On June 6, 1761, Russian scientist Mikhail V. Lomonosov anxiously waited at his Saint Petersburg home to catch a cosmic apparition: the passage of Venus, Earth's closest neighboring planet, across the giant, yellow disk of the Sun. If he missed this chance, he would have only one other opportunity to witness Venus's rare trip. The planet's infrequent journeys across the Sun's face, called a transit, had become so fascinating that several countries paid their astronomers to travel around the world to get the best view. But Lomonosov didn't have to travel. The transit of 1761 was right in his back yard. (Interestingly, the next transit of Venus on June 8, 2004, will follow a similar path across the southern hemisphere of the Sun.)
As the Russian scientist watched the tiny, rocky planet begin its fleeting trip across the edge of the Sun, he noticed something odd: a wisp of light, which he called "hair-thin luminescence," around the trailing edge of the planet that hadn't as yet passed in front of the Sun. This was a surprising phenomenon. What could have caused this bright light to appear on the silhouetted disk? Lomonosov reasoned that this luminescence was actually sunlight that had been bent by the planet's thick atmosphere. Scientists had theorized that Venus possessed an atmosphere. But Lomonosov was the first to observe it.
Nearly 250 years later, astronomers took advantage of another planetary transit to make the first detection of an atmosphere around a planet outside our solar system, called an extrasolar planet. The planet is a puffy gas giant and slightly less massive than Jupiter. It orbits the Sun-like star HD 209458. Before this landmark discovery, no astronomer had detected an atmosphere around an extrasolar planet. And they had found plenty of extrasolar planets, nearly 80. All of them are massive planets like Jupiter, but until the Hubble observations, their gaseous atmospheres were guessed at, not seen. The planets were detected indirectly by finding their ghostly gravitational footprints, a slight wobble in their parent stars as the planets orbited around them.
But scientists have witnessed only the planet orbiting HD 209458 journeying across the giant face of its star, briefly dimming the starlight. Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph, a team of astronomers made their discovery by measuring how the starlight filtered through the planet's atmosphere. The team is led by David Charbonneau of the California Institute of Technology in Pasadena, CA, and the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, Timothy Brown of the National Center for Atmospheric Research in Boulder, CO, and Robert Noyes of the Harvard-Smithsonian Center for Astrophysics. Imprinted on the light was the spectral fingerprint of sodium, a chemical element found in the atmospheres of stars and brown dwarfs, and common in the Earth's crust.
The detection opens a new frontier in the planet-hunting chase, paving the way for astronomers to find chemical elements such as oxygen, the signature of life, on other planets.
"To search for evidence of life on planets, we must chemically sniff out their atmospheres," said Ronald Gilliland of the Space Telescope Science Institute and a member of the team that made the discovery. "Hubble has shown that the transit technique actually works to make those detections."
In the Hubble study, finding sodium does not indicate that life exists on the planet orbiting HD 209458. But then, the conditions on this extrasolar giant seem inhospitable for life. How could life thrive on a gassy planet that is 20 times closer to its parent star than the Earth is to the Sun? The star heats the planet's atmosphere to a scorching 2000 degrees Fahrenheit (1100 degrees Celsius).
Using current telescopes, the transit technique can only locate large gaseous planets like Jupiter, the largest planet in our solar system. These gas giants are hundreds of times more massive than Earth and, unlike Jupiter and Earth, reside dangerously close to their parent stars, completing an orbit in a matter of days. Jupiter completes a circuit around the Sun in 12 years; Earth, in a year. About a fifth of the known extrasolar planets are called "hot Jupiters" because they are at least 10 times closer to their stars than the Earth is to the Sun.
Finding planets that can sustain life - terrestrial planets like Earth - will require refining the transit technique and building more sophisticated ground- and space-based observatories.
Although astronomers may be years away from detecting terrestrial planets, they are anxious to probe the atmospheres of the massive planets they have been discovering with such a flourish since the first detection in 1995.
So far, however, astronomers only have one extrasolar atmosphere to sample - the gas giant orbiting HD 209458. That's because the view from Earth does not allow astronomers to find every transiting extrasolar planet. For astronomers to detect a transit, the planet's orbit must be tilted edge-on as seen from Earth. In fact, astronomers have only a 10 percent chance of catching these gas giants transiting a star. The few planets that are caught crossing in front of their much larger companions cause only the tiniest dip in starlight - about 1 percent.
So, astronomers must search a myriad of stars for several days to find a few transiting planets. The entire crossing lasts only a few hours. Several teams of astronomers have taken on the challenge, planning surveys of thousands of stars to track down the elusive transits.
To find transiting planets, the teams use simple, ground-based telescopes with commercial camera lenses. Brown leads one such team, called STARE. Charbonneau also is building a telescope, which should be ready in the spring.
"Our goal is to find 10 transits per year," Brown said. "We're looking at relatively bright stars, ones where we can easily do follow-up studies with large telescopes."
Once the team pinpoints the transiting gas giants, perhaps within the next two years, they'll propose using more powerful telescopes such as Hubble to probe their atmospheres. Detecting an extrasolar planet's atmosphere, even with Hubble, is extremely difficult. The planet may block 1 percent of starlight, but its thin atmosphere filters less than one thousandth of the glow from its parent star.
Astronomers hope to tackle many questions about the nature of these gas giants. What are their atmospheres like? Are they all the same? How different are they from Jupiter's atmosphere? Why are many of them living on the edge, orbiting so close to the searing radiation of their parent stars? Astronomers believe that these planets formed farther away from their stars. So, how did they migrate inward?
"We'd like to study the atmospheres of gas giants that are at different distances from their stars," Charbonneau explained. "Because they're at various distances, and orbit different kinds of stars, they will have a range of temperatures. Certainly temperature plays a dominant role in the makeup of a planet's atmosphere. These studies will help us figure out the forces that shape these planetary atmospheres."
Because many of these planets are perilously close to their stars, their dayside (the side closest to the star) is bombarded with 20,000 times more radiation than Jupiter receives from the Sun. These planets are heated to thousands of degrees Fahrenheit. How do they throw off the intense heat?
"Perhaps there's a huge flux of energy from the dayside to the nightside in the form of 5,000 mile-an-hour (7,900 kilometer-an-hour) winds," Brown said. "Or maybe there's another mechanism. We're trying to understand the processes that operate on these hot gas giants. They may give us insights into our own solar system."
Ultimately, astronomers want to know whether other solar systems - where Earth-sized planets flourish - exist in the universe. If so, do the planetary atmospheres include oxygen, the chemical ingredient of life?
"Oxygen is really the key," Charbonneau said. "The large quantity of gaseous oxygen in the atmosphere of the Earth is due entirely to biological activity. We know of no geological process that can mimic this on other planets. So, if we were to observe oxygen on an extrasolar planet, we would think that it might be due to life on that planet."
But astronomers have to find Earth-like planets before they can study them for signs of life. They have proposed several satellites that would search the heavens for terrestrial planets that cross in front of their solar companions. These planets are smaller and dimmer than the gas giants and would block a mere one ten-thousandth of the starlight. They also reside tens of millions of miles farther away from their stars than the gas giants now studied. Earth, for example, is a cozy 93 million miles from the Sun, completing a roundtrip around the star in 365 days.
"We have no idea right now how common Earth-like planets are," Gilliland said. "Statistically, a large search of about 100,000 stars a moderate distance away should tell us if Earth-like planets are common or rare. Using this method, there's a half-percent probability of detecting an Earth-like planet in an edge-on orbit. So, we must observe 200 Earth systems before we see one transiting planet."
Examining an Earth-like planet's atmosphere takes even more precision than the current technique used to sample the atmosphere of a gas giant. Its atmosphere filters only one millionth of the starlight. Using the transit method to probe such a tiny amount of starlight for the elements of life will require a sophisticated space-based observatory dedicated to monitoring suspected Earth-like planets for several years. Astronomers believe the observatory could be either a 10- to 20-meter optical telescope or a constellation of telescopes.
"Only a decade ago planets outside our solar system were still in the realm of science fiction," Charbonneau said. "Looking for unseen planetary companions was crazy. Hoping to see their atmospheres was even crazier. The last five years have resulted in a huge shift in thinking among astronomers. New planets are announced monthly, and now even their atmospheres are within reach. Suddenly, discussing searches for Earth-like planets seems quite reasonable."