The diagram above shows what typically happens when a planet the size of Jupiter passes in front of its parent star. From Earth, it appears as though the star gets slightly dimmer for the several hours it takes the planet to cross the star. For a hot Jupiter, the planet will block about 1% of the light from the star. The goal of a photometric transit survey like KELT-North is to identify these planets by looking for this tell-tale 1% dimming. Since there is only a 1 out of 10 chance that a hot Jupiter will transit it star, and since not every star is orbited by a hot Jupiter, KELT-North needs to look at tens of thousands of stars over the course of several years to be able to find transiting planets.

    Specifically, KELT-North looks at bright stars along a 26-degree wide strip of sky that is overhead from North America during the year. Our typical targets have apparent visual magnitudes of 8 to 11. This means that these stars are 20 to 100 times fainter than can be seen with the naked eye. In comparison, most other transit surveys look for planets around stars 100 to 200 times fainter than the naked eye limit. KELT-North is able to effectively survey brighter stars because we have a much wider field of view and smaller telescopic aperture than other surveys. Planets around brighter stars between magnitude 8 and 11 are of prime scientific interest, because these are the systems that are the easiest to look at with current and next-generation ground- and space-based telescopes.

    Once we have identified a star which shows a dip in brightness like in the diagram above, there still remains several weeks worth of follow-up work. Through a twist of physics, hot Jupiters are the same size (though not the same mass) as the smallest stars. This means that a small star passing in front of a larger star will appear the same as a transiting hot Jupiter in the KELT-North data. In fact, because small stars are more frequent than hot Jupiters, most of what KELT-North finds are in fact double star systems.  We separate the hot Jupiters from the double star systems with the help of our follow-up collaborators. Based across the United States and Europe, our follow-up team takes high-quality imaging and spectroscopic observations of our best planet candidates. By looking for flat-bottomed achromatic transits and small amplitude radial velocity orbits in these data, we are able to pick out which stars host actual planets.

    KELT-North is a photometric survey for transiting exoplanets. We take images (photometry) of tens of thousands of stars every night in an attempt to see planets outside our Solar System pass in front of (transit) the star that they are orbiting. Over the past 17 years, astronomers have discovered hundreds of planets orbiting other stars - in the same way that the eight planets of our Solar System orbit the Sun. Some of these planets are what are known as “hot Jupiters.” These planets are as big as Jupiter, the largest planet in our Solar System, but orbit their parent star much closer than our Jupiter does: typically taking anywhere from 2 to 10 days to complete an orbit. When these hot Jupiters were discovered, astronomers realized that there was a chance that they would appear to pass in front of, or transit, their star from our perspective on Earth.