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Lowell Observatory image of an exoplanet transiting its parent star

Exoplanet transiting its parent star. Lowell Observatory.

In Depth

Kepler’s ability to detect transiting planets allows us to see Earths trillions of miles away.

The figure below seems like a humble or even inscrutable graph, but it’s a triumph of patience and sensitivity. This is a light curve — a graph of brightness over time — of one of the stars Kepler is observing. Because we already knew this star had a large, Jupiter-like planet, it was used in the early part of Kepler’s science mission as a proof of concept. In this case, the planet is aligned, as viewed from Earth, so it passes directly in front of and behind the star as it orbits. When the planet crosses the disk of the star — a phenomenon called a transit — the star’s brightness dips slightly. (We occasionally see transits of Mercury and Venus across the disk of the Sun, too.)

Kepler transiting planet

Kepler transiting planet. Courtesy of NASA's Kepler mission.

The deep dip in the figure shows where the planet crosses in front of the star. But you can also see a smaller dip where the planet disappeared behind the star, and that’s important because it’s about the size of the effect an Earth-size planet would have when it transited. We already know there are huge, inhospitable, Jupiter-like worlds out there, but Earths are much harder to see. This figure proved Kepler’s ability to detect distant Earths.

Even more exciting is the slant in brightness between the transits. After the deep, primary transit, the star-planet system gets slightly brighter. What we’re seeing here is the phase of the planet, going from crescent to half to gibbous to nearly full before it disappears behind the star. Exactly as our Moon appears brighter at it approaches full phase, the planet gets brighter as we see more of it illuminated.

In this case, however, it’s not like measuring the change in brightness of the Moon. This is an exquisitely precise measurement, reaching hundredths of a percent in sensitivity. Kepler represents the true state of the art in detecting miniscule changes in brightness and our first opportunity to detect planets the size of Earth trillions of miles away.

Are any of these planets habitable, perhaps with oceans of liquid water and hospitable atmospheres? Finding candidate planets is the first step to addressing this age-old question, and Kepler is leading the way.

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