However Far Away
I'm a couple days late on this one, but that's OK. Those of you with some interest in astronomy have no doubt heard about the recent launch of the Kepler spacecraft, which was designed to systemically detect and catalog the variety of extrasolar planetary systems. The launch went well; "first light" was a success, and as a bit of a warmup exercise, Kepler then gathered its first real science data by measuring the light curve of an already-known exoplanetary system called HAT-P-7. And what a measurement is was! On August 6th, the Kepler team called a press conference to announce the results:
Source: Kepler Mission
It's amazing how much detail this shows, considering that HAT-P-7 is around 1,000 light years away.
First off, this system would qualify as a "easy" target (if anything Kepler is designed for could be called easy). HAT-P-7 b is one of the larger "hot Jupiter" exoplanets - about the same size as its namesake planet, but orbiting its parent in just over 2 days. Because it's so close to the star, its outer atmosphere is puffed up (making it large), and intensely heated (making it bright). Both those factors conspire to make the planet contribute a relatively large amount to the system's overall variation in brightness.
This great animation (by Sara Seager of MIT) explains the shape of the light curve. The deep minimum is caused by the planet transiting the face of its parent star, with the rounded bottom due to the fact the star isn't quite as bright near its edges, since the surface curves back away from our line of sight. The technical term for this is limb darkening. Half a period later, there's a small dip in brightness as the planet passes behind the star and we no longer see the light reflected from it. Superimposed on the blips is a small oscillation that's due to the phases of the planet! And sure enough, the primary transit happens when the oscillation is lowest (a "new" planet), and the secondary transit happens when the oscillation is highest (a "full" planet).
Source: Kepler Mission
Also notice how the brightness during the secondary minimum is almost exactly equal to the brightness just before primary minimum - perfectly in line with expectations. During secondary, the planet is behind the star and we see the full brightness of the star, nothing more or less. Just before primary minimum, the planet is nearly new and contributes no extra light to the system, so again we have just the full brightness of the star.
It works! And now, we know Kepler is ready to go on to make bigger and better discoveries.
Science aside, I think it's totally amazing that we're able to do something like this - to discover and record the phases of a planet over 5,800,000,000,000,000 miles away. Thirty years ago, it wasn't possible, and wasn't even considered to be possible. Read the quote from Robert Burnham below, and then imagine what we might be discovering another thirty years from now.
One final question is certain to occur to us - Is the Sun's family unique, or are other stars surrounded by similar "solar systems"? The answer is quickly given. We do not know. Among the billions of stars composing our Galaxy, which is itself only one among millions, it appears certain that other solar systems must exist - somewhere. There may, in fact, be millions of other planetary systems. But, because of the vastness of space, we shall never be able to observe them directly. Even at the distance of the nearest star - a mere four light years - a planetary system such as ours would be completely beyond the range of the greatest telescope in the world; which is in the nature of a comment on both the incredible immensity of our Universe and the almost microscopic insignificance of our home in space.
And having returned once again to Earth, let us leave our travelers alone with their thoughts.