An extrasolar planet expert at NCAR's High Altitude Observatory (HAO), Brown has been applying a technique known as spectroscopy to piece together atmospheric data on a planet orbiting star HD209248, located 150 light years from Earth. He found sodium in the planet's atmosphere in 2001 and is now searching for water and carbon monoxide. HAO director Michael Knoelker, who specializes in precision solar spectroscopy, is a coinvestigator on the Venus project.
During next week's transit, Brown will apply the same technique to examine regions of the solar spectrum that are strongly absorbed as they pass through Venus's atmosphere between 65 and 85 kilometers (40 and 53 miles) altitude--a region above the planet's thick cloud layer.
Each wavelength absorbed will indicate the presence of a specific gas and its height in the atmosphere. Brown will also be able to construct wind patterns based on the gases' Doppler shifts, that is, their motion toward or away from Earth.
"This is Venus's first transit in front of the sun since quantitative astronomical spectroscopy was invented," explains Brown, "so it's our first chance to use the technology to observe close up the transit of a planet with an atmosphere." Venus, Mars, and Earth constantly orbit the sun. Only when the orbital planes and positions align in a particular way is a transit across the solar disk by Mars or Venus visible from Earth. Mercury's transits are visible from Earth every few years--its last trip was in November 1999--but whatever atmosphere it may have is far too thin to be observed.
Brown will examine the Venusian atmosphere using a vacuum tower telescope located on Tenerife in the Canary Islands, a Spanish province near the northwest coast of Africa. The 70-centimeter solar telescope and its infrared spectrograph are operated by the Kiepenheuer Institute in Freiburg, Germany. The spectrograph works well for the occasion since the planet's atmosphere is composed almost entirely of carbon dioxide, a gas that absorbs near-infrared radiation.
Scientists have learned much about Venus's atmosphere since its discovery two hundred years ago. In recent times, probes from orbiting spacecraft have measured its chemical composition and temperature at various heights. These data have enabled scientists to construct computer models of the planet's atmosphere. But the region that Brown will be observing has escaped close scrutiny until now.
"I hope to make a substantial addition to knowledge of an atmospheric region not well observed in the past," says Brown. "But also the experiment will be a source of great inspiration and motivation for investigating the atmospheres of extrasolar planets."
Applying spectroscopy to a distant planet too tiny to see directly is a challenge far greater than observing a neighboring planet in our own solar system.
"Observations of extrasolar planets as precise and compelling as those of Venus will not happen in my lifetime or that of my children," says Brown. "What we can see on Venus today previews what will be seen on far distant planets long after I'm gone. You could say it's cheating on time."
Brown believes his own observations of a planet circling the star HD209248 will reveal whether there are significant clouds, dust, or particles in that planet's upper atmosphere, how big the particles are, and what the dust is made of. His observations using the Hubble Space Telescope in 2001 revealed the presence of sodium, and he thinks carbon monoxide may show up soon. Later this year he will continue his search for water vapor around the same planet, again using the Hubble.
The planet is so close to its star that its annual orbit lasts only 3.5 days, and it is perpetually blasted by ultraviolet radiation and thousand-degree heat.
"Whatever can withstand those temperatures is what's there," says Brown. "In other words, no little green men."
Knoelker was a scientist with the Kiepenheuer Institute for many years before coming to NCAR. He has been using the telescope on Tenerife for solar spectroscopy, and he was part of a team that observed the impact of the Shoemaker-Levy comet on Jupiter in 1994.
Brown's other collaborators in the Venus observations are Wolfgang Schmidt and Helmhold Schleicher, both of the Kiepenheuer Institute, and Roi Alonso Sobrino, a graduate student at the Astrophysics Institute of the Canary Islands. NASA and NSF fund Brown's extrasolar planet research.