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Planetary News: Extrasolar Planets (2005)ESPs: California Scientist Discovers First Extrasolar Planet Under Three SunsBy A.J.S. RaylJuly 14, 2005 A planetary scientist at the California Institute of Technology has discovered the first extrasolar planet under three suns in the constellation Cygnus. Maciej Konacki, a senior postdoctoral scholar in planetary science at Caltech, discovered the unusual hot Jupiter orbiting the main star of the close, triple-star system known as HD 188753, using the 10-meter Keck I telescope atop Mauna Kea in Hawaii. A hot Jupiter is a giant planet with an orbital period of between 3 and 9 days. "Like many other hot Jupiters from binary or multiple stellar systems, it has a larger mass than a typical hot Jupiter orbiting a single star. Otherwise, this system is unique," Konacki wrote in his report, published today in Nature. The three stars are about 149 light-years from Earth, and are about as close to one another as the distance between the Sun and Saturn. "With three suns, the sky view must be out of this world-literally and figuratively," Konacki mused. If the planet had a surface, and you were standing on it, you would see three suns in the sky -- in brilliant displays of yellow, orange, and red. The main star of HD 188753, which is similar to our own Sun, would be yellow and appear as a very large object in the sky since the planet's "year" is only three-and-a-half days long, Konacki pointed out. The larger of the other two suns would be orange, and the smaller red. "The environment in which this planet exists is quite spectacular," he added, referring to it as a "Tatooine" planet -- because of the similarity to the dramatic view Luke Skywalker had of the dusk sky from his home planet in the first Star Wars movie. [In Star Wars lore, Tatooine is generally regarded as a planet in a binary star system, but Skywalker's view of the nightfall is a commonly known visual that lends itself to what things might look like from this newly found giant planet. No one has yet found a habitable planet like the Tatooine in the movie.] Double stars, or binary systems, and multiple star systems are common in the Milky Way galaxy, outnumbering single stars by some 20 percent, and scientists have long known that there are three-star systems that exist in gravitational harmony. But this gas giant, which is slightly larger than Jupiter, is the first to actually be found in a trinary system, and its presence comes as something of a surprise. "The fact that a planet can even exist in a multiple-star system is amazing in itself," Konacki said For one thing, the planet has to contend with the gravitational pull of three other bodies [the suns]. For another thing, the theories and models of solar-system formation suggest that planets are very unlikely to form in such environments. On the other hand, no one's really been looking for planets in trinary systems. Most of the extrasolar planets discovered so far have been found by using a precision velocity technique that is easier to employ on studies of single stars. And so, with limited time on telescopes, many scientists have avoided close-binary and close-multiple stars for reasons of cost-effectiveness. Konacki, however, has developed a "novel method" that allows him "to precisely measure velocities of all members of close-binary and close-multiple-star systems," a breakthrough that led him to conduct a survey in 2003 "for planets in binary and multiple stellar systems." The newly-found planet in the HD 188753 system is the first one from this survey with the Keck I telescope. Konacki may have gotten lucky, or there just may be many more planets out there than we ever imagined, forming in ways we do not yet understand. Scientists discovered the first "hot Jupiter" in 1995. Today, more than 20 such planets are known to orbit other stars. Hot Jupiters are believed to form in a disk of gas and condensed matter at or beyond three astronomical units (AU), three times the 93-million-mile distance between the Sun and Earth. A sufficient amount of solid material exists at 3 AU to produce a core capable of capturing enough gas to form a giant planet. After formation, these planets are believed to migrate inward to their present very close orbits. If the parent star is orbited by a close stellar companion, then its gravitational pull can significantly truncate a protoplanetary disk around the main star. In the case of HD 188753, the two stellar companions would truncate the disk around the main star to a radius of only 1.3 AU, leaving, essentially, no space for a planet to form. Therein lies the mystery of this giant planet -- and the reason this ESP is challenging the conventional wisdom on how planets come into existence. "It appears that within the favored scenario for the formation of hot Jupiters, the planet around HD 188753A could not be formed (unless for some reason the orbit of HD 199753 AB was very different during a planet formation phase)," Konacki wrote in his paper. "One possible explanation is that the snowline [the minimum distance from a star at which ice grains can condense] can indeed be as close as ~1 AU from the star. Another possibility is that hot Jupiters form in situ, near their current orbital locations." "This is another wild system that tests our understanding of how gas giant planets form," said Alan P. Boss, of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, and chair of the IAU's Working Group on ESPs. "Because the two stellar companions are on an eccentric orbit, with e = 0.5 and a = 12 AU, their closest approach to the primary star is about 6 AU. That means that particles with orbits more distant than about 1.5 AU are not stable. Hence as [Konacki reports], if this was the stellar configuration present when the planet formed, the planet-forming disk must have been smaller than 1.5 AU or so in radius. There just is not enough mass in such a disk to make a gas giant planet by either core accretion or disk instability, besides the fact that such a little disk would be too hot to make planets that close to the solar-mass primary." It could be, however, that the planet formed before the system stabilized, Boss suggested. "The only way out is for the planet to have formed in a somewhat larger (5 AU? 10 AU?) disk that existed soon after the multiple star system formed, but that was disrupted when the multiple star system evolved into the current stable configuration, consisting of a very close binary and a more distant (12 AU) third star, i.e., a hierarchical system," he theorized in an email interview with The Planetary Society from Japan, where he is lecturing at a summer school. "Such systems typically form as more evenly spaced multiple systems, which are orbitally unstable and evolve over a period of order 100 orbits and close encounters to kick one or two stars out on hyperbolic trajectories or out to become the more distant member of a hierarchical system." The protostars in the multiple system would each start off with disks capable of forming planets, Boss noted, but these disks will be "distorted and possibly destroyed" during the close encounters that make the stellar system evolve into a stable configuration. "In the case of HD 188753, at least one of the disks must have survived long enough to form a gas giant," he surmised. "However, this time scale must have been quite short, perhaps 100 x 100 yrs = 10,000 yrs (for an initial multiple system with an orbital period of 100 years)." Making a gas giant planet in 10,000 years via core accretion would seem to be impossible, at least based on the modeling done so far. "But it might be possible via disk instability," Boss proffered. "One thousand years is generally all that is needed for a solar-mass primary [sun]. Hence, I suspect that HD 188753's planet probably formed via a rapid formation process, before the multiple system evolved to its present state, and that seems to imply disk instability was at work. Once it formed, it would have been subject to tidal forces during the close encounters that might have helped push it into its current hot Jupiter orbit." Moreover, Boss added, "[t]here may have been other planets formed in this system that did not survive this process." Konacki is continuing his search and is currently in Hawaii making more observations. "There is yet much to be learned about how giant planets are formed," he reflected. But this much is certain: "Planets from complicated stellar systems will put our theories of planet formation to a strict test." Konacki's paper: "An extrasolar giant planet in a close triple-star system," was published in Nature today, Vol. 436, p. 230-33. |
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