What the transit of Venus tells us about alien planets

The transit of Venus will help astronomers on the hunt for planets outside of our solar system.

By Clara Moskowitz,?SPACE.com / June 1, 2012

Illustration of the Venus transit from James Ferguson's book Astronomy Explained.

NASA Goddard Space Flight Center Sun-Earth Day

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A rare opportunity to see the planet Venus cross in front of the face of the sun is coming up next week.

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On June 5 to 6, Venus will "transit" the sun for the last time until 2117, joining the ranks of the handful of planetary transits that have occurred since the dawn of modern astronomy.

From our vantage point on Earth, we occasionally have the chance to see two planets ? Venus and Mercury ? pass in front of the sun, as these are the only two planetary bodies between us and our star.

Transits of Mercury are more common than Venus transits, with an average of 13 occurring each century. Venus transits come in pairs separated by eight years, with more than a century usually elapsing between one pair and the next. [Gallery: Transits of Venus Throughout History]

"The first transit ever observed was of the planet Mercury in 1631 by the French astronomer [Pierre] Gassendi," Fred Espenak, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., wrote on the NASA Eclipse website. "A transit of Venus occurred just one month later, but Gassendi's attempt to observe it failed because the transit was not visible from Europe. In 1639, Jeremiah Horrocks and William Crabtree became the first to witness a transit of Venus."

Planetary transits through history

Historically,?planetary transits have offered a rare chance for scientists to learn about the solar system.

In the 18th century, transits of Venus provided astronomers with the first way to measure the absolute size of the solar system, including the distance from the Earth to the sun, which wasn't known at the time. Astronomer Edmond Halley first came up with the method of comparing measurements made from various locations on Earth to triangulate the distances to Venus and the sun.

This technique was successfully put into practice during expeditions to observe the Venus transits of 1761 and 1769 from around the world.

And even as recently as 2006, the transit of Mercury was used to measure the size of the sun. A group of astronomers from Hawaii, Brazil and California used NASA's Solar and Heliospheric Observatory (SOHO) to time the transits of Mercury across sun in 2003 and 2006, enabling the most precise measurement yet of the diameter of the sun.

"Transits of Mercury occur 12 to 13 times per century, so observations like this allow us to refine our understanding of the sun?s inner structure, and the connections between the sun?s output and Earth?s climate," one of the members of the team, University of Hawaii astronomer Jeff Kuhn, said in a statement.

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