  Theories of the formation and early evolution of planetary systems postulate
that planets are born in circumstellar disks, and undergo radial migration
during and after dissipation of the dust and gas disk from which they formed.
The precise ages of meteorites indicate that planetesimals - the building
blocks of planets - are produced within the first million years of a star's
life. A prominent question is: how early can one find fully formed planets like
those frequently detected on short orbital periods around mature stars? Some
theories suggest the in situ formation of planets close to their host stars is
unlikely and the existence of such planets is evidence for large scale
migration. Other theories posit that planet assembly at small orbital
separations may be common. Here we report on a newly-born, transiting planet
orbiting its star every 5.4 days. The planet is 50 per cent larger than
Neptune, and its mass is less than 3.6 times Jupiter (at 99.7 per cent
confidence), with a true mass likely to be within a factor of several of
Neptune's. The 5-10 million year old star has a tenuous dust disk extending
outwards from about 2 times the Earth-Sun separation, in addition to the large
planet located at less than one-twentieth the Earth-Sun separation.
