What is the threshold for a detection as a planet transits a star?
Transits, which cause dips in a star’s light, are minuscule compared to the brightness of the star, and challenging to detect. For an Earth-size planet transiting a Sun-like star the change in brightness is only 84 parts per million (ppm). That is less than 1/100th of 1%. For a Jupiter-size planet, the transit causes the star light to dip 1 to 2%. The figure shows to scale both a Jupiter transit across an image of our sun on the left and an Earth-size transit to scale on the right. The size of the effect for an Earth is similar to the dimming one might see if a flea were to crawl across a car’s headlight viewed from several miles away.
