Discovering New Worlds

Most people think of the local bus or tram service when transit is mentioned. But talk to an astronomer, and you’ll hear an entirely different definition. Astronomically, we observe transits when a planet crosses the face of the Sun, or an extrasolar planet crosses the face of its parent star. When the planet passes through our line of sight (LOS) to the Sun, we see the transit progress as a small, dark dot moving against the brilliance of the Sun.

The Mercury transit in perspective with sunspot
Credit: ESO

From our point of view on Earth, both Mercury and Venus can transit the Sun when the LOS is right. This is not a frequent event. During the 21st century, Mercury will transit the Sun only 14 times even though its orbit carries it past the Earth about every 115 days. That means that out of almost 3200 possible line-ups between Earth-Mercury-Sun in this century, only 14 times does Mercury lie along our LOS to the Sun resulting in a transit.

Venus is farther from the Sun than Mercury, and the opportunity to see a transit of Venus is much more rare. In fact, over the next two centuries, there will be only four transits of Venus across the Sun: June 8, 2004, June 6, 2012, December 11, 2117 and December 8, 2125. The remainder of the time, Venus and Mercury are above or below the Sun as they pass between the Earth and Sun.

Why are astronomers interested in transits?

Dip in brightness as prospective planet transits in front of parent star Credit: ESO

They are both historical and cutting-edge research tools. Since the days of Copernicus, astronomers have been able to estimate the distances of the planets from the Sun in terms of the astronomical unit (AU), the average distance between the Earth and the Sun. It’s a matter of geometry.

But, the problem was that they could not measure the length of the AU accurately. In 1716, Edmond Halley of comet fame figured out how to use a transit of Venus to measure the Sun’s distance. In 1761 and 1769 astronomers observed the transits of Venus, and got a first accurate measurement of the Sun-Earth distance.

Subsequent expeditions repeated the observations, but Venus is tricky. It has an atmosphere, and the timing of the beginning and end of the transit was not as precise as needed as a consequence of Venus "fading" rather than abruptly disappearing from the orb of the Sun. In modern times, radar reflections were used to measure the distance from Earth to Venus, and from that the AU was determined with precision.

Transit observations are also cutting edge research tools. Since 1995, more than 100 extrasolar planets have been discovered by measuring the subtle shift in the spectrum of a star as it wobbles back and forth as the star-planet dance around their common center of gravity. This observed Doppler shift offers evidence of planets we cannot see in the glare of their star’s light.

From these data, we can describe the orbital characteristics and something about the planet: from near circular to wildly elliptical. But we can only estimate the minimum mass for the planet, because we do not know the tilt of the orbital plane.

If the planet transits the star, the uncertainty of the tilt is removed and we can then calculate the mass as well as the radius. Of the 100 plus systems so far discovered, one, HD209548, has been observed in transit by both professionals and amateurs. It has been observed by all manner of telescopes, from amateurs using modest off-the-shelf telescopes and low-end CCD cameras, to major ground-based observatories with custom high-grade cameras, and the Hubble Space Telescope.

Working Class Star

On Labor Day, September 1, professional and amateur astronomers gathered together in Monterey, California for a workshop on observing transits as a part of the Division of Planetary Sciences’ annual conference.

HD209458 was the "star" of the show, to be sure, but the hunt is on for others. The workshop centered on the challenge of measuring starlight with high precision, the art and science of photometry. To see a transit of HD209458, a Jupiter-size planet that whips around it’s sun in just 3.5 days, you have to measure the light from the star to one part in 100, or "1% photometry". Several people present had attempted or actually accomplished these measurements with professional or amateur equipment.

The doppler effect
The farthest known planet ever discovered is a strange world indeed. Whizzing around its star every 29 hours, it is shrouded in clouds made not of water droplets but of iron atoms. This is a world of iron rain.
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics

Dr. Tim Castellano, NASA Ames Research Center, led the workshop, and is a key person on TransitSearch, a project that seeks to enlist students, teachers, and amateur astronomers is searching for the next transiting extrasolar planet. The project runs a web site that offers training, instructions, and targets of opportunity. Tim is there to help you through the teething pains of learning the nuances of doing precision photometry. The group is working down the list of the known extrasolar planet discoveries, seeking evidence of transits to refine our knowledge of these jovian-size worlds.

What’s Next

Transit observations are at the heart of a current NASA Discovery mission named Kepler, after the astronomer who discovered the laws of planetary motion now named for him. The Kepler Mission is being designed and developed by scientists and engineers at NASA Ames Research Center, Jet Propulsion Laboratories, and Ball Aerospace Technology Corporation with a host of universities and research organizations, including SETI Institute, working in close collaboration with the team.

When launched in 2007, the Kepler satellite will view more than 100 square degrees of the sky (500 times the area of the Moon) to continuously measure the brightness of 100,000 stars for at least four years. Kepler will seek evidence of planets by searching for the subtle drop in brightness as a planet transits its parent star. It is looking at lots of stars to assure that some planets will be seen in transit-they will be on our LOS to the star.

In space, the Kepler Mission can achieve 100 times better photometric precision than Earth-based telescopes looking through the atmosphere, and this will allow the team to search for much smaller planets, like Earth. The Earth is 100 times smaller in area than Jupiter. By observing for at least four years, Kepler has the opportunity to discover and confirm through detection of repeated transits that there are Earth-size planets in Earth-like orbits around other suns. The Kepler Mission may be the discoverer of worlds like our own.

Observing transits once offered us the tools to measure our own solar system, and now, transits are at the heart of a cutting-edge NASA Mission seeking planets that could host life elsewhere in our galaxy.

Related Web Pages

Mercury and Venus Transits
Transit Search
NASA Kepler Mission
European Southern Observatory
Astrobiology Magazine New Planets
Transit Search
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Eddington Mission
Darwin Mission
Herschel Mission
Space Interferometry Mission