SANTA CRUZ, CA — Of all the extrasolar planetary systems detected by
astronomers in recent years, the star 47 Ursae Majoris and its known
companions, two Jupiter-sized planets, is the one that most closely
resembles our own solar system. Computer simulations now show, however,
that Earth-sized planets are unlikely to form in the so-called
"habitable zone" of 47 Ursae Majoris (47 UMa).

The new findings are being reported at the American Astronomical
Society meeting in Washington, D.C., by Gregory Laughlin, an assistant
professor of astronomy and astrophysics at the University of
California, Santa Cruz; John Chambers of NASA Ames Research Center;
and Debra Fischer of the University of California, Berkeley.

Planet hunters have detected nearly 80 planets orbiting nearby
stars, but most of them have elongated, or "eccentric," orbits.
The two planets around 47 UMa, which is located in the Big Dipper
constellation, have nearly circular orbits, like those of Earth and
other planets in our solar system. Their orbits are farther from the
star than Mars is from the Sun, but closer than Jupiter.

The striking similarity between the pair of planets orbiting 47 UMa
and the Jupiter-Saturn pair in our solar system led Laughlin, Chambers,
and Fischer to investigate whether smaller, Earth-sized planets could
have formed and survived in the habitable zone around 47 UMa. The
habitable zone is the region surrounding a star where liquid water
could exist on the surface of a planet — a region roughly equivalent
to the space between the orbits of Venus and Mars in the solar system.
Earth-like (or "terrestrial") planets are too small to be detected
with present-day planet hunting techniques.

The researchers performed a large set of computer simulations and found
that Earth-sized planets have a very hard time forming in 47 UMa’s
habitable zone. The combined gravitational forces of the two large
outer planets conspire to prevent Earth-sized planets from building up
in orbits where temperatures are clement, Laughlin said.

"Our simulations suggest that terrestrial planets can readily form
around 47 UMa in orbits that are roughly half the size of Earth’s
orbit," he said. "Out in the habitable zone, an Earth-sized planet can
survive in a stable orbit, but it is very hard to see how such a planet
could be assembled."

During the formation of a planetary system, terrestrial planets such
as Earth or Mars are believed to form from successive collisions of
small asteroid-sized bodies which stick together to form progressively
larger bodies called "planetary embryos." This process is known as
planetary accretion. It is likely that our solar system went through
a such a phase, in which hundreds of moon-sized planetary embryos
emerged from numerous collisions among a much larger number of small
precursor bodies, Laughlin said.

The researchers used a highly efficient computer program developed by
Chambers to simulate the further development of planetary accretion
from this stage. A typical calculation followed the long-term evolution
of a swarm of 280 moon-sized planetary embryos in the presence of the
two giant planets orbiting 47 UMa, and spanned a time frame of 50
million years near the beginning of 47 UMa’s history. The simulations
showed that embryos starting in the habitable zone tend to jostle each
other into orbits where the gravitational tugs from the two outer
planets either fling the embryos from the system or drive them into
the star itself. On the other hand, the embryos that started inside the
habitable zone were always able to consolidate into several Earth-sized
planets, all having an orbital period of half a year or less.

At the end of several simulations, a single tiny survivor was left
in the habitable zone. In other simulations, the habitable zone was
entirely cleared, Chambers said.

The survival of isolated remnant embryos in the habitable zone of
47 UMa suggests a possible parallel to the asteroid belt in our own
solar system. Within the asteroid belt, many orbits are stable, but
certain locations within the belt contain unstable "resonances" where
objects experience rapid orbital instability. Planetary embryos tend
to scatter each other into these unstable zones, leaving behind a
smattering of survivors — the asteroids — which are much smaller
than the terrestrial planets.

"Because these two giant planets orbiting 47 UMa are more than twice
as close to the star as Jupiter and Saturn are to the Sun, the 47 UMa
system looks like an overweight, scaled-down version of the solar
system. Any terrestrial planets or an analogue to the asteroid belt
around 47 UMa would likely be about twice as close to the star as
well," Chambers said.

The researchers’ assessment of habitable planet formation was part of
a larger theoretical investigation of the two planets orbiting 47 UMa,
in which the team narrowed the range of possible orbital configurations
that the planets might occupy. They also showed that the two outer
planets have undergone very little orbital modification since their
formation. The research was funded by the NASA Origins of Solar
Systems Program.

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Editor’s note: Reporters may contact Laughlin at (831) 459-3208 or; Chambers at (650) 604-5514 or; and Fischer at (510) 643-8973 or .

Images can be downloaded from the web at


[Figure 1: (34KB)]
This figure shows the relative scale of the orbits of the two planets
(‘b’ and ‘c’) orbiting the star 47 Ursae Majoris. The sizes of the
planets are not to scale with the size of the central star. The
habitable zone around 47 UMa is shown as a light blue band. For
comparison, the orbital radii of Venus, Earth and Mars are shown as
dotted lines. A series of numerical simulations have shown that any
terrestrial (Earth-size) planets in the 47 UMa system are likely to
lie closer to the star than the habitable zone.

Credit: Gregory Laughlin

[Figure 2: (67KB)]
Six snapshots in time of a planetary accretion simulation showing the
hypothetical development of terrestrial planets in the 47 UMa system.
The simulation begins with 280 lunar-mass planetary embryos with
initial orbits lying between 0.4 and 2.0 astronomical units (an
astronomical unit is the distance from Earth to the sun). Embryos
are removed rapidly from much of this region as mutual gravitational
perturbations scatter them into unstable resonances associated with
the giant planets. The inner giant planet, 47 UMa ‘b,’ is seen at
the right of each panel. The outer giant planet, 47 UMa ‘c,’ is off
of each panel to the right, and the central star, 47 UMa, is off of
each panel to the left. The distance of each object above the bottom
of the panel corresponds to the eccentricity (or elongation) of its
orbit. The buildup of terrestrial planets is confined to the region
inside the inner edge of the habitable zone. At the end of this
simulation, a single planetesimal remains in the habitable zone, but
this object would be much too small to retain an Earth-like atmosphere
and would not be habitable.

Credit: G. Laughlin, J. Chambers, and D. Fischer