How Venus Transit of Sun in June May Help Find Alien Planets

Venus Transit Sun
Three views of Venus' solar transit in 2004, taken by NASA's sun-observing TRACE spacecraft. The top image is in visible light; the view on bottom left is in the ultraviolet, and the one on bottom right is in the extreme ultraviolet. (Image credit: NASA/LMSAL)

Observations of next month's historic Venus transit may eventually help astronomers spot and study alien planets circling faraway stars, one prominent researcher says.

On June 5, Venus will cross the face of the sun from Earth's perspective — the last time it will do so for 105 years. But the upcoming Venus transit of the sun is more than just a rare skywatching treat; it's also a great opportunity to hone our techniques for hunting down and characterizing alien planets.

"We're trying to do as much as we can to use the transit of Venus to understand exoplanets and their atmospheres," Jay Pasachoff, of Williams College, told

Pasachoff wrote a commentary in this week's issue of the journal Nature, which appeared online today (May 16), detailing the research opportunities the Venus transit provides. [The 2012 Venus Transit: Complete Coverage]

Searching for exoplanets

Looking for transits is one of the most productive ways to find alien planets. NASA's Kepler space telescope, for example, has detected roughly 2,300 exoplanet candidates using this method, which flags the telltale dips in a star's brightness caused by a transiting planet.

The vast majority of these potential planets still need to be confirmed, but Kepler scientists estimate that at least 80 percent of them will end up being the real deal.

One potential issue with the transit method is that brightness dips can be caused by a variety of factors other than light-blocking planets. For instance, dark patches known as starspots — akin to the sometimes massive sunspots seen on our own sun — can reduce a star's luminosity slightly.

Next month's Venus transit comes during an active period in the sun's 11-year activity cycle, and it's likely that some sunspots will darken the solar surface on June 5. So the transit could give astronomers practice in picking up a planet's signal around a spotty, variable star, Pasachoff said.

The last Venus transit, in 2004, didn't offer that opportunity, since it occurred during a quiescent phase in the solar cycle when the sun's face was largely spot-free. (Venus transits occur in pairs eight years apart, but these dual events happen less than once per century.) [Venus Transit of 2004: 51 Amazing Photos]

Characterizing planets

Venus' trek across the solar disk may also eventually help researchers better understand exoplanets and their atmospheres, Pasachoff said.

For starters, careful study of the transit will allow astronomers to calculate Venus' diameter, which is already known. By comparing the two numbers, scientists may get a better idea of how accurately this technique can be used to estimate exoplanet sizes.

Further, scientists who train their instruments on the transit can learn a great deal about the nature and composition of Venus' thick atmosphere. Because researchers already know quite a bit about Venus' air, the transit could serve as a sort of calibration exercise for future exoplanet studies.

Pasachoff and his colleagues are planning to do some work of this sort. To look for carbon dioxide, a major component of Venus' atmosphere, they'll put a new filter over a massive spectrograph at the National Solar Observatory in New Mexico.

"This will provide a unique, detailed spectrographic study of a relatively well-known atmosphere during a transit, which we can compare to studies of unknown exoplanet atmospheres," Pasachoff writes in the Nature commentary.

And it's not as if scientists know all there is to know about Venus' air. Pasachoff is working with researchers around the world to observe the transit with multiple instruments, in an effort to learn more about the nature and evolution of Earth's hellishly hot "sister planet."

"We can use this in liaison with the data coming from the European Space Agency's Venus Express [spacecraft] to understand an intermediate level of Venus' atmosphere better than we can from the spacecraft alone, or from the transit alone," Pasachoff told

A call to action

Pasachoff's commentary in Nature is essentially a call to action. He's urging his colleagues to take full advantage of the Venus transit, even if they're not exactly sure how useful their data will end up being.

"It is too soon to know exactly how the study of transits in our solar system will help us to interpret observations of distant exoplanets, but transits are so rare that to squander these opportunities would be a crime," Pasachoff writes.

"We owe it to future astronomers — especially those who will observe the next transit of Venus, in 2117 — to collect as much data as possible," he adds. "One never knows what will prove vital to future research."

Some other astronomers apparently feel the same way. NASA's Solar Dynamics Observatory spacecraft, for example, will watch the seven-hour transit, partly to calibrate some of its instruments and partly to learn more about Venus' atmosphere.

And NASA's Hubble Space Telescope will observe the transit too, albeit indirectly. Because Hubble is too sensitive to be pointed anywhere near the sun, it will use the moon as a mirror, studying light that bounces off Earth's nearest neighbor. The goal is to determine the makeup of Venus' atmosphere, testing out a technique that astronomers could use to study far-off exoplanets.

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Mike Wall
Senior Space Writer

Michael Wall is a Senior Space Writer with and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.