Newly released NASA Hubble Space Telescope
images of a vast debris disk encircling the nearby star Fomalhaut and a
mysterious planet circling it may provide forensic evidence of a titanic
planetary disruption in the system.
Astronomers are surprised to find the debris
belt is wider than previously known, spanning a section of space from 14 to
nearly 20 billion miles from the star. Even more surprisingly, the latest
Hubble images have allowed a team of astronomers to calculate 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, and the outermost point of its orbit is
27 billion miles 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 at Berkeley and
the SETI Institute in Mountain View, Calif.
The Fomalhaut team led by Kalas considers this
circumstantial evidence there may be other planet-like bodies in the system
that gravitationally disturbed Fomalhaut b to place it in such a highly
eccentric orbit. The team presented its finding Tuesday at the 221st meeting of
the American Astronomical Society in Long Beach, Calif.
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
co-investigator Mark Clampin of NASA's Goddard Space Flight Center in
Greenbelt, Md. "This could be the planet that gets thrown out."
Hubble also found 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
co-planar 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 very dim in infrared light. It is possible the planet's
optical brightness originates from a ring or shroud of dust around the planet,
which 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 very large dust ring produced when
meteoroids hit the outer moon Phoebe.
The team has also 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 own solar system.