![]() ![]() While transits can reveal much about a planet’s diameter, they cannot place accurate constraints on a planet’s mass. ![]() Number of extrasolar planet discoveries per year through September 2014, with colors indicating method of detection – radial velocity (blue), transit (green), timing (yellow), direct imaging (red), microlensing (orange). However, the rate of false positives drops off for stars where multiple candidates have been detected. This necessitates that follow-up observations be conducted, often relying on another method. It also suffers from a substantial rate of false positives in some cases, as high as 40% in single-planet systems (based on a 2012 study of the Kepler mission). For this reason, the transit method is most effective when surveying thousands or hundreds of thousands of stars at a time. ![]() As a result, this method cannot guarantee that a particular star being observed does indeed host any planets. Only about 10% of planets with short orbital periods experience such an alignment, and this decreases for planets with longer orbital periods. The probability of a planet’s orbit coinciding with an observer’s vantage point is equivalent to the ratio of the diameter of the star to the diameter of the orbit. For one, planetary transits are observable only when the planet’s orbit happens to be perfectly aligned with the astronomers’ line of sight. Transit Photometry also suffers from a few major drawbacks. This allows for measurements of the planet’s temperature and can even determine the presence of clouds formations in the planet’s atmosphere. On this occasion, astronomers measure the star’s photometric intensity and then subtract it from measurements of the star’s intensity before the secondary eclipse. Last, but not least, the transit method can also reveal things about a planet’s temperature and radiation based on secondary eclipses (when the planet passes behind it’s sun). Artist’s impression of an extra-solar planet transiting its star. ![]()
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