# Impacts of a stellar Jupiter

In the book 2010: Odyssey Two by Arthur C. Clarke, Jupiter is turned into a star. Presumably this is done by making it more dense, so if Jupiter were to reach that density and become a star, what would be the impacts on the Solar System?

• FYI: astronomy.stackexchange.com/questions/6052/…. This is a) better suited-for Astronomy and b) possibly off-topic here. But I'll write an answer here if it's decided that this is on-topic. – HDE 226868 Oct 19 '14 at 17:54
• It's not off topic here, however I agree astronomy might be the better place to ask the first part of the question so it's up to the OP if he wants to delete this one and ask it there or ask here. The second part of the question is definitely a better fit for this site. – Tim B Oct 19 '14 at 18:25
• @TimB My comment was partly that the question had already been answered at Astronomy - i.e. it would be almost exactly a duplicate. – HDE 226868 Oct 19 '14 at 18:54
• @HDE226868 Ahh, my bad. Should have followed the link :) – Tim B Oct 19 '14 at 19:43
• @HDE226868 I have read your excellent answer on the astronomy site, but I am curious why there is no metion of a possible disruption to planetary orbits. My physics is not strong, but naively one imagines a ball of gas collapsing to form a star and initiate nuclear fission/fusion would create a considerable shock wave (sound waves, "gravity waves" if there are such things, intense radiation across the spectrum). Could this not disrupt the orbits of (at least) nearby planets, thus changing all orbits over the longer term. Jupiter's moons, if they we not consumed, would surely be blasted. – Nick Oct 19 '14 at 20:26

The impact would vary greatly depending on where you are in the Solar system.

Firstly, assuming the actual mass of Jupiter has not changed then it will still hold its current place in orbit, although there may be some odd effects while it is first collapsing.

As the calculations here show the actual impact on earth would be minor, some nights would be a little brighter. There would be a tiny bit more heat.

The effect on the other planets would also be small, they would get more heat and light a they passed Jupiter in their orbital paths but then that effect would fade as they moved further apart.

The place where you are going to see a change is the moons of Jupiter. Assuming they survived the process that ignited the gas giant they would immediately have a large sun immediately above them in the sky. This sun could easily be so bright it turned them all to cinders, however if the brightness was tuned appropriately it could actually move some or all of the moons into the habitable zone.

You can expect to see the frozen surface of Europa melting, potentially even forming an atmosphere. The other moons too would greatly increase in temperature, and the possibility of them getting to the correct temperature for life as we know it cannot be ignored.

Io, Europa, Ganymede and Callisto are all sizable moons. Ganymede as the largest has twice the mass of our own moon and would be a promising place for life to develop if it was able to hold onto an atmosphere.

One interesting point though is that all of these moons are tidally locked, if Jupiter turned into a star they would all always have one side pointed towards the new star. That would make for some very odd weather patterns on the surface as discussed in some other questions on this site.

• Essentially, if we assume no mass change, than Stellar Jupiter would not affect orbits of anything. Density is not a component of Gravity. For another example, the orbital effects on the Earth... and everything really in the solar system... if the Sun became a Black Hole would be non-existent. If we aren't in the process of falling into the Sun as a Star, then we won't fall into it as a Black Hole. The mass is still the same, but just compacted into a smaller space (in the case of the sun, it would be a Black Hole the size of Quarter). – hszmv Aug 9 '18 at 19:25

## Physical properties

If Jupiter turns into a star, it would likely turn into the least massive star possible. This would make it a red dwarf - in particular, a red dwarf of spectral type M9V. We can infer a few key properties from this:

• Mass: $0.79M_{\odot}$ ($82.8M_J$)
• Luminosity: $2.69\times10^{-4}L_{\odot}$
• Surface temperature: $2400\text{ K}$
• Radius: $0.095R_{\odot}$ ($0.924R_J$)

In other words, Jupiter would get much more massive but a little bit smaller. It would be fairly dim in comparison to the Sun.

## Jupiter's moons

What would be affected first? I'd put its moons on the table. By Kepler's third law, the period of an orbiting body is $$P=\sqrt{\frac{a^3}{M}}$$ where $a$ is its semi-major axis and $M$ is the mass of the body it orbits. Since Jupiter has increased dramatically in mass, the periods of it satellites will drastically decrease, and they'll speed up quite a lot - both by a factor of about 9.

So the moons would probably be orbiting at a much quicker rate. But what else? Well, there's always the Roche limit to contend with. Inside the Roche limit, a satellite will be torn apart. Now, $R\propto M^{1/3}$, where $R$ is the Roche limit and $M$ is the mass of Jupiter. Therefore, the Roche limit would increase by a factor of 4.36. This would tear apart many of Jupiter's inner moons, likely forming a more dramatic ring system (Jupiter already has a ring, but it's not very massive).

## The other planets

Let's go even further out. Would the other planets be affected? Technically, yes. I'd worry the most about Saturn. At its closest, it's $4.4 \text { AU}$ away from Jupiter. Is that an issue? I'd say yes; if Jupiter became a star when it was this close to Saturn, there's a chance Saturn could be gravitationally captured. On the other hand, if it became a star when the two were furthest apart ($15 \text { AU}$), there wouldn't be any trouble - at first. But once Saturn caught up to Jupiter in its orbit, it could get caught.

The other planets, too could be affected. I don't know just how many (if any) others would get captured by Jupiter; it depends on how close (or far) they are from the Sun. But the solar system would certainly be an interesting place. At least, more interesting than it already is.

## Habitability

This new Jupiter would not change the Sun's habitable zone much, nor would it create a substantially large habitable zone of its own. Keep in mind that M-dwarfs are really dim, and this particular dwarf would give off a fraction of a percent of the energy that the Sun does. To give an example of just how drastic this is, here's a model (code here) of a binary system of a Sun-like star and an M5V red dwarf (slightly hotter than our new stellar Jupiter):

The separation is 5 AU, and the small dot in the upper half is the habitable zone around the dim star. The stellar Jupiter's habitable zone will be even smaller, and won't affect the rest of the Solar System, although some of its moons may be habitable - at least, the ones that haven't been torn apart by tidal forces.

• You have to figure that all the outer planets past Jupiter are lost as normal planets. Sure, the new star's mass is a 10th of the suns, but it is between those planets and the Sun. Even the inner planets might be seriously affected. – Oldcat Nov 18 '14 at 23:30
• @Oldcat That's only applicable if those outer planets were behind Jupiter when it became a star. They could just as easily have been on the other side of the Sun. – HDE 226868 Nov 18 '14 at 23:43
• Then it will just get them at the next opposition... – Oldcat Nov 18 '14 at 23:45
• @Oldcat I disagree. Jupiter would migrate inwards, since it is a lot more massive. Besides, it's pretty far away from all the other planets, even Mars. – HDE 226868 Nov 18 '14 at 23:47
• The perturbing factor is still 100x what it was when Jupie was 1/1000 solar masses rather than a 1/10. I was assuming that this new star would keep the same orbit as Jupiter now as part of this magic operation. – Oldcat Nov 18 '14 at 23:53