47 moons and counting: a space update

So, planets. Let's get back to the ones we're most familiar with. How many planets are there in our solar system? Yes, historically there are nine planets, with Pluto being the farthest from the sun, at least during most of its orbit. And yes, there may be good reason not to call Pluto a planet, as its off-center orbit, low mass and icy composition make it much more like an icy body than a planet.

We've found other large icy things out there, like Quaoar, which is much like Pluto (about half the size), only a bit farther out from the sun (a billion miles farther than Pluto). Currently, the farthest planet-like object we know of in our own system is Sedna. At the moment, Sedna is 8 billion miles away from the sun, and that's about the closest its ever going to get. Sedna has a weird, elongated orbit that takes it way out into space, about 84 billion miles at its farthest point, and takes about 10,000 years to make one pass around the sun. We don't have a great measurement of Sedna's size yet, but we know its bigger than Quaoar, probably about three-quarters the size of Pluto.

Since we're talking about things that are very far away, why not take it to the limit? Another dynamically changeable subject: the farthest object we can see. The farthest object you can see with the naked eye is the Andromeda Galaxy, which looks like a dim smudge just above the middle of the V shape that makes up the constellation Andromeda (assuming you can get to a very dark place). That smudge is really the combined light of about 100 billion stars shining over 2 million light years away (one light year is 5,865,696,000,000 miles).

But hey, we've got much better things than the human eye to observe with. What's the most distant object that our giant, painfully sensitive telescopes can see? For this question, I couldn't find a single, exact answer. There are several objects that are about 13 billion light years away, but there are some decent errors associated with estimating the distance to something that far away. At the moment, there are a couple of young, bright galaxies vying to be the farthest observable object. Sometimes these galaxies are called quasars, although again, astronomers are arguing about definitions.

A quasar is the unimaginably bright core of a young galaxy (remember, when you look billions of light years away, you're also looking billions of years into the past), probably powered by huge amounts of matter getting pulled down a massive black hole. Quasars are hundreds of times as bright as our Milky Way Galaxy, which, simply put, is why we can see them from really far away. Here, as you might guess, the time lag is astonishing. When we look at these distant quasars, were seeing the universe as it looked 13 billions years ago, or when the universe was only about half a billion years old.

If quasars are lit up by vast amounts of matter streaming in toward a massive black hole, how massive are we talking about here? We're not sure about what's lurking in the heart of the brightest quasars, but in nearby active galaxies (galaxies that almost certainly were quasars when they were younger), we have observations of stars whipping around central black holes of up to 3 billion times the mass of our sun. A black hole gains mass by swallowing material, so you guessed it, these monsters have devoured the equivalent of billions of suns.

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