Andrew Collier Cameron stretches a hand towards the charcoal skies over the Kingdom of Fife, eastern Scotland, and talks of a planet that lies far beyond the clouds. It has never been seen, even through the most powerful space telescopes.
It revealed itself to Cameron only by casting the tiniest of shadows as it strayed in front of its parent star, 1 000 light years from his office at St Andrew’s University.
Though distant, the planet has given up secrets. It shows only one face to its sun: half of it basks in permanent warmth while the other lies in eternal darkness. It moves so swiftly round its star that a year lasts only a few Earth days. It is a giant that would dwarf Jupiter, yet is incredibly light in comparison: it has the density of willow. ”It would float,” says Cameron, ”if you could only find a bath big enough.”
Cameron and his team are part of a new wave of astronomers scouring the galaxy for glimpses of unknown worlds and, in recent years, their success has been astounding.
They have found more than 250 new worlds, in solar systems far, far away.
Almost every week, new discoveries are made. American space agency Nasa has announced a newly discovered solar system strikingly similar to our own.
Each new planet we discover adds a fragment to our understanding of the galaxy beyond our celestial backyard. But, collectively, these scattered worlds point to something more profound.
Scientists’ ability to spot distant planets has accelerated more rapidly in the past 20 years than many dared imagine. And, for the first time, they are talking seriously about searching for a planet that could recast the role of humanity in the universe: a second Earth.
”There’s a strong desire in humans to know how special our planet is,” says Cameron. ”There’s a lot of talk about how finely tuned Earth is for life, but just how special is it? How special is life? Are we alone? This is what gets people into this.”
Nearly all the planets discovered beyond the solar system are enormous gas giants, like hotter and bigger Jupiters. They orbit so blisteringly close to their suns that they carve out a year in just a few Earth days, their surfaces approaching a searing 2 000°C.
These huge worlds are the easiest to spot, though most are still detected indirectly through their effects on their parent stars, measured by the famous Doppler effect.
When a planet orbits a star, its gravitational field makes the star move in a little circle of its own. If the planet is big enough, astronomers can pick up this cosmic dance by peering at the starlight that reaches Earth tens, hundreds, even thousands of years later.
If the starlight shifts to a more reddish colour, the light waves are being stretched out, so the star is being pulled away from Earth. As the star is pulled back towards us, the light we see turns more bluish as the waves are squashed together.
By watching the shift from red to blue and back again, astronomers can deduce the mass and rough orbit of the planet doing the wrenching.
Cameron spotted his giant willow world another way. The team is part of a consortium called SuperWasp — Super Wide Angle Search for Planets — run by Don Pollacco at Queen’s University in Belfast.
Together, they operate ground-based telescopes at the summit of La Palma in the Canaries and at Sutherland in the Karoo.
Every evening, automatic covers roll back from the telescopes to reveal the stars, which are snapped every 30 seconds as they curl over the sky from sunset to sunrise. In a 10-minute period, each telescope will capture the light of 3,5million stars.
In the morning the night’s images from La Palma are piped down the internet to Belfast, while those from Sutherland are stored on magnetic tapes and shipped back in bulk.
At Pollacco’s lab, a bank of computers then spends 12 hours a day examining every star, looking for telltale dips in the brightness caused by a planet moving across the face of its sun.
The most promising candidates are then beamed out to researchers such as Cameron for confirmation.
In 2005 the SuperWasp team found two huge planets this way, the willow world, in the constellation of Andromeda, and a smaller, more dense planet circling a star in the constellation of Delphinus.
The team has since discovered three additional giant planets more than 700 light years away, which pass so close to their suns that their surfaces boil.
The SuperWasp technique, known as the transit method, has its shortcomings. Only a tiny percentage of planets orbit their stars in the right plane to cast their shadows across the Earth, and only giant worlds that come close to their stars produce clear enough signals to reveal themselves.
But this is a fast-changing field: improvements and refinements are bringing astronomers ever closer to their ultimate goal of finding a second Earth.
Earlier this year scientists led by Stephane Udry at the European Southern Observatory in Geneva discovered a planet orbiting a faint red dwarf star in the constellation of Libra, 20 light years away. Their measurements suggest the planet, Gliese 581c, is small, rocky and only one and a half times the size of Earth.
Its orbit has led some scientists to speculate that liquid water might be stable on its surface, raising the tantalising possibility that it could support life.
If life does exist on Gliese 581c, it has yet to generate enough noise to be noticed. Scientists at the California-based Search for Extraterrestrial Intelligence (Seti) project have looked for radio waves emanating from the star system and found nothing.
Just three months after Udry’s announcement, a landmark discovery by Giovanna Tinetti and her team at University College London sent a further ripple of excitement through the field. They used Nasa’s Spitzer Space Telescope to peer at infrared radiation from a star called HD 189733, lying 63 light years away. When a newly discovered planet moved close to the star, they noticed a familiar pattern in the infrared signal.
The discovery, reported in Nature, marked the first incontrovertible evidence of water in the atmosphere of a planet beyond our solar system. Nasa’s mantra for finding extraterrestrial life is ”Follow the water” — the one compound scientists believe is crucial for life. — Â