Graduate student and astronomy writer

Archive for October, 2014

How to detect planets outside the solar system: The Transit Method

Hey everyone out there! This is the first in a series of posts about how you detect planets around other stars: the aptly named extra-solar planets, or exoplanets. There will most likely be 3 more posts like this dedicated to the other three major detection methods: radial velocity, gravitational microlensing, and direct imaging.

But for now, we’ll start with my favorite method, as this is what got me interested in exoplanets in the first place. As always, I appreciate comments and questions about content, as that always helps me improve my writing.

Now sit back, relax, and enjoy the following feature on….

The Transit Method

The transit method for the detection of planets is possibly the most well known way to find extra-solar planets. This is in large part due to the resounding success of NASA’s Kepler mission, which has revolutionized the search for and study of exoplanets (planets outside of our solar system) since it first started observing more than 150,000 stars near the constellation Cygnus. Kepler detects planets using the transit method, which looks for the minuscule dips in the light from a star when a planet moves between the star and our telescope.

This method is almost like looking for the shadow of the planet: imagine you are staring at a lit flashlight (no, please don’t actually do this, this is bad for you). The flashlight is very bright, very constant light that doesn’t change. Now, imagine that as you are looking at the flashlight and a bug flies right in front of the flashlight. Now, you can’t actually see the bug because the flashlight is so bright in comparison, but you can see that the flashlight’s light is slightly dimmer than it was before. That is a transit.

If something transits a star at regular intervals, Kepler scientists flag it as a planet candidate that needs further investigation. As of October 16, 2014, Kepler has detected 4234 planet candidates. Scientists then spend a lot of time making sure that the detected “planet” is not something else that can mimic a planetary transit signal. After the candidate has been vetted, it moves from a “planet candidate” to “confirmed planet” and enters the ranks of the other 989 planets confirmed by Kepler scientists.

HAT-P-3b transit

GIF of HAT-P-3b in true color, created using the function TRANSITGIF from Jason Eastman ( As the planet moves in front of the star, the light detected from the star decreases, then levels out in the middle of the transit, then increases back to the original level as the planet moves away from the star.

What makes the Kepler mission so remarkable and so unlike any other transit detector of its kind is that it was able to stare constantly at the exact same 150,000 stars for 3 years straight, obtaining a very large amount of extremely precise data. It was able to do so due to its very precise guiding system, which gave out for good in May 2013. Still, astronomers have found a way to make use of the perfectly functional telescope using an alternate guiding system — see K2 link below. Unfortunately, the reaction wheels gave out right before Kepler would have obtained enough data to detect an Earth-like planet around a Sun-like star. However, we still have detected more planets than previously thought possible, so I still call Kepler a resounding success!

Video courtesy of NASA Ames/SETI/J Rowe: Blue planets are non-Kepler planets discovered. Red planets are Kepler planets from before Feb. 2014, and gold planets are just those confirmed in February 2014!  The Feb. 2014 planets are from Rowe et al. (2014).

The planets detected by Kepler range in size from larger than Jupiter to smaller than Mercury, and can orbit their stars anywhere from almost a year to under a day! With the most recent releases of the Kepler data we have started to detect Earth-sized planets in the habitable zones of their stars — amazing! Though we have yet to detect a true Earth analog, we are starting to close the gap and we will most likely detect one in the next decade. Keep a lookout!

Kepler Planets

Plot showing the distribution of planet discovered by Kepler and other detection methods. The distance from the planet to its star is on the horizontal axis, and the planet’s mass is on the vertical axis. The planets’ masses for Kepler planets are largely calculated by relations between the planet size and mass from a selection of stars that have both properties measured.

Summary of the Transit Method:

Direct/Indirect: Indirect. Uses the changing light from the star to infer the presence of a planet.

What you learn from a transit: Planet size, orbital period, orbital distance, orbital inclination, eccentricity.

Other things you could learn: if there are other planets in the system you haven’t detected! If there are other planets there that are interacting with your detected planet, that planet can tug on the detected planet and vary the transit time that you detect. These tugs, called Transit Timing Variations, change the time that the detected planet transits, and can indicate the presence of one or more undetected additional planets in that system.

Other telescopes that use the transit method: K2, CoRoT, HATNet, WASP, TESS (future)


Well folks, that’s it for now. This was only a (very) short summary about a method that has broken tremendous ground over the past decade. There is a lot more information about transit detection, transit analysis, and various follow up and false-positive analysis methods that I haven’t even touched on here. But that’s a different post.

Hope you enjoyed learning a bit about the transit method. Next up: Radial Velocity.

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