Lost Star

Seven years ago this week I was preparing one of my favorite lectures for The Formation and Evolution of Planetary Systems, a class I frequently teach at Caltech. “Preparing” is probably the wrong word here, because this lecture, called The Edge of the Solar System, was one I could give even if instantly wakened from a cold deep sleep and immediately put on stage with bright lights in my eyes and an audience of thousands and no coffee anywhere in sight. The lecture explored what was known about the edge of our main planetary system and the ragged belt of debris called the Kuiper belt that quickly faded to empty space not that much beyond Neptune. Conveniently, one of my most active areas of research at that time was trying to figure out precisely why this ragged belt of debris had such an edge to it and why there appeared to be nothing at all beyond that edge. I could wing it. So instead of preparing the lecture, I really spent that morning doing what I did whenever I had a few spare moments: staring at dozens of little postage-stamp cutouts of pictures of the sky that my telescope had taken the night before and my computer had flagged as potentially interesting. Interesting, to my computer, and to me, meant that in the middle of the postage stamp was something that was moving across the sky at the right rate to mark it as part of the Kuiper belt. I was not just lecturing about this debris at the edge of the solar system, I was looking for more of it, too.

Ever since the Voyager 2 data confirmed the nonsymmetrical shape of the solar system scientists have pondered its cause (i). In summary, the edge of the heliosphere (the place where the solar wind slows to sub sonic speeds) appears to be 1.2 billion kilometers shorter on the south side of the solar system (and in the general direction of the winter solstice, the direction of Voyager 2), than it is on the edge of the planetary plane (where Voyager 1 exited approximately a year earlier). This indicates the heliosphere is not a sphere at all but a bullet shape. More data is required to determine the exact shape in all directions.

The initial explanation was there must be some sort of gas cloud pressing against one side of our solar system. While this hypothesis is plausible there is another possibility that deserves consideration; stellar wind.

The sun’s solar winds are primarily driven by its magnetic field. When magnetic storms arise on the sun it produces coronal mass ejections (CME’s), which are like waves or ripples on the solar wind. The solar wind is constantly pushing on the daylight side of the earth’s magnetosphere squashing it in a pattern similar to the way the sun’s magnetic field seems to be squashed where Voyager 2 exited the solar system. Thus it is possible that the dented solar system might be due to the same type of cause; stellar winds from a not too distant star.

Some indication of this might reside in the data recently received by NASA’s sun-focused STEREO spacecraft. The twin STEREO spacecraft were launched in 2006 into earth’s orbit about the sun to obtain stereo pictures of the sun’s surface and to measure magnetic fields and ion fluxes associated with solar explosions. Between June and October 2007, the STEREO spacecraft detected atoms “originating from the same spot in the sky: the shock front and the heliosheath beyond, where the sun plunges through the interstellar medium”, and found “energetic neutral particles from beyond the heliosphere” that are moving toward the sun (ii). While this might be due to other causes such as “charge exchange between hot ions and neutral atoms” as hypothesized by scientists at UC Berkeley, it may also indicate the source of the asymmetrical solar system is due to the stellar wind from another star rather than an interstellar gas cloud. More data is needed and should be forthcoming with the pending launch of the Interstellar Boundary Explorer (IBEX), due to begin receiving data some time in the next year.

(i) Science Daily, Voyager 2 Proves Solar System is Squashed, December 13, 2007
(ii) E Science News, First Images of Solar System’s Invisible Frontier, July 2, 2008

“There is something out there” by Mike Brown • Part 1

Asymmetrical Shape of Heliosphere Raises Questions