Wednesday, January 9, 2013

Star Fomalhaut has a mysterious planet circling it


WASHINGTON -- 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.

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