System 15
This very large star system consists of 4 stars and 76 planets! However, all but 10 of these planets are rogue planets drifting into space
(these are shown, along with their major moons, in the diagram outside the orbiting bodies). What can account for this? Originally we may
have had four separate star systems, orbiting about their common centre of gravity. However, two of those stars are now
white dwarfs.
These were likely the larger of the four stars initially and so aged more rapidly. As they left the main sequence they would have evolved
asymptotic red giants and then planetary nebulae, blowing off their outer layers and filling the system with gas and plasma. Some of
the original planets may have been destroyed as these stars swelled to many times their original size before blowing off their
atmospheres. Any remaining planets would have had to readjust their orbits and many may have broken away from their parent stars,
which were much reduced in mass by this stage.
Binary systems may also have unstable orbits which eventually lead to escape of some
of the planets from the system; this system is complex with four stars so we would expect some orbital instability. This could account for
some of the rogue planets. Another possibility, however, is that new planets have begun to form. The binary nature of the central star is
expected to have rapidly propelled the planetary nebula gas into space, clearing it from much of the system. Some of this gas still forms a
torus around the star system and we suspect that some of this matter may have condensed into new planets.

Some of the matter lost from the older stars may also have been accreted by the remaining two stars. In the central binary, the ejected
matter would have imposed drag on the orbiting red dwarf partner, causing it to spiral closer to its white dwarf companion. Eventually this
pair formed a close binary in which matter is streaming from the red dwarf toward the white dwarf, which is accreting this matter via an
accretion disc. The pair have formed a cataclysmic variable star in which the white dwarf is feeding parasitically upon the red dwarf which
has filled its
Roche lobe.

The rogue planets are most likely frozen, or else too young to have evolved life. Of course we can never rule out the possibility of
sub-surface lifeforms, or of life on satellites orbiting giant and supergiant planets being heated by tidal forces, but the most interesting
feature of this system appears to be the second planet orbiting the outermost white dwarf. This planet (15-gamma-2) is a medium-sized
(M class) planet, a frozen world, but one with an anomaly in its spectral signature. Also, the third planet (15-alpha-3) orbiting the yellow
dwarf G-star is worth investigating further; this world is the closest to being a conventional habitable world, and although considered
inhospitable due to its hot temperatures (mean 46.5 degrees C) and dry conditions, it has a thick atmosphere, there are regions on the
surface that we might expect to be suitable for life. The two gas giants just beyond 15-alpha-3 (15-alpha-4 and 15-alpha-5) are also
within the habitable zone and their satellites are possible candidates for life. We recommend a fly-pass of these systems, or launching a
remote probe, to gather further data.

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