The First Successful Prediction of an Exoplanet
AAS PRESS RELEASE: 9 Jan 2008
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
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
Full Resolution: TIFF,
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
specific planet orbiting a third system, HD 38529. So far, no
planet has been discovered there (see Fig. 4). However, the scientists
future observations to provide more information, perhaps even
another successful prediction by the Packed Planetary Systems
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).
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
Full Resolution: TIFF,
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.
Full Resolution: TIFF,