Feature: ET, when will we see you? (1)

Are we alone? Scientists say they are on the cusp of answering the age-old question about extraterrestrial (ET) life.

"We are lucky to be in a special era, with the next generation of giant telescopes on the way. There may be some exciting discoveries in the following 10 to 20 years," says Mao Shude, director of the Center for Astrophysics of the Beijing-based Tsinghua University.

According to mainstream scientific opinion, it is possible that extraterrestrial life exists, as the Earth is not unique in the universe. Our galaxy has hundreds of billions of stars, many with solar-like planetary systems, and there are hundreds of billions of galaxies in the universe. So it' s reasonable to infer that Earth-like planets might be common, and the universe could teem with life.

"I think primitive life is likely to be abundant, but intelligent life might be rarer," says Mao, also director of the Galaxy and Cosmology Division of the National Astronomical Observatories of China (NAOC), Chinese Academy of Sciences.

The next generation of giant telescopes might help astronomers resolve some long-standing issues, such as analyzing the spectrum of distant planets, making it possible to detect biomarkers of life.

Biomarkers are certain elements that might indicate the existence of life. An important biomarker, oxygen molecules, without supporting life, can only last for a short time compared with the 13.7 billion-year history of the universe.

Oxygen easily reacts with other elements, and Mars appears red as a result of oxidation. "If we find a lot of oxygen molecules in the atmosphere of an extraterrestrial planet, they are probably produced by life activities," Mao explains.

WILD IMAGINATION

The most common method of searching for extraterrestrial life is to search first for planets similar to Earth, with plenty of sunshine, liquid water and a protective atmosphere.

However, that method is constantly questioned as some speculate that life elsewhere in the universe might be quite different from that on Earth.

"We don't know how to start if we don' t know what kind of life we are looking for. At least, we know what conditions are needed for life on Earth," Mao says. "So scientists tend to search for planets around Sun-like stars and put forward the concept of the 'habitable zone' ."

The "habitable zone" is the distance from a star where a planet could have liquid water.

"Nature, however, has a much more wild imagination than we do. For example, whether life can exist in ice at extremely low temperatures is unknown to us," Mao says. "It' s easy for scientists to start with familiar conditions and then gradually expand the search to an unknown territory."

Over the past decade, astronomers around the world have identified more than 3,000 extra-solar planets. However, most are giant planets, probably composed of gas, because they are more easily detectable.

A few dozen are Earth-like planets, which are likely composed of silicate rocks or metals and may have water on them. "Astronomers first pick sample planets suitable for life, and then think about further study and analysis, or even communication with them," Mao says.

To further analyze the atmospheric composition of a planet, the telescope must be very sensitive to the sharp contrast of the light intensity between the star and the planet. The Earth is about one billionth of the Sun in brightness in the optical, a contrast still far beyond the range of current telescopes.

For more effective observation, astronomers are also striving for breakthroughs in another critical technology - adaptive optics.

When light from a star or any other astronomical object enters the Earth's atmosphere, it can be distorted by atmospheric turbulence, which can blur images produced by any telescope larger than tens of centimeters.

This annoyed Isaac Newton more than 300 years ago when he discovered that larger telescopes could not form clearer images due to atmospheric distortions.

But today, after years of development, many observatories around the world, including those with telescopes of 8 to 10 meters in diameter, are equipped with adaptive optics systems.

Adaptive optics is a technology that aims to correct the distortions induced by atmospheric turbulence. The system is mainly composed by three parts: a wavefront sensor, a deformable mirror and a real time controller. The wavefront sensor is like the eye of the system, and it measures the light distortions a few hundred or even thousand times in one second, so that the turbulence distortion almost looks like a cartoon played frame by frame to the system; the real time controller acts like a super fast brain, and it calculates how correction should be applied and sends commands to the deformable mirror; and finally, like the hand of the system, the deformable mirror actually carries out those commands, and changes its surface shape accordingly to correct the distortions before the arrival of the next command.

The next generation of ground-based telescopes, including the Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT) and the European Extremely Large Telescope (E-ELT), will have better adaptive optics systems, so they will be central to future observations, says Feng Lu, an associate researcher with the NAOC. (mo