Today’s Astronomy Picture of the Day is a great visualization of what is involved in studying exoplanets—planets circling other stars. The picture (shown to the right in small form) has a large number of stars with tiny shadows representing the passage of an exoplanet across the star’s disc. The Sun, with shadows for Jupiter and Earth, is included for reference.
The picture is constructed from data from the Kepler exoplanet-hunting mission, and uses the transiting of exoplanets across their star’s disc to measure not only the orbital properties—how large is the orbit, how long does it take to orbit, etc.—but the physical size of the planet, a piece of data that isn’t available to most other methods. Despite the picture, Kepler isn’t seeing the actual disc of the star or the real shadow of the planet; instead, Kepler is measuring the amount of light the start produces with and without the planet in our line of sight. The tiny eclipse may not be very large in terms of the star’s total light output, but it’s measurable.
Details about how transits work are under the fold….
(Click on the figure to see it in a larger version.) The cartoon shows five different points in the orbit of an exoplanet as it moves across the disc of its host star. When no eclipse is occurring, we get all the star’s light unimpeded; when the planet is partly blocking the star, we see a gradual decrease in intensity until the planet is fully in between us and its host. Unlike the Moon eclipsing the Sun, we’ll never see an exoplanet fully blocking its star’s light—the only reason this happens on Earth is because the Moon is so close to us. Given the vast distances between us and the nearest exoplanet system, even the largest planet is going to be a tiny speck, too small to see unless we’re very fortunate.
Transits don’t tend to stand alone for characterizing exoplanets; though I won’t elaborate in this post, generally another method is used in combination. Several things have to work in concert for transits to even be visible: we have to see the orbit of the planet “edge on” (think of how a plate appears differently if you see it from the top or from the side). If the planet’s orbit is too tilted, we’ll never see it cross its host star’s disc. Also, the amount of light blocked has to be enough that we can detect the difference, so as with any other exoplanet detection method, we will tend to see larger planets—or at least planets that are large in comparison with their host star.
By correlating the depth of the eclipse (how much light is blocked) with the time it takes for the planet to move fully over the star’s disc, we can determine the relative sizes of both planet and star. By timing between eclipses, we can determine how long it takes the planet to complete one orbit and (using other methods) determine how large the orbit is. Altogether, this is a very powerful and effective way to locate planets when we have little hope of imaging them directly within the next decade or so.
Galileo's Pendulum 
14 responses to “Exoplanet Transit Authority”
[…] find another planet that can sustain life again, Earth has shown life is possible in our universe. Given the amazing exoplanet discoveries over the last decade, I’d be surprised now if no other planet is life-sustaining, but […]
[…] many people, I’ve been watching the Kepler telescope’s exoplanet search with a great deal of interest. An exoplanet is a planet orbiting a star other than our Sun; the vast majority of known planets […]
[…] I still have a soft spot for the biggest planet. I followed the Galileo mission avidly; even years later, analysis of the Galileo data continues to reveal new things about Jupiter and its moons, including the violently volcanic moon Io. Juno holds out a tantalizing promise: if all goes as planned, it may be able to reveal exactly what the core of Jupiter is like — solid rock, or compressed hydrogen? In other words, is the core of Jupiter like the terrestrial planets (Earth et alia), or is it more like a star? Though this may seem like an abstract question, it also may help us learn how our Solar System formed, and allow us a stronger basis for comparison with other star systems. […]
[…] make no secret about my love for exoplanet research and discovery. When you consider that a mere 25 years ago, our Solar System hosted the only known planets, and […]
[…] I wrote in a previous post, Kepler finds exoplanets by watching for fluctuations in light as a planet eclipses its host star. In this case, the eclipses came mostly from the stars crossing each other, relative to Earth, but […]
[…] In 2008, astronomers analyzing Hubble Space Telescope images found several containing an object apparently in the disc that moved significantly over time — a distinctly plausible planet candidate. Called Fomalhaut b, the object was estimated to be about 3 times Jupiter’s mass, with an orbit of about 115 times farther than Earth is from the Sun. Readers with some astronomy knowledge will probably already see where this is going: that’s a long way out for a planet that size! It’s actually closer to the edge of the dust disc than it is to the host star. However, other exoplanet systems have challenged our planet formation models before, so maybe Fomalhaut is no different in that respect. Also, its large distance from the host star actually made it easier to spot, since the closer in a planet is, the more the star’s light will overwhelm it. Fomalhaut b was warmly acknowledged by many to be the first directly-imaged exoplanet, a bit deal, since the vast majority of exoplanets can only be detected indirectly. […]
[…] clear that even if it has a solid surface, it’s not going to be particularly Earthlike. (The Kepler mission finds exoplanets using transits, when the planet crosses the disc of its host sta… this method is often better at finding sizes than masses, at least initially. To measure masses […]
[…] Kepler mission is capable of measuring the period of orbit (the exoplanet’s year) and the diameter of the […]
[…] how did the Kepler mission even find these planets? It’s not easy: the basic method uses the transit of the planet across its star, creating a momentary dip in the brightness of the star. When the exoplanet is relatively large, […]
[…] March: “The Aharonov-Bohm Effect, or How is a Coffee Cup Like a Donut?” and “Exoplanet Transit Authority“ […]
[…] better detectors and improved techniques, the hunt for exoplanets—planets outside our Solar System—has switched […]
[…] not to say the process will be easy. Despite the huge number of exoplanets that have been discovered (763 confirmed discoveries as of this afternoon), it’s still not an […]
[…] A transit is an eclipse, much like a solar eclipse, in which the Moon passes in front of the Sun. However, even though Venus is significantly larger than the Moon, it’s farther away from Earth, so it blocks less of the Sun’s light. It’s also a much rarer event than a solar eclipse: Venus transit happen roughly twice every century, at an interval of 8 years. This summer (June 5, 2012) also marked a Venus transit, though unfortunately the weather in Richmond, Virginia made it hard for me to see it. Transits were once used to measure the size of the Solar System; today they are used to find planets orbiting other stars. […]
[…] the movie, beaming has an effect even if we’re seeing the orbit from a steep angle, where the Kepler method of observing a transit isn’t […]