Comment by antognini | Hacker News Reader

Comment by antognini | Hacker News Reader

As a former dynamicist, the underlying reason for this correlation is that planetary systems with many planets are always on the edge of dynamical instability. In other words, these kinds of planetary systems with lots of planets are always packed with as many planets as they can support over any given lifetime. It then turns out that the stability criteria imply that the system cannot be any more packed than having approximately log-equal spacing. In fact, if you take a star and put planets around it with a log uniform distribution and evolve it for a while, you find that eventually the planets that remain roughly fit the Bode-Titus law [1]. (I don't think the reasons for this are entirely well understood.)

One consequence of this is that our own solar system has been around for about 5 billion years, and is therefore probably unstable on timescales of 5-50 billion years. Indeed, long-term simulations indicate that there is a few percent chance that Mercury will get kicked out of the solar system or crash into Venus or the Sun before the Sun dies in ~5 billion years [2] [3].

[1]: https://arxiv.org/abs/astro-ph/9710116

[2]: http://www.scholarpedia.org/article/Stability_of_the_solar_s....

[3]: http://adsabs.harvard.edu/abs/2009Natur.459..817L

I wonder what effects Mercury crashing into the sun would have - large solar flares? Nothing at all?

Nothing much. The Sun is most of the mass in the Solar system, by over 97%. Most of the rest of that mass is Jupiter, followed at quite a distance by Saturn. Jupiter has more than 5700 times the mass of Mercury. In other words, next to the Sun, Mercury is a teeny little speck that would barely even leave a “splash” as it roasted to a cinder in its corona. It would be the most underwhelming event you can imagine, for Sol. For Mercury it would be an exciting journey of vaporizing outer layers, being torn to little shreds, and those shreds vaporizing completely (all before it contacted anything like a “surface”).

What would be the effects of Venus crashing into the sun next?

Same shit. Basically a wisp of charged subatomic particles.

It’s like 400,000 times smaller than the sun. Worth maybe 5 billion coronal mass ejections, which are weekly events on the surface of the sun.

Figure the overall mass of venus is 100,000,000 years of CME ejecta all at once. But the sun’s gravity will override the planet’s hydrostatic forces and suck much of the planet beneath it’s own surface, in hours maybe. It wouldn’t dent the gravity well of the sun.

The atmosphere of venus would be gone before it reaches the sun. That might be the fun part. After that, the outer surface of rock on venus would melt and start to spew something like a comet’s tail, and then parts would peel away and drop into the sun like a large dark sun spot for maybe a day or so, as it gets closer. Eventually, a portion of liquified the mass would uneventfully fall into the center, and join in the sun’s general fusion reaction.

This may be disappointing, but it would be the same for all of the planets, even Jupiter. Ignoring the effects on orbital dynamics from the planets changing positions or falling past other planets, they’d just burn up. Jupiter probsbly explode, or be drawn into a rig around the sun that would eventually fall in, and burn up.

All in all, the Sun is just huge and wouldn’t be disturbed by any of it. Its gravity would tear things apart before an impact, just like the moon would be shredded if it came too close to Earth. Any sattelite has what’s called a Roche Limit, and if passes that it breaks apart from tidal stresses. That’s how you get rings around planets, and I guess a star too. Although in most cases you wouldn’t have a ring around a star, just burning fragments falling Sunward.

Dumb question, but would the asteroid belt count as a ring around the sun?

Never thought about it before until I read your comment, so I thought someone more knowledgeable than me could provide some insight here.

Everything in the Solar system is gravitationally bound by the Sun, which is what makes it part of the system in the first place. In that sense, yes, and really the whole Solar system is too. There’s some nuance lost in that formulation though, because unlike a moon that came too close to its planet, broke apart and became a ring, the asteroid belt is much more distant and diffuse. It’s also true that Jupiter sort of “shepherds” the asteroid belt, so unlike something like planetary rings, it’s a much more complex system. I hope that goes some way to answering your question, which wasn’t dumb at all.

Just wanted to chime in and say thank you. HN needs more comments like this.

I'd think the main effect would be that the orbits of the remaining planets become unstable, haven't checked that though.

97% is quite an understatement, too. The Sun makes up about 99.8% of the total mass of the solar system.

Is this with the dark matter or without?

Both. There isn't much dark matter in our solar system (estimated at 10^-18 solar masses), so well within rounding errors.

I doubt there would be much of an effect on anything. Something like that is unlikely to interact much with the Sun's magnetic fields (so it wouldn't cause any solar flares). It would probably produce a relatively large sunspot. Comet Shoemaker-Levy produced observable spots on Jupiter and the relative mass of Mercury to the Sun is much larger than Shoemaker-Levy to Jupiter (and the relative diameter is much larger as well). I bet that there would be a measurable increase in the metallicity immediately after impact. This would last probably about an hour before convection caused the debris from the planet to be spread evenly throughout the Sun's convection zone (at which point it would be unobservable).

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