TOKYO -- A recent series of discoveries suggests there may be more Earth-like planets out there than has long been believed. Will the new clues eventually lead to proof of extraterrestrial life? Scientists certainly think so, with NASA boldly claiming it will be able to obtain evidence within a decade.
In an article published in British science journal Nature in March, scientists from Japan, the U.S. and Europe said data from the Cassini space probe indicated that the oceans of one of Saturn's moons might be home to hydrothermal activity, a natural phenomenon on Earth.
Launched by NASA and the European Space Agency in 1997, the explorer gathered information on Enceladus, a moon just 500km in diameter. Hydrothermal activity occurs when seawater infiltrates and reacts with a rocky crust and emerges as a heated, mineral-laden solution.
What Cassini found was that the water on Enceladus and the steam coming off the surface contained superfine particles of silicon dioxide, a chemical compound rarely found in space but common on Earth. Researchers at the University of Tokyo and the Japan Agency for Marine-Earth Science and Technology and other institutions examined these particles and found that they were the products of the interaction of water heated to over 90 C and rock.
Yasuhito Sekine, an associate professor of planetary science at the University of Tokyo, said that based on those findings, beneath Enceladus' thick crust of ice likely lies an ocean over 10,000 times bigger than Lake Biwa, Japan's largest freshwater lake.
The existence of liquid water under the ice crust, which sunlight rarely fully penetrates, is probably attributable to a type of friction-induced heat arising from expansion and contraction of the moon due to the strong gravitational pull of Saturn. That heat would keep the water relatively warm and prevent it from freezing.
It has been also found that Enceladus has organic matter containing carbon. In addition, energy -- hot and cold -- has been detected coming off the water. The factors represent the minimum requirements for the existence of life. Sekine said further study of the moon by a probe may help shed light on how life evolved on Earth.
It is also thought that oceans probably exist on three of Jupiter's moons -- Ganymede, Callisto and Europa. ESA plans to launch the Jupiter Icy Moons Explorer, or JUICE, in 2022 and hopes it will be positioned near the planet by around 2030. NASA, meanwhile, aims to launch a manned Mars explorer in the 2030s.
Billions of Earth-like planets
It used to be believed that the stars beyond our solar system hosted only a few Earth-like planets -- that is, composed mostly of rock -- and that the bulk of the planets were shrouded in gas, like Jupiter. The general view was that building a civilization on these gas-type planets was impossible, and scientists largely doubted the existence of intelligent life on such heavenly bodies. But recent data suggests there are billions of stars with potentially habitable, Earth-like planets.
The Kepler space telescope, launched by NASA in 2009, enabled stargazers to discover a number of planets that were undetectable using terrestrial telescopes. Observation of these bodies showed that there were more planets like our own than expected.
Junichi Watanabe, a professor and vice-director general of the National Astronomical Observatory of Japan, the country's national center of astronomical research, said observing extrasolar planets is extremely difficult. He likened it to being in Tokyo and trying to ascertain the pattern on the wings of a fly perched atop Mount Fuji.
Data gathered from studies of these distant planets indicates that of the roughly 2,300 that have been observed, 900 are Earth-like, 200 are similar to Jupiter and 1,200 are midsize bodies closer in nature to Neptune. Further studies on Earth-like planets may yield clues to the existence of life on them.
It may also become possible to detect the existence of ozone on a planet by noting the existence of a particular element in the atmosphere. If there is any algae on a planet, for example, it will produce oxygen. And if that gas is exposed to sunlight, it will react with the ultraviolet light to create ozone.
The Kepler space telescope has transmitted an enormous amount of data to Earth, but its reaction wheels, components that help maintain a satellite's precise attitude, failed in 2013. NASA plans to launch a successor to Kepler in 2017 and expand the area under observation by a factor of 400.
Written in the stars
But terrestrial telescopes are still hugely helpful in unlocking the mysteries of space. These include the NAOJ's Subaru Telescope, which, with a diameter of 8.2 meters, is one of the largest in the world. It was built on the summit of Mauna Kea, a dormant volcano on the Big Island of Hawaii.
Academic and other institutions in five countries, including Japan, the U.S. and China, plan to collaborate on the construction of a massive Thirty Meter Telescope near Subaru. The exceptionally high resolution of the telescope, which is expected to be operational by the end of 2020s, will give scientists a clearer view of planets through Earth's image-distorting atmosphere.
More specialized studies are also being carried out what conditions a star needs to host habitable planets capable of giving rise to and sustaining life. Scientists previously thought that for life to exist on a planet, it would need to be relatively small, with a mass 10-50% that of our sun. Some 70-80% of the planets in our galaxy fit that description.
However, computer simulations by a team of researchers from the Tokyo Institute of Technology and China's Tsinghua University concluded that a majority of these planets were covered completely by oceans, with no land, or were essentially deserts.
The mainstream view now is that life is much more likely to be present on a planet that has more or less the same mass as the sun. Shigeru Ida, a professor at the Tokyo Institute of Technology, said a good balance of land and water is indispensable for the emergence of life.
Though defining what constitutes "life" can be difficult, it basically means a being that has the ability to take in nutrients and grow by itself. This definition applies to a variety of life-forms, ranging from primitive types, such as germs, to more intelligent ones, such as human beings.
When asked about how to determine if there is life out there, Carl Sagan -- the late American cosmologist, author and pioneer in space life science -- answered with a question of his own: How would an extraterrestrial being determine if there is a life on Earth by observing the planet?