![Hot Jupiter](https://www.astronomy.com/wp-content/uploads/sites/2/2023/02/asy-20011127-01739-orig-lg.jpg)
Many of the known extrasolar planets are “hot Jupiters,” gas giants that orbit very close to their host suns.
NASA/Greg Bacon (STScI/AVL)
Mercury is not a leftover core of a gas giant planet. Although extrasolar systems have revised the planet formation rulebook, we still think gas giants form far from their stars, about as far as Jupiter is from ours.