The phrase 'Hot Jupiter' was first suggested in 1995 and covers very large (.i.e. Jupiter-sized) planets that are very close to the star which they orbit. With a typical orbit time of just a few days.

Their existence had been predicted in the 1950s - though at the time there was no way to reliably detect them. In the 1990s astronomers Michel Mayor and Didier Queloz won a Nobel Prize for discovering the first examples. They also came up with the name.

New space-based planet-scanning technology has already found many hundreds of such planets.

They are a cosmological puzzle, because they are so close to the host star that its intense radiated heat would presumably have 'evaporated' their structure many millions of years ago. Or have prevented their formation in the first place.

There are currently three theories about their formation ;

[1] They formed far away from the parent star and then somehow their orbit progressively became smaller (ref.)

[2] They managed to form in situ without getting evaporated (ref.)

[3] The 'core' formed in a remote orbit and then migrated inwards - subsequently accreting more material to form a Jupiter-sized planet. (ref. as above)

To date, none of the explanations has been fully accepted.

Note that the question of how any planets form has still not been fully agreed - see Planet formationplugin-autotooltip__plain plugin-autotooltip_bigPlanet formation

"The origin of planets is a vast, complex, and still quite mysterious subject. Despite decades of space exploration, ground-based observations, and detailed analyses of meteorites and cometary grains (the only space samples available in our laboratories),