Mark Anderson has an M.S. in astrophysics, is a contributor to Discover, and has written about science and history for many other publications. His new book The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus has just been published by Da Capo.
Also see Paul Raeburns’s explanation of what investigating Venus can teach us about our own planet.
The 2004 Venus transit at sunrise
On Tuesday afternoon—for those in North, Central and parts of South America—the planet Venus will pass directly in front of the sun for seven hours. This rare spectacle, called the Venus transit, occurs twice within a decade, then not again for more than a century. But as fleeting as they are, transits of the past provided invaluable information about our place in the solar system—and, astronomers hope, this transit could help us glean more information on planets elsewhere in the galaxy.
In the 1760s, some of the age’s top explorers and scientists collaborated on dozens of expeditions across the planet to observe the Venus transit. These voyages launched the legendary careers of Captain Cook and the surveyors Mason and Dixon. The expeditions also represented the world’s first big science project—forefather to today’s Large Hadron Collider and Human Genome Project, in which an international community of hundreds or thousands collaborates on a single fundamental scientific problem at the frontier of human knowledge.
In the balance hung two of the greatest scientific and technological puzzles of the 18th century: discovering the Sun’s distance from the Earth and finding one’s longitude at sea. The transit itself briefly enabled astronomers to become interplanetary surveyors triangulating distances across the solar system. The first problem could be solved. And the voyages to observe the transit—because of both their globetrotting nature and the precision timekeepers and navigational aids each expedition carried with them—became the perfect test labs for the second problem. So while the observations made from the destinations of these voyages solved the first problem, the journeys themselves helped solve the second. Astronomy was the essential way-finding science of the times, cracking open the field of navigation and heralding a new age of global expedition, trade, and conquest.
Today, the distance to the Sun is known with pinpoint accuracy and global navigation is easily guided by GPS. But at approximately 6:10 p.m. EDT on June 5, astronomers around the world will still be training their telescopes on this year’s transit, this time with a new objective in mind.
New planets outside the solar system are being discovered practically every week. Some 770 such exoplanets have been confirmed, with three times as many possible discoveries awaiting further investigation. This is a staggering menagerie of alien worlds across the Milky Way—especially considering that only 20 years ago the known exoplanets in the universe totaled just two.
From among that ever-growing list, astronomers hope to find out which planets are like most similar to our own—and which might have suitable conditions suitable conditions to support life. To know that, they have to determine not just each planet’s size or how far it is from its star, but what’s in its atmosphere. Venus, for instance, has 82% of the Earth’s mass, measures 95% of its size and orbits 72% of Earth’s distance from the sun. So judging by these facts alone, it might appear that Earth and Venus are close cousins, if not quite twins. But whereas Earth’s atmosphere is more than one-fifth oxygen, with much of the rest nitrogen, Venus’s is made up of more than 95% carbon dioxide—-making the daily high on Venus around 870 degrees Fahrenheit (470 degrees C), far too hot for liquid water or life as we know it.
So learning about an exoplanet’s atmosphere is vital to determining its habitability. Here is where present-day Venus transits can help the modern exoplanet search just as the 18th century transits proved so essential to understanding our own solar system.
When Venus passes in front of the Sun, a little bit the Sun’s light also passes through the tiny ring of Venus’s atmosphere at the planet’s outer edge. Just 0.001% of the Sun’s light during the Venus transit zips through our neighboring hothouse planet’s atmosphere on its way to Earth.
But isolating and examining that 0.001% of the sun’s light in detail will be very important. When Venus crosses the sun’s face during a transit, that 0.001% of the light carries spectral signatures—absorption lines—as tiny mementos of its passage through Venus’s atmosphere. During the 2004 transit, a team of nine French, Swiss, American, Spanish and German astronomers discovered by examining these absorption lines not only a strong signature of carbon-dioxide in the Venusian atmosphere but even the characteristic windspeeds at various altitudes above the planet’s surface. They spied on Venus’s weather from across the solar system.
In 2009, NASA launched the Kepler spacecraft, a dedicated exoplanet-finding, space-based telescope. Kepler performs a kind of exoplanetary stake-out, continually keeping its field of view trained on 145,000 stars in the constellations Cygnus, Lyra, and Draco. It primarily looks for exoplanets transiting their host stars. Astronomers then perform follow-up studies of the candidate exoplanets that Kepler finds. Using observations such as the 2004 Venus transit data as a guide, they can then examine the composition and characteristics of exoplanets’ atmospheres—wherever such atmospheres might be found. Using spectroscopic techniques developed and tested during this century’s two Venus transits—the one that happened in 2004 and the one happening this week—astronomers may in the future be able to make exoplanetary weather reports not just across the solar system but across the galaxy.
In December 2011, nine American and French astronomers wrote a letter to the journal Astronomy & Astrophyisics urging their colleagues to perform careful measurements of this week’s Venus transit. “The June 2012 transit will be a unique occasion,” these scientists note, to study “a planet that was long believed to be the Earth’s twin sister—[and] to discriminate between Earth-like and Venus-like atmospheres of exoplanets transiting their stars.”
Even the Hubble Space Telescope, which would burn out its optics if it looked directly at the Sun, will be studying the Sun’s light reflected off the surface of the moon during the transit. Hubble too will be combing through the light it collects for the 0.001% of the signal that contains telltale fingerprints of Venus’s atmosphere.
Perhaps by the next transit of Venus, in December 2117, the present-day problems that the transit illuminates might be solved. The galaxy might teem with sister earths, or it could turn out to be astonishingly barren: Anything is possible. But one profound fact remains. After three centuries of Venus transit observations, much is achievable with a little help from a nearby planet.
Tips for Transit Spotters
This year’s Venus transit begins on June 5 at approximately 6:00 p.m. Eastern Daylight Time and will last just under seven hours. NASA has set up a website providing more information about where and when the transit can be seen.
A word of warning for would-be Venus transit watchers: Like the Hubble’s delicate cameras and optics, the human eye can sustain serious and permanent damage from looking directly at the sun. Don’t do it. (Crucially, if you have the advantage of viewing the transit through a telescope or binoculars, only do so if a solar filter is attached to the device. Solar filters typically look like a piece of aluminum foil that fits over the telescope or binoculars’ front end.)
It’s best to see the transit through a telescope. So for most people, that means seeking out local astronomy clubs, schools or other groups who will be setting up their own equipment to view the transit. Or, if you have access to #14 welder’s glass, that’ll do the trick for naked eye observing. And there are easy projection techniques using a pinhole camera or binoculars, too.
Image courtesy of Jimmy Westlake