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. An animation of the system as seen from a hypothetical moon is available here.