Feature: ET, when will we see you? (2)

Compared with space telescopes, ground-based telescopes can be larger and connected to more instruments, and can work longer and look deeper into space. The adaptive optics technology will improve their resolution close to or even above space telescopes, making them capable of observation tasks previously impossible on the ground, such as tracking extra-solar planet candidates, Feng says.

For example, with the assistance of adaptive optics, TMT will have a resolving power and sensitivity much greater than the Hubble Space Telescope when it goes into use around mid-2020s. One of its major tasks will be to analyze the spectrum of extra-solar planets.

But the next generation of space telescopes will also revolutionize astronomy. The Transiting Exoplanet Survey Satellite (TESS), to be launched by the U.S. National Aeronautics and Space Administration (NASA) in 2017, is one of them.

Kepler, TESS' predecessor launched by NASA in 2009, is the world's first space observatory dedicated to the search for planets outside our solar system. So far, Kepler has confirmed 2,325 extra-solar planets, more than 70 percent of the total. Twenty-one of them are Earth-like, staying in habitable zone and within twice the size of Earth.

While both can monitor planetary transits, TESS is capable of carrying out all-sky surveys, while Kepler can only observe a small part of our region of the Milky Way.

"More importantly, TESS will look for extra-solar planets in orbit around the brightest stars, which will help analyze the physical nature of these planets. Kepler's planets are often around dim stars, which makes it difficult to carry out follow-up studies," Mao says.

"The number of planets that TESS will detect is not necessarily much more (than Kepler), but the quality will be more advanced."

LISTENING TO SPACE

As well as searching for Earth-like planets, scientists also scan the skies for extraterrestrial signals via radio telescopes, as some believe other civilizations will inevitably produce and release radio waves, as we do, during their evolution.

Frank Drake, a U.S. astronomer and astrophysicist, was the first to test the method. In 1960, he started Project Ozma, a pioneering SETI (search for extraterrestrial intelligence) experiment, at the U.S. National Radio Astronomy Observatory at Green Bank, West Virginia. The objective was to examine nearby Sun-like stars for signs of life through interstellar radio waves.

Although the project ended in vain, it triggered dozens of later SETI programs.

However, the work of selecting possible meaningful signals from massive background noise consumes time and resources. The Space Sciences Laboratory, at the University of California, Berkeley, launched the SETI@home ("SETI at home") project in 1999, to search for possible radio transmissions from extraterrestrial intelligence via public volunteer computing.

Part of the worldwide SETI effort, the project uses observational data from Puerto Rico's Arecibo Observatory, the world's current largest telescope, and the Green Bank observatory. The data, taken "piggyback" or "passively" while the telescopes are used for other scientific projects, are divided into small chunks and sent to millions of home computers for analysis. The software searches for signals with variations that cannot be ascribed to noise, and hence possibly contain information.

So far, about 9 million volunteers in 226 countries have analyzed a total of 15 years of data from the Arecibo telescope, according to Dan Werthimer, co-founder and chief scientist of the SETI@home project.

"We have identified about 100 very short radio bursts, about one millionth of a second long, that we don't fully understand," Werthimer said in an e-mail interview, Radio communications from Earth still haven't been ruled out as the source of these signals.

So far, no signals have been confirmed as from extraterrestrial civilizations. "We've only had radio for 100 years and lasers for 60 years," Werthimer said. "We are just getting in the game and just beginning to explore the potential different frequencies and signal types that another civilization might use. There's a long way to go before we can do a thorough search.

"The good news is that the capabilities of earthlings are growing. The computing power is developing quickly and the FAST telescope will be very powerful," he added.

FAST, now under construction deep in the mountains of southwest China's Guizhou Province, will become the world's largest radio telescope once completed in September this year.

With a dish the size of 30 football grounds, FAST is 500 meters in diameter and made of 4,450 panels. Scientists have depicted it as a super-sensitive "ear", capable of spotting very weak messages - if there are any - from "cousins" of human beings.

It will be 10 times more sensitive than the telescopes in the Breakthrough Listen project, a 100 million-U.S.-dollar initiative by Russian tycoon Yuri Milner to search for extraterrestrial civilizations, says Li Di, chief scientist with the radio astronomy department of the NAOC.

Werthimer is seeking cooperation with Chinese astronomers to develop a SETI@home project for FAST. "We hope to work with China to do SETI at the same time while the telescope carries out sky surveys to search for pulsars, fast radio bursts and to map the galaxy as planned."

Though unsure of how such cooperation would work, Li Di is interested in working with the SETI@home project. "With their experience and advanced technologies, they will help us improve the telescope's scientific capabilities and operating conditions. It's like standing on the shoulders of a giant," Li says. (mo