Published On Thu Nov 13 2008
Circling the star HR 8799, 128 light years from Earth, three newly discovered planets are between seven and 10 times the mass of Jupiter.
This July, Christian Marois, a young astrophysicist with Canada’s National Research Council, was on a plane over the Pacific poring over telescopic images of the star HR 8799 – an unremarkable pinprick in the Pegasus constellation – when things suddenly fell into focus.
“I thought `This is crazy, this is amazing,'” the 34-year-old research associate says. “I discovered there was, not one, but two objects around this star.”
And for the first time in the history of creation, a creature on a planet in our solar system was looking at an image of planets orbiting in another. The discovery was released yesterday in the journal Science.
“It was the first image of another planet system orbiting another star,” says Marois, who is completing a post-doctoral stint at the council’s Herzberg Institute of Astrophysics in Victoria.
“I’ve been dreaming about astronomy since my childhood … so this was a childhood dream come true.”
His dream would capture three planets as it turned out.
Circling HR 8799, 128 light-years (that’s 128 x 9.5 trillion kilometres) from Earth, the planetary trio is between seven and 10 times the mass of Jupiter.
Some 50 per cent more massive and five times more luminous than our sun, HR 8799 is visible to the eye – look toward Pegasus near its zenith in the northern latitudes.
Seeing the planets? That took a bit more work.
Some 300 planets have been detected around distant stars over the past 10 years. But these orbiting bodies have been inferred, rather than photographed, largely by the “wobble” they create in their stars as they pull it gravitationally in different directions during their circling orbits.
And while there have been “exoplanet” sightings reported in the literature – another is reported in this same Science edition – Marois says his images are the clearest and definitely the first to show a distant solar system.
“It might be seen in the future as the first unambiguous (images),” Marois says. “It has all the elements that are strong arguments that these things are planets. There’s no missing link, there’s no missing information.”
Getting the images was no easy task. “We had to look at a lot of stars in order to be able to see these,” says Marois, whose planetary quest began eight years ago.
Indeed, Marois says 80 stars, painstakingly pared down from the billions available, were scrutinized before the HR 8799 planets appeared. Because stars are often 25 times brighter than their planets – which simply mirror the starlight – they typically drown out any of the planetary glow that might be glimpsed by earthbound lenses.
Thus, Marois says, researchers needed to find stars with planets that were far enough out of their sun’s glaring shadow to have some visible shine of their own.
This meant turning away from targets that resembled our sun – where Earthlike objects might exist – and toward larger A-Class stars that are able to gravitationally hold on to larger planets in more distant orbits. “Everybody had neglected those stars because, since they’re more massive, they’re brighter.”
But more massive stars, he says, can hold larger planets at wider separations, giving astronomers big optical targets that are farther away from the central light source.
Second, Marois says, the star had to be located outside of the galaxy’s cluttered plane, where the light from backing stars would simply drown out any planetary light.
Our milky way is a spiral galaxy and most of the stars we can see from Earth are located within the circling arms that give a clear night sky its lactic hue.
“HR 8799 is well away from the galactic plain so there was not a dense star field. And there was nothing else in the field, there was nothing, so it had to be a bound (planet).”
Also, Marois says, the star needed to be relatively young to ensure any orbiting planets would retain the formational heat that could add to their visibility.
The HR 8799 solar system is only about 60 million years old, compared to the 4.6 billion years our planetary neighbourhood has been around. The trio of planets are 5.3 to 6.6 times hotter than Jupiter.
As important as meeting these stellar criteria, Marois also needed a new way to observe the skies that would separate the brightness of the stars from the puny, planetary glow. That problem was solved by a software program he developed as a PhD candidate at the University of Montreal, which allowed planetary bodies to stand out from their sun’s bright shadow far more readily than before. That “enabled us to extract very well the light from the star by a factor of 10 to 100, so these were the deepest images ever obtained on any telescope,” he says.
While hundreds of other exoplanets have been inferred, seeing is believing, says University of Western Ontario planetary geologist Gordon Osinski. “You’re always wary in any kind of science about basing everything on assumptions and inferences,” says Osinski, deputy director of Western’s Centre for Planetary Science and Exploration.
“So for sure having direct observations is a very big thing.”
As well, Osinski says, actual sightings of other planets may eventually help detect extraterrestrial life, through atmospheric analysis.
Richard Gray, an astronomer at North Carolina’s Appalachian State University, says the ability of visual images to give atmospheric information gives this study its true excitement. “It’s certainly proof of concept, but more than that, if you can actually image a planet, then that means you can study its atmosphere in detail,” says Gray, formerly of the University of Toronto. “We can learn much, much more about these planets than ever before.”
Marois’s images were compiled from two of the world’s largest telescopes: the W.M. Keck and Gemini observatories on the summit of Hawaii’s Mauna Kea.
Marois says these earthbound telescopes, with their 10-metre apertures, are better than the space-based Hubble telescope at planet detection. Indeed, the Hubble already eyed HR 8799 in the 1990s and failed to reveal its planets.
There is, however, almost zero chance that anyone on the planets will be looking back at us, Marois says. As gas giants far more massive that our behemoth Jupiter, the planets have virtually no capacity to support life. They orbit 25, 40 and 70 astronomical units from their star (an astronomical unit is the distance between our sun and the Earth, about 150 million kilometres).
“They are similar in distance (from HR 8799) as the outer planets in our own solar system are from the sun,” Marois says, referring to Neptune and Uranus.
There is a distinct possibility, however, that smaller planets, perhaps even rocky ones, may be orbiting closer to HR 8799, he says.
“There’s definitely a probability that something is there … but sadly we don’t have the instrumentation yet to tell if there’s anything closer.”
It’s still up for debate, experts say, whether Marois’s images represent the first optical sightings of any “exoplanet,” or planet surrounding another star.