A newly discovered planet
has bountiful sunshine, with not one, not two, but three suns glowing in its
sky.
It is the first extrasolar
planet found in a system with three stars. How a planet was born amidst
these competing gravitational forces will be a challenge for planet
formation theories.
"The environment in which
this planet exists is quite spectacular," said Maciej Konacki from the
California Institute of Technology. "With three suns, the sky view must be
out of this world -- literally and figuratively."
The triple-star system, HD
188753, is located 149 light-years away in the constellation
Cygnus. The primary star is like our Sun, weighing 1.06 solar
masses. The other two stars form a tightly bound pair, which is separated
from the primary by approximately the Sun-Saturn distance.
"The pair more or less
acts as one star," Konacki told SPACE.com.
The combined mass of the
close pair is 1.63 solar masses.
Using the 10-meter Keck I
telescope in Hawaii, Konacki noticed evidence for a planet orbiting the primary
star. This newfound gas giant is slightly larger than Jupiter and whirls
around its central star in a 3.5-day orbit. A planet so close to its star
would be very hot.
Although other so-called hot
Jupiters have been found in such close-in orbits, the nearby stellar pair
in HD 188753 likely sheared off much of the planet making material in the disk
that would likely have existed around the primary star in its youth.
Since this proto-planetary disk holds the construction materials for planets,
there does not appear to be any safe place for this far-off world to have been
assembled.
Snow line and
migration
The heat coming from a
nearby star frustrates the initial stages of giant planet formation -- the
gluing together of planetary seeds, called cores. Therefore, the typical
hot Jupiter is thought to form farther out -- beyond a theoretical limit called
the snow
line.
"Past about 3 AU, it is
cold enough to form ices and other solid material for building cores,"
Konacki said. An AU is the distance between the Sun and the Earth --
about 93 million miles.
Once a sufficiently large
core is built outside the snow line, the planet can start accreting gas and --
if the conditions are right -- migrate
toward its sun.
Although this scenario
appears to work in most stellar systems, it has difficulty explaining the newly-discovered
planet in HD 188753. Of all the planet-harboring stars known, this is the
closest that a stellar companion has ever been found.
"The problem is that
the pair is a massive perturber to the system," Konacki said.
"Together, these two stars are more massive than the main star."
Moreover, the pair goes
around the primary along an oblong orbit that stretches from 6 AU out to 18 AU
over a 26 year period. This eccentricity
increases the instability of the disk around the primary. Konacki
estimates that due to the gravitational perturbations from the pair, the
proto-planetary disk was truncated down to 1.3 AU, far within the snow line.
"How that planet formed
in such a complicated setting is very puzzling. I believe there is yet much to
be learned about how giant planets are formed," Konacki said.
Targeting multiple
stars
Konacki hopes to find more
planets around stars with companions. About 30
extrasolar planets have been found around double-star systems, or
binaries. This is a small percentage of the total number of extrasolar
planets, even though multi-star systems outnumber single star systems.
The reason for this
disparity is that the main technique for locating planets -- the
radial velocity method -- is not well-suited for finding planets with more
than one star.
"Single stars are much
easier to work with, since the shape of the spectrum stays the same,"
Konacki explained.
By watching for wobbles in a
star's spectrum, astronomers can infer the gravitational tug from a nearby
planet. But when there is a companion star, its light competes with that
of the main star. Konacki has developed a method to extract the planet
wobbles from this messy, combined spectrum.
He found this triple-sun
planet in the first 20 stars he looked at. He plans to survey about 450
stars in the future.
The discovery is reported in
the July 14 issue of Nature. Animations created by JPL's
PlanetQuest show the orbital
motion of the system, as well as what it looks like from a hypothetical
moon.
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