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Planet Hunting Toolkit IV: Transits March 18, 2009

Posted by CosmicThespian in Toolkit.
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Radial velocity and astrometric searches can tell us what stars have planets, how far these planets are from their stars, and how massive each planet is. Unfortunately, that’s about it. If we want to really start to understand these other worlds, and possibly look for clues hinting at the presence of life, we need more. This is where the Transit Method can help.

Transits are astronomer-speak for the event of an object passing between you and a star. You may know them by another name: eclipses.  The most common type of transit that people are most likely familiar with has to do with our Moon.  When the Moon passes between us and the Sun, we find ourselves momentarily in the lunar shadow.  The Moon blocks the light of the Sun, the stars become visible in the middle of the day, and we have ourselves a solar eclipse.  This is the most dramatic example of a transit.

There are two other types of Sun-based transits Earth-bound observers can witness: those of Mercury and Venus.  Whenever either of these two planets pass between us and the Sun, we can watch their shadow trek across the face of the Sun using solar-filtered telescopes.  You can find a list of future transits of Mercury and Venus here.

Transit of Venus

Montage of 2004 transit of Venus (Image Credit: NASA)

Under the right conditions, we can also observe extrasolar planets transit their host stars!  This won’t be possible for all, or even a large fraction of, other planetary systems.  The only time we can see this occur is when we are looking at a distant solar system “edge-on”.  If the planet’s orbit is too steeply inclined relative to the Earth, the planet will never appear between us and its star.  The chance of finding a planet that passes between us and its star is dependant on the size of the star and the size of the planet’s orbit.  The smaller the star or the larger the orbit, the less likely are we to find a transit candidate.  Typical numbers are around 1 percent.  While the chances for finding a transiting planet may seem unlikely, if one studies hundreds of thousands of stars one can potentially find thousands of planets!

When a planet does transit its host star, we can detect the event by a dip in the star’s brightness.  The dip occurs because as the planet passes between us and its star, it blocks some of the light.  For a time, we are in the shadow of a distant world.  How much the light dims depends on the size of the planet, but 1 % is a pretty typical value.

Illustration of a transiting planet and the corresponding dip in the star's brightness.

Illustration of a transiting planet and the corresponding dip in the star's brightness.

The power of exoplanetary transits is in what these events can tell us about these distant worlds.  First, by carefully measuring how long it takes for the transit to reach it’s minimum brightness as well as how long the entire transit event takes, one can directly measure the size of the planet!  When one combines this with the mass as determined by radial velocity measurements, the planet’s density can be calculated.  This is an important first step in determining the bulk composition of the planet and is crucial in distinguishing gas giants like Jupiter from rocky worlds like our own.

Secondly, when the planet transits its star, some of the starlight passes through the planet’s atmosphere while en route to Earth.  In my article on spectra, we learned that measurements of a star’s spectrum can tell us what the star is made of.  Well, the same is true for any starlight passing through the atmosphere of an orbiting planet.  By comparing the spectrum of a star before, during, and after a planetary transit and looking for what changes, astronomers can measure the chemical makeup of exoplanetary atmospheres! Our first indication of life in the Universe will probably not come from a radio signal blaring across the galaxy, but from the subtle chemical effects of biological activity on a distant planet’s skies.

The big disadvantage to relying on transits is that they are very rare.  Not only will most exoplanets never transit their stars, but the ones that do only do so briefly.  If a planet takes years to orbit its star, the transit event itself may only last for hours or days.  That means, in order to be successful, astronomers must stare at the same stars for years at a time if they want to increase their chance of success.  The Kepler space mission will, in fact, do just that.  It will stare at one patch of space, monitoring the brightness of roughly 100,000 stars for four years.  The payoff could be huge: it is currently the only instrument sensitive enough to detect the presence of an Earth-sized planet in an Earth-like orbit!

Transit searches can also be easily tricked by binary stars and normal stellar variability.  Because of this, transit detections require follow-up, typically using radial velocity measurements to confirm the presence of a planet.

Despite the disadvantages, transiting planets are poised to take the exoplanetary community by storm.  The COROT and recently launched Kepler space missions are using transit detections to look for potentially thousands of planets.  By combining the findings of these missions with ground-based follow-up radial velocity measurements, we are standing on the brink of a revolution in understanding the innumerable worlds that orbit other suns!

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