A planet
with two, three or even four suns, while strange for us earthlings (except for
in movies such as Star Wars), is not entirely unusual in the universe.
More than half of all planets form with multiple stars. But a recent planet
found in a triple-star system perplexes astronomers because, according to current
models of planetary formation, it should not exist.
An artist’s interpretation shows
the view from a hypothetical moon in orbit around a planet that has three suns.
The gas giant travels around a single star, both of which are orbited by a nearby
system of two stars. Courtesy of NASA/JPL-Caltech.
The planet orbits at a distance of only 7.5 million kilometers from the primary
star of system HD 188753 (about 5 percent of the distance between Earth and
the sun); it circles the star in just over three days, instead of the 365 days
for Earth. But it was a binary star system (two stars that orbit in unison)
orbiting the primary star and planet that struck Maciej Konacki of Caltech in
Pasadena, Calif., as peculiar. According to Konacki’s paper in the July
14 Nature, the companion binary of HD 188753 orbits exceedingly close
to the primary star, compared to other multiple-star systems — so close
in fact that its gravity should have stripped away the materials needed to build
a planet.
Planet-building materials exist in early solar systems within a disk around
a star. Over time, the icy debris clumps together and, like a growing snowball,
eventually reaches the size of a planet. Some conditions, however, restrict
the size and location of such planet nurseries. Alycia Weinberger, an astronomer
at the Carnegie Institution of Washington, says that intense heat melts away
any potential planet-building materials close to a star. “To condense material
into a planet, it needs to be fairly cool,” Weinberger says.
Prior to the discovery of the planet in HD 188753, scientists knew of large
planets in other systems that also orbited too close to a hot star to have formed
there. To explain the existence of these planets that astronomers call “hot
Jupiters,” they calculated that the planets could form in cooler locations
farther out and then migrate in. “Basically there’s an interaction
between the planet and the disk and that causes the planet to slowly spiral
in, and eventually it stops someplace close to the star,” Weinberger says.
“That’s the accepted mechanism for all of these so-called hot Jupiters.”
Konacki says, however, that HD 188753 poses a significant complication. Even
at a distance from the primary star, gravity from the system’s closely
orbiting companions would have demolished any potential building material. “The
difficulty in this system is how you form the planets to begin with.”
It is possible, Konacki says, that cold planet-building materials can reside
closer to a star than previously believed, so that the planet formed in its
current orbit. Another possible explanation is that the orbits of the stars
were different during the time of planet formation — far enough apart as
not to affect the debris disk. “We may not know for this particular system,
ever, what its original configuration was at the time the planet formed,”
Weinberger says. But “how important this discovery is for planet-formation
theory depends on whether you believe that it formed in its current configuration
or not.”
If it turns out that the planet indeed coalesced with the companion and primary
stars of HD 188753 as close together as they are today, astronomers will have
to scrap current ideas about how planets form. Weinberger says, “We would
really have to revise planet-formation theories to be able to keep the planet
in the system.” Konacki agrees, and holds that modifications to current
theory could explain the formation of the planet. “Let’s wait for
what the theorists have to say,” Konacki says.
Konacki continues to survey 450 multiple-star systems to find out if similar
arrangements are common. Looking at systems dissimilar to our own is an important
first step in learning about the nuances of planet formation. “The planets
have turned up it seems in the utmost creative places,” Weinberger says.
“That’s lots of fun because now it’s up to us to try and figure
out how that happened.”
Kathryn Hansen
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