Yes yes I know that jupiter is too small to be a Star even a small one. (As an aside the minimum mass needed is what 0.08 solar masses? Jupiter is about half that IIRC().
But lets say it turns out that like the Frisch-Peierls memorandum the original calculations required too much more material than what was later the case (before aforementioned memo, the amount thought needed for critical mass was several hundred tonnes of U235). It turns out that Jupiter does have enough material to ignite and it does.
What happens to Earth? Star!Jupiter, will it remain at the approximately the same distance from Earth as Jupiter is (average of about a billion km). How will Star!Jupiter affect the other planets orbit. Effects on life on earth?
Well for a start there will be no effect on other planets’ orbits - Jupiter’s mass is staying unchanged in this hypothetical and gravity is solely dependent on that. The mass loss through fusion is insignificant as a proportion of the whole - the Sun’s good for ten billion years and a Jupiter-sized star would be burning even slower.
In this hypothetical, I’m assuming Jupiter’s putting out less than one percent of the Sun’s radiation and it’s a good deal more distant, so while our night skies are getting appreciably brighter for some part of each year the effect on Earth’s temperature balance is likewise negligible - the insolation we receive is already variable by more than one percent over the course of the year thanks to the eccentricity of the Earth’s orbit.
tldr; We get an interesting object close by, our lives aren’t affected very much, several of Jupiter’s satellites are now in their own Goldilocks zone but we don’t fuck about with Europa.
We can’t say anything about how bright or hot it would be, since our current physics say it can’t happen at all. Our current physics might, of course, be wrong, but if that’s the case, then we don’t know what’s right.
The sensible assumption to make, though, would involve extrapolating from known facts about brown dwarfs - which would make star!Jupiter a marginally luminous object with most of its radiation in the infrared, and even that being perhaps a millionth of the Sun’s output.
As I understand it, when a star ignites is fusion reaction due to pressure, the outer layers blow away, forming a planetary nebula. Just like the sun did, the nebula forming planets. So if Jupiter were to be brought to just barely past mass for ignition, it would lose much of its mass, and the stop undergoing fusion. SO again, according to how we understand physics, Jupiter can’t become a brown dwarf, or mini-star.
I have no idea how the sudden loss of mass would affect the balance of Earth’s orbit. Or what material would hit the earth, or when.
What you may want to know is, what if a cosmic entity quickly swapped Jupiter with a miniature star. But that’s just a what happens when there’s a little light in the sky question.
No, it’s pretty much impossible all the way around. Nuclear fusion is a function of temperature and density. Gravity is the only way to increase those for a planet/sun-sized object - as it gets bigger, it becomes denser, and there is a resulting increase in temperature. At Jupiter’s size, there isn’t enough density or heat for fusion. Heck, even at the sun’s size, fusion occurs only in a small percentage of your mass at the very center.
If you did ignite Jupiter in some way, the extra heat would cause the planet to expand, further lowering density and acting to offset heat input. Hot enough and you’d probably break up the whole planet; at the very least, you’d make it increasingly hard to maintain fusion. It wouldn’t be stable.
The only way to create stable fusion in Jupiter without changing its mass would be to put it in the universe’s biggest compression chamber. Once you posit something like that, the energy output can be virtually as high as you want it - if you want more fusion, you just squeeze harder.
Even if Jupiter’s output were equal to our own sun’s, it would be a pretty small effect on the Earth. Radiation drops off by the square of the change in distance. At it’s closest, then, a sun-like Jupiter would only provide 6% of the energy we get the from the sun now and only about 2% at it’s farthest.
A continuous 2-6% increase in solar energy hitting the earth seems pretty big to me. I don’t know how much increased solar energy retention we have due to increased CO2 (anyone have a number?), but I suspect it’s significantly smaller than this.
Yes, but that’s assuming that Jupiter vanishes and is magically replaced by a duplicate of the Sun. Hey, why not a star 10 times brighter than the sun if we’re playing cosmic engineer?
If instead we imagine that Jupiter were much more massive, but only add enough mass to barely allow fusion–Jupiter is a brown dwarf–then nothing really happens except we notice that the amount of IR coming off Jupiter is a lot higher. That might have a significant effect on MegaJupiter’s satellites, in fact all the satellites would have to be repositioned to keep orbiting MegaJupiter in circular orbits.
Even if we bump Jupiter up to red dwarf status we won’t see much effect on Earth except that Jupiter is a much much brighter light in the sky.