown dwarf has 50 times the mass of Jupiter, and the team says that its gravitational influence may have gradually stretched the planet’s orbit.
Even so, brown dwarfs are probably not responsible for most of the elongated orbits seen to date, says team member Massimo Marengo of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.
“We have done a quite extensive survey of nearby stars for which extrasolar planets have been observed,” he told New Scientist. “So far this is the only one in which we have found a brown dwarf companion, so it seems to be a relatively rare case.”
Journal reference: Astrophysical Journal, (in press)