Backlit Planet: Mercury Transits the Sun

• by Francis Reddy

• April 22, 2003; added global visibility chart May 2

On May 7, the planet Mercury will appear in silhouette on the face of the sun. It's the first of fourteen such events, called Mercury transits, to occur this century. Sky watchers over two-thirds of the globe will see at least a portion of Mercury's solar passage. The transit will begin at 5:13 Universal Time (UT), which is just before sunset in Fiji and New Zealand. It will end just after sunrise in eastern North and South America, at 10:32 UT.

The track of Mercury across the sun on May 7, 2003. Transit maximum occurs at 7:52 UT, when Mercury passes closest to the sun's center. The times shown are for a geocentric observer, but they're accurate to within three minutes for any location on Earth. North is at top; Mercury's size is exaggerated by ten percent. Inset: Europe, the Middle East, Asia and Africa face the sun at transit maximum. Click for the animated version.

The transit will be visible over much of the same area as next year's eagerly anticipated Venus transit. Mercury's tiny disk requires a telescope to see, but Venus will be visible on the sun with the unaided eye. In both cases, though, proper eye protection from the sun's damaging light is absolutely necessary.

Only Mercury and Venus, the two innermost planets of the solar system, can pass between Earth and the sun. In the seventeenth century, scientists realized that transits could be used for determining a poorly known yet essential quantity: our distance from the sun. Although scientists regard these events as little more than interesting novelties today, they will use upcoming transits to calibrate instruments on satellites that monitor the sun. And while transits tell us little about our Mercury and Venus, the discovery of a planet that transits its distant star has given astronomers an important tool for the exploration of other solar systems.

Some portion of the transit will be visible from the illuminated region of this map. It's centered on the Arabian Sea at the point where Mercury lies directly overhead at mid-transit. That's the moment when Mercury reaches its deepest position on the sun. For observers in Japan, the Phillipines, Australia and New Zealand, the sun will set before mid-transit. The transit will be visible in its entirety from most of Europe, Africa, and Asia. From western Africa, western Spain, Portugal, and the eastern Americas, the sun will rise after mid-transit. Click here for a larger image. Similar visibility charts for both of the upcoming Venus transits can be found in Celestial Delights.

For North Americans, the transit ends at sunrise along an arc from Charleston, South Carolina, in the U.S. to Thunder Bay, Ontario, in Canada.

A Posthumous Triumph

Since Mercury passes between Earth and the sun about every four months, it stands to reason that, sooner or later, it must appear in silhouette against the sun's disk. Both Claudius Ptolemy and Nicolaus Copernicus had considered the possibility of Mercury and Venus transits, but Johannes Kepler was the first to make specific predictions. Kepler noticed that Mercury would pass in front of the sun on November 7, 1631, and that, astonishingly, Venus would do the same thing a month later. He alerted astronomers to the events and encouraged observations, but died before his predictions were fulfilled.

On the appointed morning, the French astronomer Pierre Gassendi watched for the Mercury transit from his Paris apartment. He used a telescope to project a magnified image of the sun's disk onto a paper screen in a darkened room. About five hours before the time Kepler had predicted, Gassendi saw something between clouds that looked like a very small sunspot. Could this tiny black dot -- merely one two-hundredth the apparent diameter of the sun -- be the planet Mercury? Its gradual progress across the sun's disk removed all doubt.

"The crafty god had sought to deceive astronomers by passing over the sun a little earlier than was expected," Gassendi wrote. "I found him out, and saw him where no one else had hitherto seen him." The following month he looked for the Venus transit but failed to see it. We now know that it occurred a few hours later than predicted and was not visible from Europe.

Transit Zones

Mercury follows an orbit tilted with respect to the Earth's and crosses the plane of our orbit at only two points. For a transit to occur, Mercury must lie near one of those points when it passes us. To meet these conditions, Mercury must slip between Earth and sun within three days of May 8 or within five days of November 10.

The unequal size of the "transit zones" explains why we experience twice as many transits in November as in May. Mercury lies farther from the sun and moves much slower in May, reducing our chances of catching it in the right place. On the positive side, Mercury's disk appears twenty percent larger during May transits because it's closer to us.

Mercury transits turned out to be poorly suited for establishing the size of the solar system. And while Venus transits held great promise, they occur very infrequently -- on average, only twice every 125 years. Astronomers have observed just five Venus transits since the telescope was invented; the last one occurred in 1882. The rarity and expected payoff of these events prompted the first international scientific expeditions.

Transit observations provided scientists with their first glimpse of the solar system's vast scale. By the end of the nineteenth century, astronomers knew the distance to the sun to about one percent of its actual value, but the greater precision once hoped for never materialized. As astronomers turned to other methods, transit expeditions lost their scientific appeal. Now, with interplanetary spacecraft and radio ranging techniques, we know the layout of the solar system to a precision that astronomers in previous centuries never imagined.

The View from Space

Sun-monitoring spacecraft occasionally catch Mercury on the sun. Astronauts on the last Skylab mission watched the 1973 transit through the space laboratory's x-ray telescope. Twenty years later, an x-ray telescope aboard the Japanese Yohkoh satellite returned images of Mercury gliding across the sun's corona. The observations helped the Yohkoh team fine-tune its knowledge of the spacecraft's position and enabled scientists to calibrate images taken by different instruments.

Both the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE) watched Mercury's 1999 transit. From SOHO's position, the planet missed the sun itself but passed through the corona. TRACE was able to capture Mercury in front of the sun in multiple wavelengths.

Upcoming transits provide scientists with opportunities to learn more about their spacecraft. According to Stein Vidar Haugan, SOHO Science Operations Coordinator for the European Space Agency, three SOHO teams plan to view next month's event:

• Stanford University scientists will image the sun's full disk throughout the transit using the Michelson Doppler Imager. They want to better understand any distortions produced by the instrument and to improve knowledge of motion in the spacecraft's roll axis.
• Scientists at the Rutherford Appleton Laboratory in the United Kingdom will watch with the Coronal Diagnostic Spectrometer. They aim to better understand the performance of the telescope and its gratings to improve their interpretion of the data it returns.
• And NASA scientists at the Goddard Space Flight Center are expected to capture the event using SOHO's Extreme ultraviolet Imaging Telescope. The disk of Mercury would be completely black were it not for light scattered within the telescope, so these researchers will use the transit to improve their models of stray light.

SOHO lies between the Earth and the sun, circling a point where it experiences a more or less equal gravitational pull from both bodies. The transit begins about ninety minutes later for SOHO, at about 7:50 UT, because it will view the sun from a vantage point many degrees from the sun-Earth line.

Searching for Other Earths

Transits of Mercury and Venus tell us nothing about either body that we haven't already learned from spacecraft visits. But since 1994 astronomers have identified over one hundred planets in orbit around distant stars. Like astronomers in earlier centuries, they've turned to transits as a way to study worlds far beyond their reach.

Nearly all extrasolar planets have been found by looking for wobbling stars. An orbiting planet creates a periodic velocity shift in a star's spectrum. Jupiter, for example, causes a detectable wobble in our sun. These changes provide indirect evidence for the existence of planets and allow astronomers to infer their mass and orbital properties.

Transiting planet: A jovian world orbits the star HD 209458. NASA image.

In late 1999, astronomers witnessed the slight dimming caused by a planet moving across HD 209458, a sunlike star 150 light-years away in the constellation Pegasus. The transit provided the first direct and independent confirmation of the existence of an extrasolar planet previously known only from a star's velocity changes. The planet around HD 209458 proved to be a gas giant with about 63 percent the mass of Jupiter. It circles the star at a distance one hundred times smaller than Jupiter does our sun, completing an orbit every three-and-a-half days.

The transits gave astronomers a way to probe the planet, since starlight passing through an atmosphere bears the chemical fingerprints of the component gases. In 2001, astronomers using the Hubble Space Telescope announced that they had detected sodium -- the first measurement ever of any atom in the atmosphere of an extrasolar planet.

Last month, a team of researchers from the Institut d'astrophysique de Paris, the University of Arizona and the Geneva Observatory announced the results of their own Hubble investigation. They found a hot, 120,000-mile-wide envelope of hydrogen gas surrounding the planet. The gas, which is swept back by radiation pressure from the star's light, speeds away at sixty miles a second and gives the planet a long comet-like tail. HD 209458 is slowly evaportating its gas giant.

And astronomers have announced two more transiting planets this year. Both were found as part of the Optical Gravitational Lensing Experiment (OGLE), which monitored five million stars over a period of thirty-two days. The aim of the experiment was to detect brightness changes caused by starlight bent toward Earth by the gravity of an orbiting planet. The study turned up fifty-nine candidates.

Earlier this year, astronomers from the California Institute of Technology and the Harvard-Smithsonian Center for Astrophysics reported velocity changes in one of those stars. They interpreted the slight motions of the star OGLE-TR-56 as indicating the presence of a hot, close Jupiter-mass planet.

Now a team led by Stefan Dreizler of the Institut für Astronomie und Astrophysik in Tübingen, Germany, reports velocity changes for the star OGLE-TR-3. The possible planet, they say, has a mass about 60 percent that of Jupiter and orbits just 2.2 million miles from its star, which gives it the shortest orbital period of any extrasolar planet -- just 28.5 hours. Astronomers expect that the atmosphere is evaporating due to the extreme temperatures on the side of the planet facing its star. That high temperature may allow the planet to be observed directly at infrared wavelengths, in spite of the star's close proximity. If confirmed, the OGLE planets will be the first ones discovered by a search for transits.

Planet-hunters will need a dedicated space-based observatory to detect transits by small Earth-like worlds. The first mission of this type is currently slated for launch in late 2006. Its telescope will gaze steadily at a single star field in the constellation Cygnus for several years, monitoring 100,000 sunlike stars for the telltale sign of a planet in transit. And it's appropriately named Kepler, after the first astronomer to predict the transit of a planet across the face of our star.

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