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Detecting Other Worlds: Transiting Exoplanet

A planet blocking starlight
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Set of three diagrams: View from Above, View from Telescope, and Light Curve. View from Above shows a star with a circle around it to represent a planet’s orbit, and a telescope below, pointing straight up at the star. The planet is positioned along the orbit at the 8 o’clock position relative to the star, to the left of the telescope’s line of sight. View from Telescope shows the star surrounded by a flattened ellipse representing the planet’s orbit as seen from the side. The planet is positioned along the orbit, to the left of the star. Light Curve shows a line graph of apparent brightness on the y-axis versus time on the x-axis, with a solid orange circle plotted to show the apparent brightness of the star-planet system at the time illustrated in the two other diagrams. The graphed line looks like a broad U shape cut into a high flat plain. The orange circle is plotted on the flat plain, to the left of the U-shaped valley.

Set of three diagrams: View from Above, View from Telescope, and Light Curve. In the View from Above, the planet is positioned along the orbit at the 7 o’clock position relative to the star, at the left edge of the telescope’s line of sight. In the View from Telescope, the planet is positioned along the orbit, in front of the left edge of the star. On the Light Curve of apparent brightness versus time, the orange circle is plotted near the top of the left wall of the U-shaped valley.

Set of three diagrams: View from Above, View from Telescope, and Light Curve. In the View from Above, the planet is positioned along the orbit at the 6 o’clock position relative to the star, in the middle of the telescope’s line of sight. In the View from Telescope, the planet is positioned along the orbit, in front of the star. On the Light Curve of apparent brightness versus time, the orange circle is plotted in the middle along the floor of the U-shaped valley.

Set of three diagrams: View from Above, View from Telescope, and Light Curve. In the View from, the planet is positioned along the orbit at the 5 o’clock position relative to the star, at the right edge of the telescope’s line of sight. In the View from Telescope, the planet is positioned along the orbit, in front of the right edge of the star. On the Light Curve of apparent brightness versus time. the orange circle is plotted near the top of the right wall of the U-shaped valley.

Set of three diagrams: View from Above, View from Telescope, and Light Curve. In the View from Above, the planet is positioned along the orbit at the 4 o’clock position relative to the star, to the right of the telescope’s line of sight. In the View from Telescope, the planet is positioned along the orbit, to the right of the star. On the Light Curve of apparent brightness versus time the orange circle is plotted on the flat plain, to the right of the U-shaped valley.

A graph of apparent brightness on the y-axis versus time in Earth-days on the x-axis. The graph is labeled “Actual Data.” The y-axis ranges from 99.2 percent at the origin on the bottom to 100 percent at the top, in even increments of 0.2 percent. The x-axis ranges from 0 days at the origin at the far left to 7 days at the far right, labeled in even increments of one Earth-day. The data are plotted as orange circles, connected by a thin white line. The data form a straight flat line with a y-value of about 100 percent, cut by a series of three deep, narrow valleys. All three valleys reach down to a value of about 99.3 percent. The valleys are evenly spaced over time, occurring at about 1.2, 3.4, and 5.6 Earth-days.

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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

A Story Of 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: Cygnus Constellation

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.