Thursday, January 31, 2008

Pluto And Beyond #3: The Golden Age of Outer Solar System Studies

Executing a photographic survey of the ecliptic searching for unknown planets (or other outer system objects) until very recently was less than conclusive because it took so much time create the exposures and work around the Zodiac that it was always conceivable that an object would be slowly moving in just the right fashion to move out of an area just before that area was imaged. Now, however, ‘teams’ of robot telescopes and advanced scanning software can image the Zodiac, or, in fact, the whole sky, so quickly that it will be very difficult to find reasons potential targets might avoid detection.

However, the outer system in some ways is like Loch Ness—it captures the imagination of people, even professional astronomers, and they tend to want to see things out there . . .

And the remarkable thing about the outer system is that professional astronomers are discovering that there is vastly more to see out there than anybody ever suspected.


When I first became interested in astronomy, the outer system was typically thought of as barren. The outer system, then, consisted of the asteroid belt, the gas giants, Pluto, and a rag-tag population of comets. That was about it.

Then, as I understand the timeline, dynamic studies of comet populations that modeled solar system behavior by simulating the last few billions of years made it clear that at the observed rate of comet destruction, in order for us to see as many comets as we do today, there must be a vast reservoir of comets out there at the fringes of the solar system. Those studies created a revival of the Oort Cloud structure surrounding the solar system.

The Oort Cloud structure has proven a useful theory for explaining comets and now several other structures have been proposed for the outer system.

In addition to the classic asteroid belt between Mars and Jupiter, there is now a population of centaur planetoids known to orbit between Jupiter and Neptune.

There is even a multi-part population of trans-Neptunian objects that includes the Kuiper belt, the scattered disk, and other objects out to the Oort cloud.

There’s a hell of a lot of stuff out there and the way that stuff groups itself (and comes ungrouped) is making the outer system seem far more dynamic than the inner system.


Beyond new objects and their behaviors, even some of the classic outer system objects have proven full of surprises.

The rings of Saturn turned out to be vastly more complex than anyone suspected. And of course now we know Jupiter, Uranus and Neptune also have ring systems.

The axial alignment of Uranus is tipped almost parallel to the ecliptic. Something caused that.

The magnetic field of Uranus isn't aligned with either the planet’s rotation or the planet’s center. Nobody knows what that means or what that implies for our understanding of planetary magnetic fields in general.

The moons of Neptune are bizarre. Triton, for instance, rotates around Neptune in the opposite direction of the planet’s spin. And both Triton and Nereid rotate not around Neptune’s equator but in very inclined orbits. Something caused that, too.

Infrared observations of our galaxy have revealed vastly more brown dwarf stars than anyone had suspected. There always has been speculation that Jupiter may be a failed star. As astronomers gather more data about brown dwarf populations, if brown dwarfs are found with very low mass, speculation about Jupiter’s exact nature may become more, so to speak, heated.

Sophisticated computer software and advanced hardware make it possible to simulate all manner of oddball orbits for suspected objects beyond Pluto. Large planets that orbit far off the ecliptic nonetheless can have stable orbits which interact with trans-Neptunian objects only at extended time scales. This has been suggested as a mechanism for periodic extinctions on Earth, as TNO’s may get periodically perturbed and sent toward the inner system for possible collision with the Earth. Modern robotic telescopes and scanning software will soon be able to conduct full-sky surveys that search for planets not just near the ecliptic by everywhere in near-Sun space.

Possibly the most intriguing change to thinking about the outer system—well, to me, I guess—has been the discovery that the distinction between asteroids and comets is not as clear cut as it was thought to be. Some centaur objects have been observed to display asteroid characteristics during part of their orbit and comet characteristics during other parts of their orbit. The solar system has been around for billions of years. Comets are volatile. They not only crash into the Sun or other bodies, but they sometimes simply break up. In order to explain how a large population of comets can still exist after billions of years, astronomers were forced to imagine the Oort Cloud structure surrounding the solar system. But how can a volatile sub-set of asteroids still exist after billions of years? That’s a good question.

And one final speculative aspect of finding all these things, all these structures, all these behaviors in the outer system, raises a very intriguing question about the nature of solar systems in general. For generations solar systems have been thought of as something like islands or archipelagos in space, separated from one another by vast seas of interstellar space where distances are measured in light years. It is always possible that this was a quaint, anthropomorphic idea to begin with. It is always possible that matter of some kind and structures of some kind extend throughout interstellar space itself and link stars together, possibly with plasma dynamics of one kind or another making the galaxy itself one holistic structure of structures.


What makes this a real golden age isn’t just our awareness of the outer system but also the fact that for the first time in history astronomers have tools that can effectively probe the outer system.

We have space probes actually traveling through the outer system and sending back data.

We have space-based telescopes imaging the outer system without atmospheric degradation.

Adaptive optics make it possible for ground-based telescopes to see almost as well as space based telescopes.

Multiple-mirror telescopes combined with adaptive optics make it possible for ground-based telescopes to see better than space-based telescopes.

Advanced software and hardware make it possible to theoretically simulate any imaginable theory about objects and behaviors in the outer system.

I don’t often envy young people. Life is complicated and young people have to deal with life’s complications without the benefit of experience. However, when I think about the outer solar system, I really do wish I were a young person in college right now.

There’s a lot to find out about what’s going on in the outer system. There’s a lot to explain about what’s going on in the outer system. I think it would be great fun to be one of the people doing the finding out and explaining.

1 comment:

metapsyche said...

Exoplanetology, baby!