Technicians work on the COROT satellite, which launched in 2006 from the Baikonur Cosmodrome in Kazakhstan. The satellite is equipped with a 30-centimetre diameter space telescope designed to detect tiny changes in brightness from nearby stars, a possible sign of a planet transiting in front of a star. (Associated Press)

Twenty years ago, three Canadian astronomers published evidence of a Jupiter-sized planet in close orbit around a giant and distant star.

The University of British Columbia's Gordon Walker and Bruce Campbell and Stevenson Yang of the University of Victoria couldn't "see" anything in the conventional sense, but they were able to observe changes in the motion of the star Gamma Cephei, changes they deduced were caused by the gravitational influence of a nearby planet.

They published their findings in the Aug. 15, 1988, issue of the Astrophysical Journal, the culmination of over 10 years of work that began when the three men developed a technique for viewing small changes in the motion and velocity of a star. It would take another 15 years, however, before they would be proven right.

Today, hunting for extrasolar planets, or exoplanets, is a hot field in astronomy. The total number of planets discovered outside our solar system is about 280, with over 60 discovered in 2007 alone and new announcements coming seemingly every week.

Most recently, on April 9, 2008, Spanish astronomers announced they had discovered a rocky planet five times the size of Earth circling a red dwarf star 30 light years away near the constellation Leo, a planet they said was the smallest planet yet discovered.

But 20 years ago, Walker, Campbell and Yang were still fighting to confirm the first such planet. At the time, the search for planets outside our solar system was considered at best Quixotic and at worst a waste of valuable telescope time, said Artie Hatzes, now the director of the Tautenburg Observatory in Germany.

The same year the Canadian astronomers published their findings, Hatzes finished his PhD at the University of California, Santa Cruz, and saw a job posting to work with astronomer William Cochran at University of Texas. Cochran had just received a grant to go exoplanet hunting, but Hatzes wasn't sure whether he should take the job.

"I asked my advisor at Santa Cruz, Steve Vogt, 'Steve, you know Bill Cochran. Is he crazy?'" said Hatzes.

Mixed reaction to discovery

Walker, Campbell and Yang may have begun asking themselves the same question following the mixed reaction their paper received.

An artist's impression of the planetary system around the red dwarf Gliese 581. (Courtesy: ESO)

Their technique was innovative. It involved measuring the star's velocity by using a controlled captive gas as a reference point. Starlight passes through a cell containing the molecular vapour. As the vapour absorbs light in telltale lines — akin to a chemical barcode — it imposes these lines on the star's light. This allowed the scientists to view the starlight through a kind of ruler, helping to mark subtle changes in the light as the star moved away or toward Earth, a Doppler shift of the kind sound waves undergo as an object moves toward or away from a fixed point.

About the only questionable part of their method was the use of the corrosive hydrogen fluoride as their captive gas.

"It's extremely dangerous to use, but we all survived," said Walker.

What they had a harder time overcoming was their own concern over the data and planetary theory at the time. The planet they thought they had found didn't behave like any planets known at the time. While they estimated it was a Jupiter-sized planet, it appeared to orbit the star once every 2.5 years, far faster than any of our own solar system's gas giants (the four large planets composed primarily of gases: Jupiter, Saturn, Uranus and Neptune).

There was also the possibility that the observed changes in the starlight were the result of the rotation of the star itself, and in 1992, they published a paper arguing that was the more likely scenario.

"They had found it, but I think Gordon convinced himself out of it," said Hatzes. "Theorists at the time said Jupiter-sized planets don't behave this way."

Exoplanet research takes off

It wasn't until three years later, when Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting the star Pegasi 51, that extrasolar planet research really took off.

As it turned out, huge Jupiter-sized planets that closely orbit stars weren't so uncommon. Or, at the very least, such planets were the easiest to find. Not only did their large size have a greater gravitational pull on the stars, but in instances where the planet actually passed in front of — or transited — our view of the star, such planets could actually alter the light from the star in ways astronomers could observe.

University of Toronto associate professor Ray Jayawardhana, who now studies exoplanets, brown dwarf stars and planet formation, was a graduate student when the Geneva team announced its discovery. At the time, his research focused on the large scale structure of the universe, the kind of big-brained field popular among astronomers interested in the early days of the universe.

"Planet hunting wasn't on my radar," he said. "But after Geneva, it was clear the search for exoplanets was much more on the horizon."

The new discoveries were driven by a confluence of favourable conditions. Larger telescopes, both in space and on Earth, were able to view stars with a larger field of view and were thus better able to find stable points of reference in nearby stars. Increased computing power allowed greater production of data. And the spectrometric process Walker, Campbell and Yang pioneered was honed to greater precision, with safer iodine replacing hydrogen flouride as the gas of choice.

Perhaps more importantly, success bred interest, which in turn allowed astronomers to spend more time at telescopes looking for planets and spurred further innovation.

New telescopes to focus on Earth-like planets

More recently, space telescopes, one in orbit and one set to launch next year, will be employed in the search for signs of transiting Earth-like planets. The French-led COROT 30-centimetre space telescope launched in December 2006, with its view trained on rocky planets like Earth or Mars rather than gas giants like Jupiter or Saturn. NASA's Kepler telescope, set to launch next year, will be even more focused, limiting its search to transiting Earth-like planets in habitable zones of space.

While ground-based observatories have much larger, more powerful telescopes than those in space, astronomers working with them have to account for the interference from our own atmosphere, said Jayawardhana. Advances in a field called adaptive optics have helped astronomers compensate for those issues and create sharper images.

Near-infrared sensors, like those found on the Hubble Space Telescope, have allowed scientists to learn more about the chemical composition of planets by observing how starlight was altered as it passed through the planet's atmosphere. In March, Hubble detected methane and confirmed the presence of water vapour on a planet called HD 189733b, a hot Jupiter-sized planet about 63 light years from Earth.

Public interest, said Hatzes, has also fueled the research.

"Planet hunting is something that strikes the interest of the layperson," he said. "It's easier to grasp a planet orbiting a distant star than trying to explain the discovery of a tiny, subatomic particle. It helps us answer the question of whether our planetary system is unique. Are we special? This is something that is accessible to the public."

Public interest, in particular, has focused on the search for Earth-like planets at a distance similar to our own planet's distance from the sun. Astronomers see this "sweet spot" as the best place to look for planets since the temperature and conditions might make the presence of liquid water possible, a key ingredient for supporting life.

Astronomers hoping to hit the jackpot

Finding transiting planets is far more of a crap shoot, said Hatzes, because astronomers have to be lucky enough to have their telescopes at the right angle to view the passage of a planet in front of a star.

"The odds that we'd be able to see a planet like Earth pass in transit in front of a star is slim, something like 0.5 per cent," he said. "That's assuming we can actually spot a planet that size."

It's proven far more difficult to find Earth-like planets than finding gas giants in close proximity to bright stars, but astronomers in the field say recent advances make such findings more likely than 20 years ago. Last year's discovery of a planet with a diameter about 50 per cent larger than Earth lying in the habitable zone of the star Gleise 581 is the closest discovery yet. However, observations using the Microvariability and Oscillations of Stars (MOST) telescope, the Canadian Space Agency's suitcase-sized device, didn't show the planet in transit.

As University of British Columbia astronomer Jaymie Matthews said in June of last year, "It's like playing the lottery, but we wanted to play the game, because the jackpot is huge."

For the three Canadian scientists who helped get the ball rolling, the jackpot would come in 2003. Using data from the McDonald Observatory in Texas, the Canadian team worked with Hatzes and Cochran to answer a 15-year old question: was there a planet orbiting Gamma Cephei?

The answer was yes: 20 years of data showed the same periodic movement of the star, the kind of motion that could only be explained by the gravitational tug of a planet. It was vindication for those who worked on the project, and it remains the first published exoplanet discovery to be subsequently confirmed.

Almost 300 planets later, Walker is looking forward to the next big challenge.

"If we start to detect Earth-mass planets, that will galvanize an effort to follow-up," he said. "The next step would be to try to get an image. Boy, that would be a real challenge."

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