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Transiting Planets

The effects of 3 different types of transiting planets on a Kepler light curve. (Illustration: H. & M. Giguere)

Hi I’m Matt, a graduate student at Yale University and a member of the Science Team. We’re really impressed with the turnout so far on and users have already pointed out some really amazing objects! Quite a few people have asked for some clarification on what transits look like, so I’ll address that in this post.

In the figure above, we’ve taken a Kepler light curve from a star that’s about the same size as the Sun and have simulated what the effects would be if a few different types of planets were to transit.

The white dots show the amount of light from the star measured with Kepler with no planets transiting. The blue points show what we would see if a planet just like Jupiter orbiting this star were to transit. This Jupiter-size planet, at about 11.2 times the size of the Earth and one tenth the size of the star, is shown to scale transiting its parent star in the top left blue box.

The green dots show what a planet just like Neptune would look like transiting. Since it is much further away from the star than Jupiter, it would have a slower orbital speed so it would take longer to transit the disk of its parent star, which is what explains the longer duration, or wider width, of the transit event. With Neptune’s much smaller size than Jupiter, at 3.9 times the radius of the Earth, it doesn’t block out as much light, which is why the depth is much shallower.

Both of these events are very noticeable, compared to the effects of an Earth-size planet. The tiny speck on the star in the far right red box shows, to scale, what a transiting Earth-size planet would look like if we could see it. Now you get an idea of how difficult finding Earth-size planets is going to be! If that transiting planet had an orbital period of 1 year just like the Earth, then the dip in light observed from the parent star as the planet transits would be similar to the red points in the light curve. Since the Earth is much closer to the star, it has a much faster orbital speed, which then makes the duration of transit much shorter than the duration of either Jupiter or Neptune. Because the Earth-size planet is much smaller than either Jupiter or Neptune, it also blocks out less light making the dip in light we receive here on Earth barely discernible from no transit at all.

We don’t expect people to see these events all the time, so don’t worry about missing them. That’s why we’ve introduced fake planets into the mix. The fake, or synthetic, planets will help us determine the completeness of Planet Hunters, or how likely we are to detect planets of different sizes and with different orbital periods if they exist.

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