ON JUNE 6, 2012, the planet Venus will pass between Earth and the Sun. The start of this event, known as a "transit of Venus," can be observed from everywhere in North America. To see the entire transit you'll need to be in the western Pacific Ocean or in the countries on the west shore of the Pacific. If you miss this rare event, you'll have to wait 105.5 years for the next one.

That's the headline news. But as usual, the headline is not the whole story. In this column, we'll tell you about the transit of Venus, one of the rarest planetary alignments. We'll tell you what past transits revealed about Venus and our solar system. Then we'll tell you how transits of planets outside our solar system are helping astronomers answer one of the biggest science fiction questions of all: Is there life on other planets?

THE SAD TALE OF GUILLAUME LE GENTIL

Here on Earth, we can see transits of Venus and Mercury when these planets pass between Earth and the Sun. If you want to see a transit of Earth, you have to be on Mars or some more distant planet. In fact, Arthur C. Clarke published a short story called "Transit of Earth," about a dying astronaut on Mars who observes the transit of Earth and Moon across the face of the Sun. Clarke did his homework—astronomers confirm that an astronaut on Mars on May 11, 1984 would have been able to observe a transit of Earth.

Since planets orbit in a regular pattern, transits can be predicted. The orbits of Earth and Venus are such that transits are oddly spaced in a repeating cycle. Transits of Venus are separated by 8 years, then 121.5 years, then 8 years, and then 105.5 years. So transits occurred in 1761, 1769, 1874, 1882, 2004, and will occur in 2012.

It takes several hours for the small black disk of Venus to cross the face of the Sun. Back before the 1761 transit, astronomers recognized that the transit of Venus offered an opportunity to learn more about the size of the solar system. If they could precisely time how long it took Venus to complete the transit, they could use Kepler's laws of planetary motion and an understanding of parallax to calculate the distance from the Earth to the Sun more accurately than ever before.¹

In one of the first examples of international scientific collaborations, eighteenth-century transit-watching expeditions traveled to Siberia, Norway, Newfoundland, Madagascar, the Cape of Good Hope, Baja California, and Saint Petersburg. Captain James Cook and his crew observed the transit from Tahiti.

Pat's favorite transit observer was an unfortunate French astronomer named Guillaume Le Gentil. He planned to observe the transit from a French colony in India. But various difficulties delayed his arrival in the colony and Guillaume ended up watching the 1761 transit from a rolling ship where taking accurate measurements was impossible. Rather than return home to Paris, Guillaume decided to wait in India for the next transit in 1769. When the day of the 1769 transit finally arrived, clouds covered the Sun and poor Guillaume didn't see a thing. Guillaume made his way home to Paris (delayed further by dysentery and storms), only to find he had been declared dead, his wife had remarried, and his relatives had divided up his estate.

THE BLACK DROP

Astronomers with better luck than Guillaume made observations of Venus's 1769 transit from 76 locations around the globe. Sadly, even the astronomers who had good viewing conditions could not get precise measurements of when the transit began and ended. Their measurements suffered from an unexpected problem known as "the black drop effect."

Astronomers call the moment when the disk of Venus just touches the Sun "first contact." Venus moves farther and farther into the bright solar disk until it becomes tangent to the edge of the Sun, a point in time called, of course, "second contact." However, as the edge of the planet moved past second contact, it appeared to be connected to the darkness of space next to the Sun by a black bridge. The black disk of Venus looked like a drop of water detaching from a faucet, and so the black bridge was named the "black drop." The black drop made it difficult to get the timing of second contact precisely right. The black drop occurred again at "third contact," when the edge of the planet reached the far side of the Sun.

You can see something like the black drop without waiting for an astronomical event. Close one eye and hold your finger and thumb in front of your open eye. Slowly bring the finger and thumb together. Just before they touch, you'll see a shadowy bridge jump into being in the gap between your finger and thumb.

What causes the black drop? It seems to be an optical effect caused by imperfect telescope optics looking through an imperfect atmosphere.

Though the black drop thwarted those hoping for precise measurement, Russian scientist Mikhail Lomonosov made one very important observation during the 1761 transit. He noticed that Venus developed a halo as it approached the Sun. Lomonosov realized that Venus had an atmosphere—sunlight bending as it passed though that atmosphere created the halo.

In 1874 and 1882, European scientists again traveled to far corners of the globe to observe the transit of Venus. In 1761, the black drop effect had taken astronomers by surprise. Nineteenth-century astronomers were prepared for it and decided that the true time of second contact is halfway between the formation of the black drop and the breaking of the thread, a determination that matches modern estimates. Timing the transit using this assumption provided a more accurate estimation of the radius of Earth's orbit.

For Paul, the black drop effect is a reminder that you need an excellent quality telescope to observe some astronomical events. For those observing the 2004 transit of Venus with large aperture modern telescopes, the black drop was mostly absent. Astronomers also get very clear views of transits and other astronomical events from telescopes in space that don't suffer from atmospheric interference.

Today's astronomers don't need measurements made during the transit to determine the radius of Earth's orbit. That's been calculated by measuring the distance between the Earth and Venus by bouncing radar signals off Venus. Once astronomers know the distance from the Earth to Venus they can use Kepler's laws to calculate the radius of the Earth's orbit, the very important distance known as the astronomical unit or AU. But the 2004 transit offered other opportunities to gather information. During the transit, scientists used spectroscopy to analyze the light that passed through the upper atmosphere of Venus to discover its composition (mostly carbon dioxide) and the speed of the wind in the upper atmosphere.

DISTANT WORLDS

All these observations were a great rehearsal for studying the transits of planets orbiting stars other than the Sun.

We Earthlings can see transits of Mercury and Venus—but that's not all. You see a transit when a planet passes between you and the star around which that planet is orbiting. Planets orbiting stars other than the Sun can pass between Earth and their own star. Astronomers looking for planets outside our solar system use transits to detect them.

When a planet passes between us and the star it orbits, that transit dims the light from the star just a bit. During a transit of Earth viewed from a distant star system, the light from the Sun will dim by 0.01 percent. That is, the light will drop from 1.0000 to 0.9999. An Earthlike planet transiting a distant star will do the same thing. We have become very good at measuring the exact amount of light coming from a star. If an Earth-sized planet transits a star, we can measure the decrease in light.

That brings us to the Kepler mission. As spacecraft go, Kepler (referred to variously as an astronomy probe and a space observatory) doesn't sound terribly spectacular. This craft is basically a set of light-sensing instruments that's orbiting the Sun just behind Earth, in what's called an Earth-trailing solar orbit. For the duration of its four-year mission, Kepler will detect light from a single group of stars and note fluctuations in that light.

The spacecraft may not sound very exciting, but the mission is a science fiction writer's dream. Through those fluctuations in light, astronomers can determine when planets are passing between the Earth and one of those stars. The Kepler mission is to find Earth-sized planets, worlds that could be similar to ours. Essentially, we're looking for planets that could support life.

SO MANY POSSIBILITIES

Keep in mind that the probability that Earth will be positioned to observe the transit of any given planet is low. When looking for transits of extrasolar planets, the good news is that we are farther from their sun than any of these planets—so we can see a transit. But the bad news is that it is unlikely that the orbit of the planet will actually take it across the disk of its sun as viewed from Earth.

Since transits are so unlikely, how can Kepler expect to find anything at all? The answer is in the numbers. The Kepler spacecraft has a ninety-five-megapixel camera that allows it to measure precisely the intensity of 150,000 stars at the same time, repeating this measurement every thirty minutes. Since a transit of Earth would usually last over ten hours, there's plenty of time to catch a transit in progress.

The first Kepler data was released in 2009 after a mere six weeks of observing. In this period, Kepler detected five possibilities that have been confirmed to be planets by follow-up ground-based observations. (Two objects in transit that were hotter than their stars are probably white dwarf stars, not planets.)

The orbital periods of the first five planets found by Kepler are all under five days. These planets whiz around their parent stars in small, tight orbits. They are big and hot, with masses that range from 25 to 630 times the mass of Earth and temperatures over 1500 K. These planets are called "hot Jupiters," and their existence is messing with what astronomers think of as a "normal" solar system.

By June 2010, Kepler scientists had found 706 possible planets and released their observations of 306 of them. In February of 2011, they will release the data on the remaining 400. The 306 candidates include possible planets ranging in size from the size of the Earth to ones larger than Jupiter. They include five possible multi-planet systems. Scientists will follow up on these observations with more Kepler data and data from Earth-based observatories to see if the candidates really are planets. From this preliminary data, scientists estimate that there are at least 100 million habitable planets in our galaxy.

The Kepler spacecraft waits until it has seen three successive transits before it reports a candidate planet. The timing of successive transits gives us the length of the year on the transiting planet. All the candidates released in the first Kepler study completed three transits in less than two weeks. They all have very short orbital periods. During its lifetime of three-plus years, Kepler should be able to find planets with a one-year orbital period—just like our Earth.

BUT WAIT…THERE'S MORE

Kepler detects planets by watching for transits, but that's not all. With the information that Kepler gathers, scientists can infer a great deal about a planet.

Stars on the main sequence (where stars spend a long portion of their life) come in spectral types O, B, A, F, G, K, and M. A star's type is determined by analyzing the light that the star emits. (The Sun is a G type star.) Spectral types are ordered according to the temperature of the star, with O type being the hottest (over 30,000 K) and M type stars being the coolest (under 3700 K). Once scientists know a star's spectral type, they know its approximate temperature, mass, and size.

If we know how long it takes a planet to orbit a star and we know the mass of the star it orbits, Kepler's laws can be used to calculate the radius of the planet's orbit. In addition, the larger the planet, the bigger the area of its disk, and the more light it blocks. From this and our knowledge of the type of star, which tells us its size, we can find the radius of the planet.

The spectral type gives us the mass of the star, but what about the mass of the planet? Planets orbit their stars because the stars exert gravity forces on the planet. Newton's third law states that for every action there is an equal and opposite reaction. So when the star's gravity pulls on the planet, the planet also pulls on the star. This causes the star to move in a small circle as the planet orbits the star. As the star circles, sometimes it moves toward Earth and sometimes it moves away.

If the orbiting planet is the size and mass of Earth, the star doesn't move much. But it does move. When the star moves toward the Earth, the light from the star is shifted toward the blue end of the spectrum. When the star moves away, its light is shifted toward the red end of the spectrum. This shift is known as the Doppler shift, and it's the same phenomenon that makes a siren seem to change pitch as the cop races past you to nail some other speeder.

The star's very small movement makes for a very small change in the light—a change that observatories on Earth can detect. The Doppler shift lets scientists determine how much the star is moving. And that, in turn, leads to a very important inference. The distance the star moves depends on the mass of the planet, the mass of the star, and the angle of the orbital plane with respect to Earth. (A planet orbiting in a plane perpendicular to the line of sight from the Earth would produce no Doppler shift.) The transit observations give the inclination of the orbit.

Putting all this together, scientists can estimate the true planetary mass. In combination with the data on the radius of the planet from Kepler, scientists can find the density of the planet—and that reveals whether the planet is made mostly of iron or rock, or of water, or of hydrogen and helium gas.

LOOKING FOR GOLDILOCKS

So astronomers can figure out the temperature, size, and mass of a star and the distance from the star to the planet. With all that info, they can estimate the planet's surface temperature (or, for planets that don't have a clearly defined surface, the temperature of the cloud tops). The temperature is just an estimate, of course—the reflectivity of the planet's surface and the greenhouse effect of any atmosphere will affect the actual temperature. But the estimate is a start.

Astronomers and science fiction writers are particularly interested in planets whose surface temperatures are between the freezing and boiling points for water. We want to find more planets like the one we call home. Planets too close to their stars are too hot to allow liquid water on the surface. Planets too far are too cold and covered with ice. Planets at just the right distance are "just right" and are commonly called "Goldilocks Planets." (You could also say they "orbit within the habitable zone," but that's not nearly as colorful and cozy.)

Once we find a planet of the right size and composition that is orbiting a star at the right distance, we can look for evidence of life there—even if the star is hundreds of light-years away. We just have to wait for a transit and observe the spectrum of the starlight and how it changes when the planet is present. A change in the star's spectrum will be due to the starlight passing through the atmosphere of the planet. Specific changes in the spectrum reveal the makeup of the planet's atmosphere.

So far astronomers have seen evidence of water vapor, sodium vapor, carbon dioxide, and methane in the atmospheres of extrasolar planets. But what we are really looking for is oxygen.

Why oxygen? Because oxygen is a reactive gas. It oxidizes iron to make rust, oxidizes copper to make a green patina. Because oxygen is reactive, it doesn't stay in a planet's atmosphere for long—unless it's constantly replenished.

The Earth's atmosphere contains oxygen. That's mainly because plants have evolved to use the energy in sunlight to release oxygen into the atmosphere. If we see evidence of oxygen in another planet's atmosphere, that implies the presence of life—and that's what we are really looking for. Not because we're on the lookout for future real estate. The mantra of real estate speculators is "location, location, location," and planets that are ten or more light-years away fall into the geographically undesirable category if we're looking for a place to colonize. But because it would mean that maybe, just maybe, we're not alone. Kepler isn't looking for intelligent life. That's another story. Rather, it's just searching neighboring stars for Earth-like planets with conditions that are right for life that's kind of like ours.

Science fiction writers have been speculating about life on other planets for more than a hundred years (with Jules Verne writing about Moon men back in 1865). We figure it's about time that science caught up.

And thanks to the work of many astronomers, science is catching up. In September 2010, as we were writing this column, scientists using the W. M. Keck telescope in Hawaii discovered the first Goldilocks planet. Orbiting a red dwarf star known as Gliese 581, the planet known as Gliese 581g is four times the mass of the Earth, and it's right in the middle of the habitable zone.

Paul and Pat are monitoring the discoveries of the Kepler mission and looking forward to the discovery of many more Goldilocks planets that are just right for the development of life outside our solar system.

¹ Those who want to see the details of how this is done (a group that does not include Pat) should visit Paul's Exploratorium transit website http://www.exploratorium.edu/venus/P_question4.html

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