||"dawnbreak in the west"|
Tuesday, March 01, 2016
The stellar bubble
I think that our outer-system maps – the maps they show us hobbyists – are incomplete beyond Neptune and Pluto. There’s another border they could show and should. It's the Termination Shock.
Our inner eight planets lie within the Heliosphere, a plasma bubble breathed out by the Sun. This plasma pushes against interstellar gas, and the bubble’s inner lining – our ceiling, if you will – is the Termination Shock. Then follows a thicker plasma shell called the Heliosheath. Speedy Voyager 1 blasted into this shell at 94 AU, and then the slower Voyager 2 (having leisurely strolled past Uranus and Neptune) met it at 84 AU.
Over this difference, theories vary. First, the Sun’s relative motion through space pushes harder in one direction than another: perhaps in Voyager 2’s direction, with Voyager 1 more in our wake. Also the Sun is variable and doesn’t shine equally as bright all the time; maybe it had got calmer by when Voyager 2 got to the 84 AU mark. (Presumably back when the Sun was cooler, the shell was more consistently closer.)
Overall, “about 90ish AU” would work for the Termination Shock’s approximate radius. Now that the IBEX ribbon is contributing data along with the Voyagers, we’ve proven that the thing is out there. The next step is to map its bounds and to watch them move. With that done, I’d like this boundary put on our charts.
Reason? Because the Heliosheath is NOT the end of our solar system. For a start, Eris would appear to reside well within that shell – as of now.
One reason Eris took so long to find in the first place is that last time it was closest to us (37.9 AU) was just before 1700 AD, and for centuries later had been running away from us at, apparently, around the same rate our telescopes were improving. But at last, in 1977, the planet reached its peak, at 97.65 AU.
Our depictions of Eris tend to show it relative to Pluto and Neptune and… that’s it. Personally, I am fine with including that pair in our maps. Pluto is Neptune’s slave (2:3) and Eris isn’t; showing all three on the same map does illustrate how Newton works out there. But to add the Termination Shock would further hint at Eris’ likely surface conditions. Also, over the Sun’s existence, the star used to be cooler, so Eris has in its existence spent even more relative time in the Heliosheath than it does now.
Moving on to the very elliptical planetoids, queen of these is Sedna. Like Eris, Sedna is coming our way; Sedna was found at 89.6 AU and will be closest at 2076, 76 AU. But Sedna’s peak is MUCH further out; so it was found (luckily!) toward the end of its return to the Sun (it is not often closer than Eris, but when it is…). It would appear that Sedna has seen the inside of our bubble before, but has never spent much time here; perhaps the Carrington Event was strong enough to push the Termination Shock past it at the time. (The 775 AD Spike was probably stronger but I don’t think Sedna was close enough then to catch it.)
Lastly the big one, Planet Nine. We have thither a truly monstrous distance. 630 AU is the current best guess, about twenty times further out than Pluto now. This is so monstrous that even comparing it to Neptune’s orbit doesn’t help. But it’s “only” about seven times further out than the Termination Shock.
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