February 2011 astronomers announced the discovery of the K-11 star,
which is widely regarded as the most important discovery since the
first exoplanet, 51 Pegasi b, in 1995.
Of the 519 extra-solar planets so far found, most are gas giants.
A few rocky planets have been found, although much larger than our
Earth. Most stars so far found have only one planet orbiting them.
The K-11 star is 2,000 light
years away, is about the size of our sun but is 6 to 10 billion
years older. But the most unusual thing about it is that it has
6 planets orbiting it. They are much more closely packed than any
other known exo-planet system. 5 of the six planets orbit closer
than Mercury is to our sun and much faster, completing an orbit
between 10 and 47 days ( Mercury orbits every 88 days ).
According to the current theory
of planet formation by accretion, these planets can not have formed
where they are now. Grouped so closely together their gravity should
have interfered with the accretion process. They should have ripped
each other apart at the outset.
In other words the K-11 system
may be full of migrant worlds that have changed their orbits.