Astronomers are surprised to find the debris belt is wider than previously known, spanning a section of space from 14 billion miles (23 billion kilometers) to nearly 20 billion miles (32 billion km) from the star. Even more surprisingly, the latest Hubble images have allowed a team of astronomers to calculate that the planet follows an unusual elliptical orbit that carries it on a potentially destructive path through the vast dust ring.
The planet, called Fomalhaut b, swings as close to its star as 4.6 billion miles (7.4 billion km), and the outermost point of its orbit is 27 billion miles (43 billion km) away from the star. The orbit was recalculated from the newest Hubble observation made last year.
“We are shocked. This is not what we expected,” said Paul Kalas of the University of California, Berkeley, and the SETI Institute in Mountain View, California.
The Fomalhaut team led by Kalas considers that this circumstantial evidence may point to other planetlike bodies in the system that gravitationally disturbed Fomalhaut b to place it in such a highly eccentric orbit.
Among several scenarios to explain Fomalhaut b’s 2,000-year-long orbit is the hypothesis that an as yet undiscovered planet gravitationally ejected Fomalhaut b from a position closer to the star and sent it flying in an orbit that extends beyond the dust belt.
“Hot Jupiters get tossed through scattering events, where one planet goes in and one gets thrown out,” said Mark Clampin of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This could be the planet that gets thrown out.”
Hubble also found that the dust and ice belt encircling the star Fomalhaut has an apparent gap slicing across the belt. This might have been carved by another undetected planet. Hubble’s exquisite view of the dust belt shows irregularities that strongly motivate a search for other planets in the system.
If its orbit lies in the same plane with the dust belt, then Fomalhaut b will intersect the belt around 2032 on the outbound leg of its orbit. During the crossing, icy and rocky debris in the belt could crash into the planet’s atmosphere and create the type of cosmic fireworks seen when Comet Shoemaker-Levy 9 crashed into Jupiter. Most of the fireworks from collisions will be seen in infrared light. However, if Fomalhaut b is not coplanar with the belt, the only thing to be seen will be a gradual dimming of Fomalhaut b as it travels farther from the star.
Kalas hypothesized that Fomalhaut b’s extreme orbit is a major clue in explaining why the planet is unusually bright in visible light but dim in infrared light. It is possible the planet’s optical brightness originates from a ring or shroud of dust around the planet that reflects starlight. The dust would be rapidly produced by satellites orbiting the planet, which would suffer extreme erosion by impacts and gravitational stirring when Fomalhaut b enters into the planetary system after a millennium of deep freeze beyond the main belt. An analogy can be found by looking at Saturn, which has a tenuous but large dust ring produced when meteoroids hit the outer moon Phoebe.
The team also has considered a different scenario where a hypothetical second dwarf planet suffered a catastrophic collision with Fomalhaut b. The collision scenario would explain why the star Fomalhaut has a narrow outer belt linked to an extreme planet. But in this case, the belt is young, less than 10,000 years old, and it is difficult to produce energetic collisions far from the star in such young systems.
Fomalhaut is a special system because it looks like scientists may have a snapshot of what our solar system was doing 4 billion years ago. The planetary architecture is being redrawn, the comet belts are evolving, and planets may be gaining and losing their moons. Astronomers will continue monitoring Fomalhaut b for decades to come because they may have a chance to observe a planet entering an icy debris belt that is like the Kuiper Belt at the fringe of our solar system.