It is often said that Jupiter is a failed star – it is regularly mentioned that with a little extra material Jupiter may have started fusing and shining like a star. So what would happen if this occurred? The smallest star we know of is OGLE-TR-122b. This star is around 20% bigger than Jupiter but has around 96 times more mass. So what would happen if we replaced Jupiter with OGLE-TR-122b? Well, the first thing to consider would be how much brighter our skies might get. The answer to that is, not much. OGLE-TR-122b has a luminosity that is equivalent to 0.005% that of the Sun’s. This isn’t going to light up the sky much more than Jupiter already does.
The brightness isn’t necessarily the major issue, though. The current theory surrounding the formation of the Solar System features Jupiter quite heavily. It is believed that Uranus and Neptune couldn’t form at the distance from the Sun they are at. To explain their current location it is required that these planets migrate. This migration was heavily reliant on the relative masses and positions of all four of the gas giants. If we replaced Jupiter with OGLE-TR-122b this change in mass would have a major effect on this migration process. This could result in the gas giant planets being in different positions, or even one of them being kicked out of the Solar System completely.