The back yard is empty

Contributed by
Oct 3, 2008
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With the recent discoveries of near-planet-sized objects out beyond Neptune, it makes you wonder just how much junk is floating around out there.

Usually, when you see some big stuff, it means there's probably lots of little stuff. For example, asteroids collide and shatter. You get a few big pieces, but lots and lots of little shrapnel. Also, little guys can stick together to make bigger guys, and it takes a lot of little ones to make a big one. So if we see a handful of big objects, we kindof expect to see lots of smaller ones.

So a good thing to wonder is, how many little iceballs are wandering remote space outside Neptune's orbit?

The cool thing about astronomers -- and there're lots of cool things to choose from, because we are so very, very cool -- is that when we wonder about something, we can go and take a look to check it out.

That's just what a team of astronomers did. They created TAOS: the Taiwan-America Occultation Survey to look for Kuiper-Belt Objects (KBOs), those chunks of ice orbiting in the distant solar system.

Diagram of KBO occultations from TAOS
Diagram of how an iceball can be seen, courtesy TAOS.

The concept is actually pretty simple. Take a telescope, point it at a likely patch of sky, and start taking pictures. When a KBO passes between us and a star, we'll see the star dim for a short time, then brighten again. By making some assumptions (how many stars are in the image, the distribution of KBOs, how quickly they move, etc.) you can figure out how many KBOs are out there by measuring how many times the stars dim and brighten. You can even tell how big they are, too; TAOS is sensitive to KBOs from 3 - 28 km (2 - 17 miles) across.

What they found is rather surprising: after 200 hours of observing, they didn't see a single event! Not one star appears to have been occulted (blocked) by a KBO. This means there are much fewer of them out there in that size range than previously thought.

Why? It's too early to say. Maybe the small ones at that distance have all stuck together to form bigger objects. Maybe the small ones are sensitive to some force or event than bigger ones, making them preferentially go away (dropped into the inner solar system, or flung farther out than the survey can find them). Right now, all we know is that our back yard isn't as crowded as we first thought. The thing to do now is to make longer observations, more sensitive observations, expanding the survey and see over what total size range and what distance range this emptiness seems to hold.

I'm not sure myself what implications this has. I've often wondered if there is another more massive planet out in the distant solar system -- very far out, like 5 times Neptune's distance from the Sun. Nothing we've seen seems to preclude that, but we have no evidence for it either. Studies like TAOS help; they provide pieces of the puzzle that we need to put together so that we better understand our own neighborhood.