They call it “The Blue Marble.” It’s a photograph of planet Earth taken in 1972 by the Apollo 17 crew on their way to the moon. Snapped from a distance of 45,000 kilometres, it shows a world of continents and oceans, clouds and ice caps. A cyclone twists over the Indian Ocean, and vast patches of green hint at the abundance of life teeming on the planet’s surface.
How common is a sight like this in the universe? Are there millions of Earths orbiting distant stars, or are the circumstances that conspire to make life possible so complex and improbable that ours is the only habitable planet in the cosmos?
Despite the discovery of hundreds of new worlds, scientists have yet to observe even one truly Earth-like planet. The planets they are finding, meanwhile, are forcing them to rethink how solar systems form and are bringing them closer to answering the question, “Is there life elsewhere in the universe?”
The pace of discovery has been breathtaking.
In 1991, scientists had discovered exactly zero planets orbiting stars other than our sun. Ten years later, they had found 62 such extrasolar planets, or “exoplanets.” By the end of 2011, that number had climbed to 716, with thousands of other candidates under investigation.
In some ways, this flurry of discovery makes the Earth seem commonplace – planets abound in our galaxy. But the variety and oddity of so many exoplanets suggests that Earth might be anything but typical. It also hints that our solar system might have a stranger past than people once thought.
“Our view of planet formation is being turned on its head,” says Ray Jayawardhana, a professor of astronomy and astrophysics. “It must be a much more dynamic process than previously thought, one that ends up hurling planets around, leaving them far from where they formed.” Jayawardhana is the Canada Research Chair in Observational Astrophysics and the author of Strange New Worlds: The Search for Alien Planets and Life Beyond Our Solar System (HarperCollins Canada, 2011). He was part of a Toronto team that in 2008 captured the first direct image of an exoplanet orbiting a sun-like star.
Jayawardhana is one of many U of T–based planet hunters. Researchers in the astronomy department, the Dunlap Institute for Astronomy and Astrophysics, and the Canadian Institute for Theoretical Astrophysics (CITA) are all caught up in one of the most exciting explorations in physical science.
Quinn Konopacky, a post-doctoral fellow at U of T’s Dunlap Institute recalls the thrill of planetary discovery when she was doing a previous post-doc at the Lawrence Livermore National Laboratory in California. In 2009, she was part of a team examining images of a star in the constellation Pegasus. This star was already known to have three gas giants sweeping around it. New images, though, revealed an additional “blob” closer to the star.
“I was very excited about the new planet candidate, but we had to make sure it showed up again to be certain it was real,” she says. In 2010, the object reappeared in images from the Keck Observatory in Hawaii. Calculations showed that it was indeed a fourth planet. The team realized that, with four giant planets in wide orbits, the system was analogous to a scaled-up version of our own solar system. “This was pretty profound,” says Konopacky. “I don’t know of any other planetary systems that have such similarities to our own.”
Astronomers are now searching for new models to describe what is out there. “The big story is the incredible diversity of worlds,” says Jayawardhana. “It really does seem that nature is much more prolific and wondrous than our imagination – by a wide margin.”
Until recently, theories of planetary genesis were necessarily based on our own relatively orderly solar system, where large planets orbit far from the sun, smaller rocky planets circle in closer, all planets travel in the same direction in nearly perfect circles and their paths fall almost in one plane.
Now astronomers have found exoplanets smaller than Earth, many times larger than Jupiter and all sizes in between. Some planets orbit in one direction while their star spins in another. Some follow oval-shaped paths, swooping in close to their stars and then flying back out into cold, distant regions. Still others orbit at strange angles, far out of alignment with the original dust disks. Each new surprise changes theories of planetary system architecture, and also raises new questions about how common life-supporting planets might be.
Another big surprise has been a multitude of huge gaseous planets that orbit much closer to their stars than anything in our own system. These so-called “hot Jupiters” speak to new theories of chaotic and complicated planetary evolution.
“The observers love it because they’re making the theorists look like fools,” says CITA director Norman Murray. “When you see these discoveries, you say, ‘How could this happen? Everything we know is wrong.’”
In actuality, theorists such as Murray had predicted some of these phenomena. But the flood of new information is changing the established story.
It’s widely believed that every planetary system begins with a cloud of gas measuring light years in diameter. Over millions of years, the cloud spins and compresses under its own gravity and momentum into a whirling disk of material with a central bulge. The centre collapses into a star, while the rest of the disk provides raw material for planets.
What happens next is a matter of debate.
One long-standing but still contested theory suggests that dust particles in the cloud stick together, gradually accreting into pebbles, boulders and larger bodies called “planetesimals.” Eventually, they become massive enough that their gravity starts attracting even more material. Ultimately, these objects become protoplanets and then full-fledged planets.
There are problems with this “core accretion” theory, though. The forces that bind tiny particles together are different from those that hold a planet together. Theorists can’t fully explain how objects make the jump from chemically bound to gravitationally bound bodies. Also, core accretion is slow. It could take millions of years to grow a gas giant this way, but the stellar disks out of which planets form don’t last nearly that long.
A second theory involves “disk instability.” This theory suggests that turbulence in the dust cloud creates globs of matter that simply collapse into planets under their own gravity.
Neither theory is completely accepted, nor are they mutually exclusive. There is some evidence that smaller planets may form via core accretion, and giants via disk instability. Recent discoveries now show that, however planets first come together, their origin story doesn’t end there. “It’s not as though all these systems form the same way,” says Murray. “The hot Jupiter systems didn’t form the same way as other systems.”
Hot Jupiters are mysterious because a star and large planet that form so close to one another should theoretically fuse into a single object. This has led to new theories suggesting that planets may be much more mobile than previously thought.
A hot Jupiter may form far from its star, “migrating” over time, sweeping up gas and other planetary bodies, growing larger and larger as it edges closer to its system’s centre. The migration theory, though, can’t explain yet another surprising type of exoplanet – smaller gas giants known as “hot Neptunes.” If hot Neptunes migrated toward their stars the way scientists’ models project, they should have picked up more material along the way and ended up much larger.
In addition to gradual processes such as migration, astronomers now think more dramatic forces may also be at play in planetary system formation. Some strange architectures may result from planets crashing into one another, objects whipping planetary neighbours into new orbits and other volatile behaviour that researchers are only beginning to contemplate. Each new system seems to demand new explanations.
As varied as planetary systems have turned out to be, planets themselves have become indisputably common. In fact, it is possible that every star has planets. “Before 1996, you could have argued that planetary systems might be very rare, but today we know that planet and star formation work hand in hand,” says James Graham, director of the Dunlap Institute.
Graham is a project scientist for the Gemini Planet Imager, a cutting-edge camera that relies on a technology called “adaptive optics” to compensate for image distortions caused by the Earth’s atmosphere. When the Imager sees first light in early 2013, it will be able to pick out a planet that is one ten-millionth as bright as its companion star.
Direct imaging is just one planet-detection method; most planets are identified via indirect measurements only. For example, if a star periodically dims, it can indicate that a planet is “transiting” in front of it. A planet’s gravity can also create a telltale wobble in the motion of its star.
Despite these and other detection methods, an authoritative planetary census still faces technological limitations. In general, larger planets in younger systems are easier to detect. Direct observation works best for planets far out from their stars, while many other methods are only practical for planets in tight orbits. Small Earth-like planets are often a major challenge.
Step by step, though, new tools reveal an ever-greater diversity of planets, orbits and systems. One of the major quests in the field is to find Earth-sized planets orbiting their stars at a “Goldilocks” distance, where it’s neither too hot nor too cold for liquid water. Such planets would be the most likely candidates for finding extraterrestrial life.
In addition to ground-based observatories, planet hunters also have access to tools such as NASA’s Kepler Space Telescope, which has a mission to find planets similar to our own.
Kepler reveals variations in light intensity for thousands of stars. A single dip could have many causes, so researchers look for at least three periodic indicators before they draw publishable conclusions about the existence of a planet.
Because Kepler scientists monitor Earth-like orbits, three dips takes about three years. Kepler launched in 2009, so many people expect major announcements this year or next.
“I’ve heard people connected to Kepler hint that there are Earth-sized planets in Earth-sized orbits,” says Murray. “It hasn’t been announced yet, but I suspect they know.”
Even with current limitations, there exists an astounding depth of information about exoplanets. Scientists can calculate their diameter, orbital distance and, in some cases, whether a planet is rocky or gaseous. They can measure temperature, atmospheric composition and other surprising details. They can provide weather reports for planets hundreds of light years distant – a hot Jupiter forecast might call for temperatures in the high 700s with winds gusting up to 9,000 km/h.
Of course, not all exoplanets have such extreme environments. And with so much new information, researchers are getting closer to answering a fundamental question: Is there life on other planets?
Murray cautions that the apparent abundance of planets doesn’t necessarily mean life abounds in the universe. “There are probably a lot of stars with Earth-like planets – on average there could be at least one per star,” he says. “But astronomers and biologists lack imagination. We don’t understand how life can form without liquid water and carbon.” Many other conditions may also be necessary for life and we still don’t know exactly how precise and rare they might be. Consider the other “Earth-like” planet in our own solar system. Venus is similar in size and composition to our world, but minor differences make it completely inhospitable to life as we know it. (Mars, with a mass about one-tenth of our planet, is arguably too small to qualify as “Earth-like.”)
Still, Murray says, even if only one in 10 or one in 100 stars have planets suitable for life, “That’s still a lot of stars.” (Astronomers estimate there are at least 100 billion stars in our galaxy alone.)
The James Webb Space Telescope, scheduled for launch in 2018, may help scientists detect the building blocks of life – oxygen, ozone, water and carbon dioxide – on extrasolar planets. But researchers caution that there are still a lot of challenges to overcome to fully understand these new worlds. “We’re just at the beginning,” says Konopacky.
The existence of exoplanets went from questionable to commonplace in the space of two decades. The discovery of extraterrestrial life could happen just as suddenly – it is more likely than ever that our blue marble will turn out to be merely one of dozens spinning through the firmament. “That dramatic moment is no longer a remote possibility,” says Jayawardhana. “It may well occur in our lifetime, if not during the next decade.”
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