Discovery of a First: A World With 3 Suns
A planet far away in the galaxy is one of the strangest places ever observed by astronomers.
It orbits a Sun-like star, which also has two other stars in its gravitational embrace. So the planet, its mass slightly greater than Jupiter's, experiences the unearthly spectacles of multiple sunrises and sunsets. Its main sun, bright yellow, hovers close by. The two others, a larger orange one and a smaller red one, pirouette around each other in an orbit comparable to the distance between Saturn and our Sun.
The discovery, reported in yesterday's issue of the journal Nature, has thrown another nasty curve to theorists of the origin and dynamics of planetary systems. A planet occupying an orbit among three stars is seen as more evidence that the solar system may not be typical.
Maciej Konacki of the California Institute of Technology, who made the discovery with the Keck telescope in Hawaii, said scientists previously had no evidence that planets could form or survive in the traffic of such gravitationally complex stellar systems, which were thought to be inhospitable to them.
"This is the first time we have detected a planet in a triple-star system," Dr. Konacki, a planetary researcher, said in a telephone interview. "We have to come up with a new hypothesis of planet formation that can apply in such conditions."
Other scientists said the discovery, though difficult to account for, was encouraging because it seemed to show that planets exist in many neighborhoods previously ignored in telescopic searches for planets around other stars.
More than 60 percent of stars in the Milky Way galaxy come in pairs or occasionally threesomes. A few of the 155 planets that have been detected in the last 10 years around other stars exist in binary star systems, but only where the second star is too far away from the primary star to disturb planetary formation.
Dr. Konacki knew when he started his observations that HD 188753, about 149 light-years from Earth, was a triple system and hence not a promising place to look for planets.
He took a chance, though, that a novel technique he had developed for precisely measuring velocities of all objects in multiple-star systems just might reveal the presence of a planet or two. The problem was to separate the individual starlight from each of the three close companions in order to detect the tug of a planet causing a perceptible wobble.
Dr. Konacki detected the primary star's telltale wobble, revealing the presence of a Jupiter-like gaseous planet so close to the star that it completes an orbit, or "year," every 3.35 Earth days. The planet cannot be seen, only inferred by its gravitational effects on the main star.
Such objects are known as hot Jupiters because of their proximity to their star's blazing heat. They have been found around 30 other stars, an earlier challenge to theorists. Considering it virtually impossible for such large planets to originate that close to a star, theorists posited that they had migrated inward from regions comparable to the distance of Jupiter in the solar system.
But the paired companion stars to HD 188753's primary star, scientists now realize, would presumably have burned away the disk of gas and dust out of which a planet could have formed. According to the standard orbital migration model, the newfound planet should not exist.
In another article in Nature, two German astronomers, Artie P. Hatzes and Günther Wuchterl, said Dr. Konacki's findings presented "a conundrum to theorists." They recommended abandoning the migration theory and accepting the diversity of planetary systems they are seeing, which probably reflects a diversity of the disks of raw material that formed the planets. Some disks may have more mass closer to the star than previously thought.
The strange planet of the three suns, the astronomers said, is a reminder that not all protoplanetary disks are like the one around the early Sun.