Venus' Transit and the Search for Other Worlds

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Written by Michele Johnson, NASA Ames Research Center

Media contact:
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin@jpl.nasa.gov

News feature: 2012-155 June 5, 2012

Venus' Transit and the Search for Other Worlds

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-155&cid=release_2012-155

It's the final opportunity of the century to witness the rare astronomical reunion of
the sun, Venus and Earth. On Tuesday, June 5 or 6, 2012, depending on your
location, Venus will make its presence in the solar system visible from Earth's day
side. Using special eye safety precautions, viewers may see Venus as a small dot
slowly drifting across the golden disk of the sun.

Transits of Venus are very rare, separated by more than a hundred years. There
have been 53 transits since 2000 B.C., but only six have been witnessed since the
invention of the telescope in 1608. These rare events occur in pairs, with the first
transit occurring June 8, 2004. The next opportunity won't be until Dec. 10 and 11,
2117.

Jeremiah Horrocks and William Crabtree, two young astronomers from England,
recorded the first observation of a transit in 1639. In 1769, survey crews, including
Captain James Cook, gathered transit data from various locations around the world
that were later used to calculate the distance between Earth and the sun, thereby
establishing the solar system's scale.

"Throughout history, astronomers have creatively used nature's coincidences as
opportunities to learn something new about the universe," said Natalie Batalha,
Kepler mission scientist at NASA Ames Research Center, Moffett Field, Calif.
"Today is no different. As Venus crosses the disk of the sun, her shadow sweeps
across the face of Earth in the same way that the shadows cast by distant
exoplanets sweep across the face of the Kepler photometer."

Today, transit events are used to detect planets beyond the solar system. NASA's
Kepler space telescope continuously measures changes in brightness of more than
150,000 stars to detect when a planet passes or transits in front of a star. Kepler
does not directly image distant planets, as they are too far away.

Different-size planets block different amounts of starlight. Kepler's exquisitely
precise photometer, or light sensor, is designed to detect fractional changes in
brightness. For an Earth-size planet transiting a sun-like star, the change in
brightness is only 84 parts per million. That is less than 1/100th of one percent, or
the equivalent of the amount of light blocked if a gnat crawled across a car's
headlight viewed from several miles away.

Transit data are rich with information. By measuring the depth of the dip in
brightness and knowing the size of the star, scientists can determine the size or
radius of the planet. The orbital period of the planet can be determined by
measuring the elapsed time between transits. Once the orbital period is known,
Kepler's Third Law of Planetary Motion can be applied to determine the average
distance of the planet from its stars.

Using the transit method, the Kepler mission has identified 61 planets and more
than 2,300 planet candidates during the spacecraft's first 16 months of observation
from May 2009 to September 2010.

NASA's Ames Research Center in Moffett Field, Calif., manages Kepler's ground
system development, mission operations and science data analysis. NASA's Jet
Propulsion Laboratory, Pasadena, Calif., managed the Kepler mission's
development. For information about the Kepler Mission, visit:
http://www.nasa.gov/mission_pages/kepler/main/index.html .

For more information about the worldwide events, safety precautions for viewing,
educational content and social media activities, visit:
http://venustransit.nasa.gov/transitofvenus/ .

The public can follow the event on Twitter on #VenusTransit and download a free
mobile app at: http://venustransit.nasa.gov/2012/multimedia/apps.php .

-end-



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