Transit Hunt Guide

Users' Guide to Kepler Exoplanet Transit Hunt

This online interactive requires a Flash enabled computer or device.

This Guide is a series of 10 short videos. The narrative text on the bottom half of this page.

To see the Transit Hunt online interactive, go here:

Users' Guide — Screen-by-screen

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Kepler Exoplanet Transit Hunt Guide Text/Script

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The video guide is a walk through the Kepler Exoplanet Transit Hunt. It's about the science of planet discovery. And it’s got lots of math, but it’s math that can be adapted for either middle school or high school levels.

The basic idea is to discover a planet by staring at a star and looking for a tiny drop in brightness when a planet passes in front it. When a planet goes in front of a star, that’s called a transit. And this planet finding method is called the transit method. We can’t even see the planet, but what can we find out, just from the light of its host star?

Here’s the Transit Hunt webpage:

Just do what it says: "Click on the image" ...That launches the interactive. By the way, it’s Flash-based so it won't run on iPads or iPhones.

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Screen 1: Telescopic View

To start things going, drag the "Telescopic View" circle over to the blinking star....

You see an animation of the planet orbiting the star. Notice the planet is orbiting horizontally. That’s one of the orientations where we can observe a transit. If the planet's orbit plane does not line up with us edgewise, the planet will be out of our line of sight with the star. [This paragraph is an update and correction to the original narration in the video]

Then click "Next" in the lower right.

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Screen 2: Select a star

Pick a star to investigate. Notice all the spectral types. Spectral type is a handy dandy way to sort stars according to properties connected with the star's spectrum. Properties like size, temperature, and mass. Type A and B are giant stars. We are not much interested in those because they have short lifetimes, way too short for forming planets. We’re most interested in Sun-like stars. Our Sun is type G2. For now, let's pick the G3 star on the left side of the screen.

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Screen 3: Star's mass, size, and temperature

The chart on the third screen shows mass, size, and temperature of various type stars. At the bottom of the screen, there’s a Notebar to keep a record of our findings. There is no export function for these notes. For a permanent record, make a duplicate of the data on paper or spreadsheets or take a screen capture near the end.

Mass and radius are both compared to our Sun. So a star with mass of “1” has the same mass as the Sun. Likewise a star with radius of “1” has the same radius as the Sun. Temperature is in Kelvin, where zero is "absolute zero" ... or no molecular motion. The freezing point of water...32 degrees on the Fahrenheit scale, 273 on the Kelvin scale. The boiling point of water is 373 Kelvin (100°C or 212°F).

To use the Notebar, either type in the number... or click and drag the matching color box in the data table - to the matching color space in the Notebar. Do that for all 3 star properties: mass, size, and temperature. ...then click next....

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Screen 4: Data collection

Now collect some data.

Keep your eye on the star that is in the blue circle. The Kepler space telescope has done this for over 150,000 stars simultaneously!

After you click the start button, every time you see the star blink, click the button that says “record blink”. If you’re in a group, have everyone say "blink" ...or “click” ...every time there is a blink, to assure the person clicking the button does it right. By the time we're done, the graph we've created ...of star brightness versus time, called a light curve.

Now we make measurements. First the planet's orbital period, ... the time it takes to go around the star once. Measure the spacing of brightness drops along the x-axis.

You can drag the x-axis to line up with particular brightness drops. That’s a really nifty way to help measure the orbital period in days. Type that number in the Notebar in the blue space that says "Exoplanet Orbital Period".

Then measure how deep the drops in brightness are. Drag the y-axis to line up with one of the drops in brightness to measure its depth. Type that number in the Notebar in the yellow space that says "% Brightness Drop".

What would a 100% drop in brightness mean? The star's light must have been completely blocked. In reality, a planet blocks less than 1 or 2% of its star. For really small planets, it can be hundredths of a percent. Be careful in reading the brightness drop scale to make sure your decimal point is in the right place and you've counted the scale divisions correctly.

Click Next.

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Screen 5: Distance from Star

Over 400 years ago, Johannes Kepler figured out 3 laws governing the motion of planets in our Solar System. One was that planets have orbits shaped like ellipses ... not perfect circles. Another described how a planet moves faster in the parts of its orbit where it's closer to the Sun. His third law says the longer a planet takes to orbit the Sun, the farther that planet is from the Sun. The NASA Kepler team uses that law, ...Kepler's 3rd Law, ... to find out how far a planet orbits from its host star —by measuring its period. The exact formula is

D3 ∝ T2


D is the planet’s distance from the Sun

T is the planet’s orbital period

…and that's also why the mission is named as it is ...after Johannes Kepler.

Even if you don’t know algebra, you can see that the greater the period, the farther the planet is from its host star.

For students of algebra, the formula is straightforward. For a star mass M equal to 1, a star with the same mass as the Sun, the formula is exactly how Kepler originally wrote it. For other stars, the star's mass must be factored in:

D3 = T2 x M

where M is the star’s mass

So D = 3√(T2 x M)

[3√ is cube root]

This formula has a built in calculator button. Just drag the right color-coded planet and star properties from the Notebar up into the equation.

But wait! The formula expects the period in years.

Oh no! ...we made our period measurement in days.

We need to convert that to years.

There is a handy dandy "Convert" button that automatically divides by 365.

Now press the "Calculate" button to crank out the planet's distance from its host star. It’s in “astronomical units,” or "AU". One AU is the average distance from Earth to the Sun. That’s about 150 million kilometers, or 93 million miles.

Before you click the Next button, drag the purple box in the formula to the purple space in the Notebard so your answer gets recorded there.

So far, we have found the planet's orbital period, and average orbital radius, or the distance from its star.

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Screen 6: Habitable zone?

Every star has a zone around it where there could be planets able to sustain life. Such a zone is called the star’s habitable zone. Such a zone must allow for liquid water. All life forms we know of require liquid water.

For a planet too far from its star, if there is water, there is only solid water ... ice. For a planet too close to its star, if there is water, there is only water vapor.

What other factor—besides distance from the star—affects a star’s habitable zone? The star's temperature. The hotter the star is, the farther away its habitable zone will be.

On screen 6 (Habitable Zone) you adjust two things: temperature, using the white up-and-down arrows, and

planet distance, using the white sideways triangular arrows.

Use those sets of arrows to match the star temperature and planet distance with the values you have in the Notebar. Then you can write “yes” or “no” ...or “y” or “n” the bright blue space labeled “Habitable Zone” in the Notebar. Then click Next.

Now we can add to our list of planet properties whether or not the planet is in the star's habitable zone.

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Screen 7: Planet temperature

What is this planet's temperature? That depends on some of these factors: star temperature and planet distance.

Tplanet = Tstar x √(R/2D)


Tplanet is the surface temperature of the planet

Tstar is the surface temperature of the star

R is the radius of the star

D is the distance from the planet to the star

This formula is known in physics as the “black body radiation formula.” There is one other factor that matters: the star's radius. You can learn about this in a physics course. Also, this formula ignores any greenhouse effect. Greenhouse gases in the planet's atmosphere might raise the temperature. So the temperature we calculate here will be a low-end estimate. The planet’s actual temperature may be higher.

To use our black body radiation formula ...Just drag star radius, star temperature, and planet distance from the note bar into the formula, and click Calculate. Then drag the calculated planet temperature into the Notebar.

Chalk up another item to our growing list of planet properties we got just from starlight: planet temperature.

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Screen 8: Planet size

Can we figure out the size of our planet?

Well, bigger planets create bigger dips in brightness, so we can figure out how big the planet is. The percent drop in brightness depends on the area of star that is blocked by the planet. If you have learned that the area of a circle is π times radius squared, then the equation on Planet Size screen will make sense: the brightness drop is simply the area of the disk of the planet divided by the area of the disk of the star. With some algebra and a square root, you get the formula this formula:

Rplanet = Rstar x 10.9 x √B


Rplanet is the radius of the planet

Rstar is the radus of the star

B is the brightness drop (%)

Even without algebra, you can see that planet's size is related to the drop in brightness and the star's size.

Just drag star radius and brightness drop from the notebar into the equation. Then click Calculate and drag the resulting exoplanet radius into the Notebar. Then click ‘Next’.

Add yet another planet property to our list: ...planet size.

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Screen 9 summary

That's a lot about the planet we learned just from starlight: the planet’s orbital period, orbital radius, whether or not it's in the habitable zone, planet temperature, and planet size! Amazing.

The ninth screen shows a summary of your investigation with an artist’s conception of the planet. The size shown is compared with Earth. A print out or screenshot of this page can be your permanent record of the Notebar.

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Screen 10 planet surface

This final window shows an artist's conception of what the surface of your planet might look like. You can pick another star to investigate too....

Good hunting!

Voice narration by Mark D. Messersmith

Script by Alan Gould, Toshi Komatsu, and Gary Nakagiri.

Graphics/animation by Dana Berry and the Kepler Education Team.