The First Successful Prediction of an Exoplanet


In a triumph for theoretical studies of the cosmos, a study of the orbits of two planets around the star HD 74156 has led to the first successful prediction of a new planet since the 1840s. The discovery was announced today by astronomers at the American Astronomical Society's meeting in Austin, Texas. Rory Barnes, currently a post-doctoral associate at the University of Arizona in Tucson, and his associates studied the orbits of several planetary systems and found that planets' orbits tend to be packed as closely together as possible without gravity destabilizing their orbits, see Fig. 1. Barnes and colleagues reasoned that this tight packing resulted from universal processes of planetary formation. But the two planets (named "b" and "c") orbiting the star HD 74156, a little more than 200 light years from Earth, had a big gap between them. If their "Packed Planetary Systems" hypothesis was correct, then there must be another planet between them, and it must be in a particular orbit.
Fig. 1 - A schematic of stable and unstable orbits in the Upsilon Andromedae system. The currently observed orbits are in black, and the yellow circle represents the central star. If planet d's orbit were changed to the red curve (a slight change), the system would be unstable, and most likely eject planet c. This system is therefore dynamically packed; no additional planets can exist between b and c.
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"When I realized that six out of seven multi-planet systems appeared 'packed,'" Barnes explained, "I naturally expected there must be another planet in the HD 74156 system so that it, too, would be packed."

Indeed when Jacob Bean and his colleagues from the University of Texas observed that planetary system carefully, they discovered a new planet located where Barnes had predicted (see Fig. 2). This planet is named, by convention, HD 74156 d.
Fig 2 - The left panel presents a comparison of the Barnes & Raymond's prediction (shaded region), with that of Bean and colleagues' discovery (the line marked with a "d"). The discovered planet lies in the zone predicted, making the new planet the first to successfully be predicted. The left panel shows the orbits of the planets in our Solar System (out to Jupiter) for reference.
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Barnes' colleagues in the successful prediction were Dr. Sean Raymond (now a post-doctoral associate at the University of Colorado) and Prof. Thomas Quinn (at the University of Washington). The discovery team, from the University of Texas at Austin, included Jacob Bean's adviser, Prof. Barbara McCarthur, as well as Prof. Fritz Benedict.

Dr. Steven Soter, astronomer with the American Museum of Natural History in New York, has been following the discoveries of extra-solar planets (planets orbiting other stars beyond our solar system). Soter has pointed out that the work of Barnes et al. represents the first successful prediction of a new planet since that of Neptune over 160 years ago. Before Neptune was actually seen, the early nineteenth century astronomers LeVerrier and Adams had each independently (LeVerrier in France, Adams in England) inferred its existence from its effects on the motion of Uranus.

In addition to providing a way to predict planet discoveries, Barnes believes that the Packed Planetary Systems hypothesis reveals something fundamental about the formation of planets. The process by which planets grow from the clouds of dust and gas around young stars must be very efficient. Wherever there is room for a planet to form, it does.

The Packed Planetary Systems hypothesis also predicts that gaps between known planets in other systems are probably occupied by additional, still undiscovered, planets. Indeed, shortly after the discovery of HD 74156 d, a different team of astronomers found a planet orbiting the star 55 Cnc, again in an orbit predicted by Barnes and Raymond (see Fig. 3). Barnes and colleagues have also predicted a specific planet orbiting a third system, HD 38529. So far, no planet has been discovered there (see Fig. 4). However, the scientists expect future observations to provide more information, perhaps even another successful prediction by the Packed Planetary Systems hypothesis.
Fig. 3 - The left panel shows a comparison of the orbits predicted for the 55 Cnc system in the shaded region, relative to the orbit of the recently-discovered planet 55 Cnc f (by Debra Fischer and colleagues). The new planet was found at the inner region of the large stable zone, leaving the possibility of detecting still more planets in this system. The right panel shows the orbits in our Solar System (at the same scale) for comparison.
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Fig. 4 - The left panel shows the predicted zone for a new planet in the HD 38529 system in the shaded region. No planet has been discovered in the zone yet. The right panels shos the orbits of planets in our Solar System (at the same scale) for comparison.
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