Newton said that all objects in the universe
exert a gravitational force on all other objects. Thus, the Sun exerts
a gravitational pull on the Earth, but so does the Moon and all of the
planets within the Solar System. The degree that a planet is affected
by the multitude of forces exerted on it depends on the masses and distances
of the objects doing the pulling, as well as the planet's own mass. The
motion of the Earth is governed primarily by the Sun - we say that the
Earth is in orbit around the Sun - and, to a smaller degree, by the Moon
and the other planets, despite the close proximity of these lesser bodies.
If the gravitational pull from the other planets in the Solar System were
hypothetically removed, Earth would orbit the Sun on a path which would
remain unchanged for as long as the Sun existed. But with the other planets
present, Earth's near-circular orbit is gravitationally perturbed so that
the shape and orientation of its orbit gradually change over time. Despite
these changes, Earth's orbit within the Solar System is remarkably stable,
and will continue to be for billions of years.
Earth-like planets around other stars may
not be as fortunate if the neighboring planets in their systems are larger
or more closely spaced than the planets in the Solar System. All of the
planets recently detected around nearby stars are Jupiter-mass planets
(~300 Earth masses) orbiting within what is the terrestrial-planet region
of our own Solar System. Around the star 70 Virginis in the constellation
Virgo, for example, is a planet more than 7-8 times the mass of Jupiter
and closer to its star than Venus is to the Sun (~65 million miles or 0.7
AU; an AU, or astronomical unit, is the distance between the Earth and
the Sun, or ~93 million miles). The highly eccentric (noncircular) orbit
of the large planet around 70 Virginis is shown in dark
blue in the above figure. The star 70
Virginis is located at the figure origin. Whether any additional planets,
big or small, are present around 70 Virginis is unknown at this time.
Could there be an
Earth in this system? The orbital integration software available at Behrend
College is an ideal tool for answering such questions because the user
can easily customize arbitrarily simple or complex systems of gravitationally
interacting bodies and put them into motion to investigate possible dynamic
outcomes. For the simulation pictured above, a 10 Jupiter-mass planet was
placed in an eccentric orbit (e = 0.40) at 0.47 AU from its star to represent
the giant planet recently discovered around star 70 Virginis. A second
planet of ~ 1 Earth mass was placed in an approximately circular orbit
(e ~ 0) at 1.0 AU, and inclined by 5 degrees relative to the first. The
two-planet system was then integrated over 1 million years. The orbital
evolution of the Earth-sized planet is shown in red
in the above figure. For clarity, only
~60 of the more than 1 million Earth orbits are shown. Large scale changes
in the shape and orientation of lesser planet's orbit bring it closer and
closer to the extremely stable orbital path of the giant planet. Once the
orbital paths of the two planets cross in the not-too-distant future, the
two planets will experience a close encounter resulting either in the destruction
or ejection of the smaller planet. This dramatic result suggests that there
can be no Earth-sized planets this close to the giant planet known to orbit
70 Virginis.
Any comments or complaints about this material can be sent to dmw145@psu.edu
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