One thing that's currently unknown— and there remain debates among the theorists and modellers— is how long it takes for planets to form. Timescales vary between 1 and 10 million years. What's more, based on our understanding of the environments around very young stars, big gas giant planets have to form a fair distance away from the star (equivalent to where you find Jupiter and Saturn in our Solar System), yet we've found lots of Jupiter-sized planets very close to their star... so, our current understanding is that those planets not only have to form, but have to migrate in.
One thing that would help us figure out the timescale for planet formation is doing searches for planets around very young stars. Below is a poster that reports the discovery of the youngest exoplanet known:
Shown is graduate student Chris Crockett of Lowell Observatory. (And, if his advisor is looking at this, no, that's not a beer!.) (Although, to be perfectly honest, I have to admit that the reason I sought out this particular poster is that the second author is Naved Mahmud, graduate student at Rice who worked with me at Vanderbilt, and who made one of the nicest comments a blogger could ever hope to get, so, see, flattery sometimes does get you somewhere!)
What they've discovered is a planet orbiting a Classical T-Tauri star. If you don't know what that means, you're like me... or, rather, you're like me several years ago. However, I was at Vanderbilt for six years, where David Weintraub and his students were doing a lot of work on T-Tauri stars, so as a result I learned something about them. T-Tauri stars are stars like the Sun, only still in the late states of formation. They're not protostars, really, any more, but they haven't yet ignited fusion at their cores. They are what's known as "Pre-Main Sequence" stars. (Yes, folks, that's what astronomers think PMS means.) There are "Classical" and "Weak" T-Tauri stars, based on the strength of the Hα emission line. Our understanding is that the Hα results from accretion of gas on to the star— and Crockett et al. have more evidence to support this. What does this mean? There is still a substantial gas disk around the star, putting the protoplanetary disk almost certainly within the first million or two years of it's life.
In other words: what these guys have discovered is a Jupiterish planetary mass condensed around a star that's at most a couple of million years old. This is an extremely relevant constraint on how long it takes to form planets, and will be big news for those who try to theoretically understand the original formation of our Solar System and other planetary systems.
This work should be submitted to a journal sometime soon, and presumably sometime thereafter it will show up on arxiv.org.