Every year, the Jet Propulsion Laboratory in Pasadena, Calif., holds Open House weekend, the only time the public is free to come in and wander around.

We JPL-ers brace for it. Usually, about 50,000 people show up, and it never ceases to amaze me how interested and cheerful everyone seems, even after having to wait much too long for parking. I spend most of my time at the Ask a Scientist booth, where four or five bonafide scientists and engineers sit ready to answer any question a member of the public would like to ask. We get all sorts, from conspiracy buffs who accuse us of hiding alien spaceships, to young children curious about how astronauts use the bathroom in space. One man even who stood there seriously talking to me for more than an hour as I tried to explain string theory.

Of course, none of us can answer all the questions. We are all assured ahead of time that 'I dont know' is a legitimate answer, hopefully followed by some suggestions as to where the information might be obtained. So I wasn't feeling too bad when a grade-school age girl asked us how many moons the planet Saturn had.

Honestly, I had no idea. I knew it was a lot, but with the Cassini spacecraft cruising around the Saturn system, making new discoveries as we spoke, I figured the number was probably subject to change anyway. Then something wonderful happened. The scientist sitting next to me at the booth picked up her cellphone and called a friend of hers who happened to be on the Cassini team. Answer: for the moment, the official count was 47 moons. The team was still arguing about three other objects, which they weren't sure were big enough to qualify as moons.

This simple exchange reminded me how much I love astronomy for how changeable everything is. I'm only in my mid-30s, but things I learned in graduate school, or even taught my own students, are now woefully out of date. With that in mind, I thought I would provide a bit of an astronomy up-date, especially for a few subjects on the cutting edge of discovery. There might be some good trivia questions in here, but beware: chances are the answers will change in a month or so.

One question I like to pose to the public is, how many planets are there? Most people guess nine, as that's the official count in our own solar system (more about that later). But what about planets going around other stars? How many of them do we know about?

As of now, and yes, I did just check this moment, we know of 154 extrasolar planets. Thirty-nine of those planets are orbiting stars you can see with your naked eye. We haven't just found solitary planets, either; we know of 97 other planetary systems with more than one planet orbiting the star. Some of the planets even orbit binary stars, giving their solar systems two suns instead of one. One planet was even found in a triple-star system. For the latest on extra-solar planets, give the Planetquest website a regular check at http://planetquest.jpl.nasa.gov.

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