The discovery of life on another planet or moon would be one of the greatest scientific achievements of all time. As astronomers build ever-more advanced telescopes and instrumentation, we could witness this event in our lifetimes. The search for Earth-like planets is an on-going campaign which tries to find the chemical signatures of life. This editorial, written by Professor Douglas Lin of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University in Beijing, highlights the importance of continuing such efforts to search for other life in the Universe.

"Are we alone in the Universe?'' is one of the most fundamental and profound issues in science. Life emerged on Earth as soon as its surface environment became tepid and well suited for complex meta-stable molecules to self assemble, reproduce, and survive. It is natural to speculate that these "packages of biochemical information'' have a tendency to proliferate throughout the Universe. Proofs of this hypothesis have motivated astronomers to search for signs of life beyond the domain of Earth. Despite many space missions to Mars and beyond, we have not yet found any direct supporting evidences of life on any other planets in the solar system.

However, our cosmic horizon has been greatly expanded by the discoveries of more than 500 planets around nearby star over the past 15 years. The first group of planets found are the relatively conspicuous "gas giants". They are probably not suitable to life because they are much more massive than Earth and have gaseous envelopes similar to those of Jupiter. Nevertheless, their statistical properties indicate that they are common around Sun-like stars, Since many have migrated over vast distances from the place of their birth, they have diverse orbital properties and internal structures. Similar to the Sun, the host stars of these planets appear to have families of smaller planets around them. In the last two years, a rich population of ``super-Earths'' have been found, and most of them are in multiple systems.

With the advent of highly accurate detectors and space telescopes, the inventory of known planets will soon be doubled. We anticipate the discovery announcement of a few planets with orbits, masses, and radii comparable to those of the Earth. These planets are commonly referred to as "habitable planets'' on the basis that water may exist in a liquid state on their surface.  This physical condition is thought to be essential for the blossoming of Carbon-based life.  Due to photosynthetic activities, the rise of primitive microbial life has led to a global enrichment of molecular oxygen in the Earth's atmosphere. The next challenge is to search for this unique bio-signature in the atmosphere of habitable planets.

The search and characterization of exoplanets have now matured into a most rapidly advancing branch of astronomy. Many initiatives such as transit and radial velocity surveys need to be carried out over a wide range of wavelength and continuously throughout the night in different longitudes around the world. These discoveries have also paved the experimental foundation for the emerging field of astrobiology. Although we have no idea on how to recognize life of unknown forms which may be teeming somewhere in the Universe, at least we now have an opportunity to quantify the probability of Earth-like life which we are familiar with.

Today this area of research has attracted hundreds of active young people from many different disciplines such as astronomy, physics, mathematics, life, earth and planetary sciences and across national boundaries in Europe, US, and Japan. So far, the engagement of the Chinese scientific community in this endeavor has been almost exclusively limited to astrophysical theories. Hopefully, strategic and timely investment in the development of telescope facilities, precision instrumentation, and most importantly, in the supports for experimentalists and theorists who are actively and fully engaged in innovative research programs through international and interdisciplinary collaborations will boost an all-around participation and contribution in this exciting emerging frontier of science.