Impacts of a stellar Jupiter

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.

posted over 9 years ago

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