There is a near-consensus in the scientific community that large, Jupiter-like planets can only form in systems with only one star (like our Solar System). When there are two or more stars in a system (binary stars, for example), the planetary orbits can get very complicated. The debris material that coalesces to form planets do not form large planets during computer simulations of such systems.
However, scientists will soon be forced to reconsider their theories:). A new planet has been discovered orbiting around a star in a triple-star system in the constellation Cygnus. The planet is a so-called hot Jupiter but it is much closer to its parent star than predicted by current theories of planetary formation.

Through the Artist's Eye: Trinary Sunset (Courtesy: CaltechIn recent years, astronomers have discovered hundreds (as of last month, the count is 155) of planets. Most of these planets are in single-star systems and are large, as the current technology can only detect larger planets with relative ease. However, we know that 60% of the stars in the universe form binary or trinary systems! So the big open question is: are there planets in those systems? The answer seems to be yes:).
The new planet (size similar to Jupiter's) orbits the main star of a triple-star system called HD-188753 every 3.35 days. Two of the stars form a binary system that orbits around the third primary star at an average distance of 12.3 astronomical units, where one astronomical unit (AU) is the average distance between the Sun and the Earth. The planet orbits the primary star at a distance of just 0.05 AU!!
The puzzle: why is the planet so close to the star? Current planetary theories suggest that such a large planet should not be able to form so close to a star in a multi-star system. So a (perhaps major) reworking of the theories might be in order.