A planet might take months or years to complete its orbit, but the transit would probably last only hours or days. The rest will never be detected with photometry.Īnother problem is that a planet's transit lasts only a tiny fraction of its total orbital period. In order for a transit to occur the orbital plane must be almost exactly edge-on to the observer, and this is true only of a small minority of distant planets.

Unfortunately, for most extrasolar planets this simply never happens. Not all planets orbiting other stars transit their stars as seen from Earth a distant planet must pass directly between its star and Earth. The main difficulty with the transit-photometry method is that in order for the photometric effect to be measured, a transit must occur. Transit surveys (both ground- and space-based) can simultaneously watch as many as 100,000 stars at a time. The color of the light emitted by a planet is a clue to its temperature and can also hint at the composition of its atmosphere.įinally, transit photometry searches can operate on a massive scale. By deducting the star's light spectrum when the planet is hidden from the spectrum when it is visible, scientists can arrive at the planet's spectrum (that is, its color). In addition to 'primary' transits, which occur when a planet passes in front of its star, scientists are also interested in 'secondary' transits, which occur when a planet completely disappears behind the star as seen from Earth. Water vapor is one molecule that can be observed using transit spectroscopy. By studying the spectrum of a star both during a transit and outside a transit, astronomers can find telltale dips in the spectrum of starlight that are diagnostic of the presence of atmospheric gases. Some wavelengths of that starlight are preferentially blocked by gases in the atmosphere. If the transiting planet has an atmosphere, some of the light from the star passes through the planet's atmosphere on its way to Earth. Since the star's size is known known with a high degree of accuracy, the planet's size can be deduced from the degree to which it dims during transit. Transits can provide scientists with a great deal of infFirst and foremost the 'dip' in a star's luminosity during transit is directly propotionate to the size of the planet. Because transiting exoplanets orbit in orbital planes that are necessarily edge-on to Earth-based observers, using both the transit method and the radial-velocity method to observe the same planet can provide the planet's mass and therefore its density and likely composition. Transits provide scientists with estimates of planet diameters, a physical property not otherwise measurable. It also can be performed from the ground with quite small telescopes the TRAPPIST telescopes only have 60-centimeter primary mirrors. It is a particularly advantageous method for space-based observatories that can stare continuously at stars for weeks or months. Transit photometry is currently the most effective and sensitive method for detecting extrasolar planets.

Astrometry vs. doppler method