Last Tuesday afternoon marked the last transit of Venus across the sun this century!
Our solar system was born in such a way that Earth is more-or-less aligned with the orbital planes of the other inner planets between us and the sun: Mercury and Venus. This makes it possible to view from Earth these planets trekking across the face of the sun in what scientists dub a transit. Though you might think this happens pretty often, actually witnessing a transit is a rare event, as Venus's orbital plane is tilted slightly relative to ours. Thus, Venus transits occur in pairs, and though there are only eight years in between the two, the next pair rolls around every 105-120 years!
This is an actual image of the transit taken by NASA!
The transit detection technique is used when a planet passes in front of its star, and there is a consequent drop in the measured luminosity emitting from the star. These drops are graphed and from them, we can discern the planet's mass and orbital period. In addition, a somewhat counterintuitive part of transits is when the planet passes behind the star, and the luminosity of the planet itself (however small) is lost. Transit detections can be so precise, they can detect a drop in the light curve of 1.5%! But what's really cool about the transit method is this: When the planet passes in front of the star, light shines through its upper atmosphere. By observing the resulting spectrum at various wavelengths, we can actually determine the composition of the planet's atmosphere!
Transit detection requires two conditions to work optimally. First, the plane of the orbit must be aligned with the plane of the observer. Second, this method is ideal for very large planets with small orbital radii, as their light curve will be much more pronounced.
The Kepler mission is a NASA project that's looking for Earth-size planets within the mysterious "habitable zone" of a star where liquid water can potentially exist. The telescope stares at about 100,000 stars for about 3.5 years, and can detect changes in the light curve up to 1/10,000, or 100 parts per million.
Check back later for more on the eponym of the Kepler mission, Johannes Kepler, and his contributions to astronomy.
"... the ways by which men arrive at knowledge of the celestial things are hardly less wonderful than the nature of these things themselves." -Johannes Kepler