Of Planetary Transits Near and Far

By Keith Cowing
November 14, 1999
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This coming week (15 November) the planet Mercury will pass directly between our line of sight on Earth, and the sun. This so-called “transit’ of the sun by Mercury happens at regular intervals every 3 to 13 years. Venus does this too – but not nearly as often.

These transits of the sun’s face by Mercury and Venus have long fascinated astronomers who have often gone to great lengths to observe them. Over the centuries, their value as astrometric tools has faded. Now, this old tool has found new uses – although this time the planets and stars are light years away.

Camp VenusOff to Tahiti – on business.

Edmund Halley, the legendary astronomer, predicted that Venus would transit the sun on 3 June 1769. The Royal Geographic Society seized upon this opportunity as a means whereby the distance between Earth and the sun could be more accurately measured. This was to be done by placing trained observers at three locations around the world and having them all time the beginning and end of the event. One of the locations chosen was an island in the Pacific Ocean: Tahiti.

The leader of the expedition was Captain James Cook who carried the astronomers to Tahiti aboard his ship “the Endeavour” (the namesake of the current Space Shuttle orbiter). Upon their arrival in Tahiti in April 1769, Cook and his men proceeded to prepare for the transit by building “Fort Venus”. The transit was later successfully observed at the time predicted by Halley.

A month later, Cook left Tahiti and proceeded to explore the Tahitian islands further, moving on eventually towards New Zealand. Sighting a new comet along the way, Cook eventually anchored in what he called “Mercury Bay” named for a transit of Mercury to be observed by Cook’s astronomers on 9 November 1769. A few days after the transit Cook was so taken with the place, that he decided that England should own New Zealand.

SOHOBack row seats can often be the best seats.

230 years later, on 15 November 1999, Mercury will again transit the face of the sun. The transit will begin at 21:15:01 UTC and will end at 22:06:47 UTC. People in North and South America will be able to see Mercury transit the sun’s face as it sets. People in the Pacific should see a portion of the event.

As was the case in 1769, astronomers will have an observer at a remote location taking measurements. This time the observer is ESA’s SOHO, the Solar and Heliospheric Observatory spacecraft that orbits between the Earth and the sun at a distance of 1.5 million km from Earth.

SOHO will not have the best seat in the solar system, however and will not be poised to see the actual transit by Mercury of the sun’s face. It will, however, be able to view Mercury as it transits across the sun’s corona. By knowing the precise location of Mercury, astronomers will be able to gauge what light (the light being blocked by Mercury’s transit) comes from the corona itself. This has significant value since the case the sun’s glare often makes it difficult to separate coronal light from light from the sun’s surface.

Old tool; new application.

Just as the predicted transits of known planets help us to further understand our local star, similar predictions of transits in other solar systems are of equal value. Up until this past week, the presence of planets circling other stars was done by indirect means. Highly precise observations of stars, and the way in which they “wobble” (changes in their radial velocity due to the gravitational interaction with object in orbit) has been used to determine the existence of companions orbiting these stars. A straightforward application of laws developed by Kepler and Newton centuries ago allows the mass and orbital characteristics of these unseen planets to be determined. We know they are there – we just haven’t seen them – until now.

Enter veteran extrasolar planet hunters Geoffrey Marcy, Paul Butler and Steve Vogt.

Artist's conceptGeoffrey Marcy is a professor of astronomy at the University of California, Berkeley. Paul Butler works at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington in Washington, D.C. Steve Vogt is affiliated with UC Santa Cruz and Lick Observatory.

On 5 November 1999 this team’s observations of the star HD 209458 showed it to wobble in the fashion indicative of having a large Jupiter-class companion. Unlike all of the previous systems discovered by Marcy and Butler, this solar system was oriented edge-on with respect to our solar system allowing eclipses and transits to be observed – if the observers were lucky enough to be watching at the right time. They were.

HD 209458 is 153 light years (1.4 million billion kilometers or 859,000 billion miles) away from us. It is located in the constellation Pegasus and is of similar size, color, and age as our sun.

The planet was calculated to orbit its sun once every 3.523 days. Given that the planet was orbiting its sun in a fashion where it could possible transit across its face, observations were immediately set up with the 0.80 meter Automatic Photoelectric Telescope (APT) operated by Tennessee State University in Nashville. On 7 November, a 1.7% decrease in observed output from HD 209458 was observed – exactly when it was predicted that the unseen planet would be passing between earth and the star. Further observations will be made so as to confirm and further refine these measurements.

Not only does this concurrence of predicted and observational data confirm the existence of a planet-sized object, it also allows a far better determination of its actual size and density to be made. While the planet’s mass is 63% of Jupiter’s, its radius is 60 percent bigger than Jupiter’s. This planet has apparently been heated by virtue of its close proximity to its parent star in such a fashion as to swell up to a size much larger than that of Jupiter.

As is the case with many previous extrasolar planet discoveries, this is another “hot Jupiter” orbiting very close to its parent star. The abundance of this type of planet among those currently discovered might lead one to think that this is all that is to be discovered. However, this is more likely an artifact of the means of discovery. Large objects orbiting close to a star produce much more noticeable wobbles than would smaller Earth-sized objects greater distances away. Moreover, given that such large planets could not form so close to their parent star (or so current theories hold) some process must be at work to cause these large planets to spiral inwards into close orbits about their parent star.

Refinements in the analysis of the stellar “wobbles”, plus existing imaging technologies such as used in this discovery – and those to be offered by the Next Generation Space Telescope, DARWIN, and the Terrestrial Planet Finder will likely lead to the discovery of smaller worlds more like our own.

What’s Old is New

“Local” transits (Mercury and Venus) are now rather unexciting but they may become popular again in the future: when humans come to live on Mars. Transits of Earth across the face of the sun as seen from Mars are sure to be popular events – especially the first one we witness – in person.

Further Information:

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.