As we learn more about the interactions of various stellar systems, it is becoming increasingly clear that planets are sometimes torn from the grasp of their central stars and hurtled into the darkness of space. The most common sources of such planetary ejection are interactions between giant planets. Although the orbits of the planets in our solar system have not changed appreciably for billions of years, they do interact with each other, and the shapes of their orbits do vary. Planetary systems in general are not necessarily grav-itationally stable for time scales of billions of years. If Saturn were closer to Jupiter or if it were more massive, the long-term game of gravitational cat and mouse that planets play could lead to ejection of one of these planets, and it would escape into the galaxy. If Saturn were lost, then Jupiter would stay trapped in solar orbit, but its orbit would be oddly elliptical. some of the giant planets recently discovered orbiting other stars have highly elliptical orbits, and the past ejection of a long-lost partner may have been the cause. Planets can also be ejected from binary star systems where two stars (and their planets) orbit each other.
Although it appears at first glance that ejection from a central sun would be a death sentence for any life on an ejected planet, such may not be the case. Again, the extremophilic microbes could survive in the cold of space. Such an ejected world would have no star, no orbital motion, and no "sunlight," and its surface might approach the frigid temperature of liquid helium.
Any planet ejected from a planetary system would find itself in a most bizarre situation without neighbors and without an external source of heat to warm its surface. The only thing to be seen from the surface of the planet would be the continual sweep of the stars across an eternally dark night sky. This sight would continue monotonously for billions of years. The surface of any solitary planet would cool to cryogenic temperature. Inside the planet, however, warmth would still be generated from a radioactive interior. In that case, a deep subsurface biosphere would be able to survive.
Although ejected planets might not be hospitable to life, the outlook is much more favorable for large moons orbiting ejected planets. If somehow a Jupiter with its four large moons could be ejected into interstellar space, it might provide a very interesting habitat not only for the continuation of microbial life but for its possible evolution as well. Consider life evolving on a large satellite like Europa in orbit around Jupiter. Europa is five times more distant from the sun than is Earth, so it gets only !/25 as much solar heat, which results in a surface temperature near 150 K. This is a frigid, ice-locked world that could not possibly have life on its surface. Yet in spite of its remote location, Europa is widely regarded as one of the more interesting possible environments for life in the solar system, because it probably has a warm liquid-water ocean beneath the ice. Although Europa is far from the sun, the flexing of its interior by the gravitational tidal effects of Jupiter and its other large moons generates appreciable heat. Europa has a significant ocean below a frozen ice crust, and this particular environment—if already endowed with life—could maintain itself in the cold of interstellar space.
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