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Detecting Other Worlds: Transiting Exoplanet

A Planet Blocking Starlight Full Story Below

Red intensity color map showing a roughly elliptical-shaped structure of looping lines emanating from a bright central region
Yellow intensity color map showing a fuzzy elliptical-shaped structure of bright regions surrounding dimmer, more diffuse areas
Green intensity color map showing a roughly elliptical-shaped structure of distinct, bright, frayed-thread-like filaments surrounding darker regions
Blue intensity color map  showing an irregular-shaped region of diffuse light decreasing in brightness from the center, and a scattering of distinct, bright to dim circles of various sizes
Purple intensity color map showing a distinct, irregular-shaped structure with a bright circle at the center surrounded by several bright rings, and a bright plume emanating from the bright circle
Multi-color intensity map in red, yellow, green, blue, and purple showing a roughly elliptical-shaped structure with details of the Radio, Infrared, Visible, Ultraviolet, and X-ray images
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Before the transit begins, none of the starlight is blocked.
As the transit begins, the brightness of the star decreases slightly.
The brightness of the star appears lowest in the middle of the transit, when the planet is between the star and the telescope.
As the planet completes its transit, the brightness of the star appears to increase.
When the transit is complete, the star is back to its normal apparent brightness.
A regular pattern of brightness dips is evidence that a planet is orbiting the star.
0 Hours
0.75 Hours
2.5 Hours
4.25 Hours
5 Hours
3 Orbits

Detecting Other Worlds: Transiting Exoplanet

We can detect planets by analyzing starlight.

Scientists have detected thousands of exoplanets: planets orbiting stars other than the Sun. We have seen only a few exoplanets directly, but we can detect them by measuring the effects they have on the stars they orbit.

One effect a planet can have is to block some of the star’s light as it passes between the star and the telescope. This is known as a transit. As an exoplanet transits its star, the star appears to dim very slightly. Although the planet itself is too far away to see, we can detect slight changes in the brightness of the star. If we observe a pattern—if the same change in brightness happens a few times at regular intervals—we can infer that a planet is orbiting the star. Analyzing the brightness pattern, or light curve, of the star also helps us figure out the time it takes the planet to orbit (the length of the planet’s year), the size of the planet, and how close it is to its star.

The light curve shown here is based on Kepler space telescope observations of the star HAT-P-7 and its exoplanet HAT-P-7 b, which was discovered in 2008. Kepler has been used to detect more than 2,300 planets using the transit method. HAT-P-7 b’s large size relative to its star, along with its short orbital period of just over two days, made it relatively easy to detect using the transit method. Because of its size, mass, and high surface temperature, HAT-P-7 b is classified as a “Hot Jupiter” exoplanet.

Quick Facts: Transiting Exoplanet

Also known as: Kepler-2b
Type: Hot Jupiter
Distance from Earth: about 1,100 light-years
Size: 1.36 times the radius of Jupiter
Mass: 1.8 times the mass of Jupiter
Transit duration: about 3 1/2 hours
Orbital period: 2.2 Earth-days
Location in the sky: Constellation Cygnus
Location in the universe: orbiting star HAT-P-7, inside the Milky Way Galaxy
Did you know: Other observations of HAT-P-7 b suggest that it has aluminum oxide vapor in its atmosphere. On Earth, aluminum oxide exists as rubies, sapphires, and other forms of the mineral corundum.

Explore More About Detecting Other Worlds

Find out more with these additional resources from NASA’s Universe of Learning

Astro vis Astroviz 5 Ways to Find an Exoplanet
Universe of learning Universe of Learning DIY Planet Search
Universe of learning Universe of Learning Eyes on Exoplanets
Astropix Astropix Illustrations of Exoplanets and Graphs of Exoplanet Data

Credits: Detecting Other Worlds


Content development by Margaret W. Carruthers, Timothy Rhue II, Dr. Brandon Lawton (STScI)

Design by Elizabeth Wheatley, John Godfrey (STScI)

Web Development by Philippe Batigne (STScI)

Transit light curves from data collected by the Kepler space telescope, with plots and interpretations based on work of Dr. Andrew Vanderburg