Change of orbit with change of mass?

In the science fiction book 2010, monoliths increase the mass of Jupiter until it becomes a star.

https://en.wikipedia.org/wiki/2010:_Odyssey_Two

The Leonov crew flees Jupiter as a mysterious dark spot appears on Jupiter and begins to grow. HAL's telescope observations reveal that the "Great Black Spot" is, in fact, a vast population of monoliths, increasing at an exponential rate, which appear to be eating the planet. By acting as self-replicating machines, these monoliths increase Jupiter's density until the planet achieves nuclear fusion, becoming a small star.

Obviously fiction. But my question is about orbits and the mass of orbiting bodies. For planets, whose mass is negligible compared to their star, I understand one can ignore mass for purposes of calculating orbits.

But if Jupiter were by scifi magic made as massive as a star, would its orbit remain the same regardless? Or if it were made to stay on its same orbital path, would it have to move more quickly, completing its orbit of Sol in a shorter time?

• According to the answer to my question... Apparently the monoliths you mention would need to be as massive as 12 Jupiters to begin any sort of fusion towards being a star as Deutrium fusion begins at 13m_jup. So basically you'd be dropping Jupiters into Jupiter to do this, given those could be hyper dense, they'd still be massive. – Magic Octopus Urn Aug 30 '19 at 17:32

What you said :

monoliths increase the mass of Jupiter

What you quoted :

these monoliths increase Jupiter's density

Increasing density is not the same as increasing mass.

By squashing stuff together you can increase it's density without affecting it's mass at all.

But if Jupiter were by scifi magic made as massive as a star, would its orbit remain the same regardless?

No, nor would any body in the solar system, including the Sun, remain undisturbed. It would be a massively complex change.

• This is a nonanswer, StephenG. I am not going to downvote because you wrere interested enough to answer and maybe you can fix it. The ? is not about the magic monoliths in 2010 but about the orbit of Jupiter and how it would change. If you know, then tell. If not, consider waiting for someone who does. – Willk Aug 30 '19 at 19:35
• @Willk please remember our "be nice" rule and stop being rude to people giving you good answers. Stephen - you cover off the relevant points well. – Rory Alsop Aug 30 '19 at 22:54
• @Willk Your question spends three paragraphs covering the basis of your question : the 2010 story. I have explained that you have confused an increase in density with an increase in mass. The details of what would happen to the orbit of Jupiter, the Sun and the other planets is way too complex. Some, none or all gas giants could merge (eventually). The minor planets would end up I have no idea where - they're like dust mites in this scenario. This is a chaotic system. I doubt anyone could work out if they end up in a stable new configuration (eventually). – StephenG Aug 31 '19 at 1:19
• @Willk And because it's chaotic, it's extremely sensitive to initial conditions. So even if someone were to run this magical mass increase scenario on a Newtonian gravity simulator, the final configuration of the Sun & Jupiter will vary chaotically depending on the rate of mass increase and the final mass. What'd happen to the other planets etc will depend on their exact locations when the mass magic occurs. BTW, it would not be possible to investigate this scenario with a general relativity simulator, due to the magical creation of mass. – PM 2Ring Aug 31 '19 at 4:55
• I apologize for being rude. I was hoping for math and I should have put some sort of tag specifying that. Thanks for laying it out further in the comment. – Willk Aug 31 '19 at 15:17

Ignoring the breaking of the laws of physics by creating mass aspect of the question, the mathematics of orbits is often simplified where the orbiting object is regarded as weightless and the orbited object's mass is all that's used.

This works well enough when the masses are very different, but it's not accurate. It's a short-cut. In reality, the mass of both objects needs to be taken into account and the math is somewhat more difficult.

Wikipedia covers this here

The more interesting change, when you add mass to Jupiter is that as it grows larger, the other planets orbits destabilize as they are influenced by two bodies and ultimately will likely get tossed out of the solar system or possibly into the sun or into Jupiter. It wouldn't be good for us here on Earth if this was done.