Are we alone?
Surely, you might assume, we cannot be alone in the universe. After, all barely a week goes by without scientists unearthing yet another distant exoplanet, as planets outside our solar system are called.
The latest discovery, reported in the US journal Science, is of an exoplanet 130 light years away with an atmosphere of water vapour and carbon monoxide. (A light year is the distance travelled by light in one year, namely 9,500,000,000,000 kilometres.)
This mysterious world, known prosaically as HR8799c, was found by splitting its reflected light into different wavelengths to uncover the tell-tale signature of molecules in its atmosphere.
Chances of finding life there, at least as we know it, are low: HR8799c harbours no methane which on Earth is emitted by many organisms.
The exoplanet is one of four planetary youngsters, estimated to be between 30 and 100 million years old. They are all hot monsters, with surface temperatures exceeding 1000 degrees and masses ranging from five to 13 times that of our solar system gas giant, Jupiter.
Some of their atmospheres contain water and methane, while others bear traces of acetylene or the deadly gas hydrogen cyanide.
Normally the light emitted or reflected from planets is too feeble to be detected when it is in the glare of a star's light, says Monash University astrophysicist Rosemary Mardling. "Most planet detection methods rely on observations of the star itself," she explains.
The method of directly detecting exoplanets involves blocking out as much light as possible from a parent star.
"To do this, we use either a 'mask' to block out the starlight or try to 'null' the starlight – which lets us observe the remaining light reflected by the exoplanet" Dr Mardling says. "This is done using filters which only let through light at infrared wavelengths, at which young exoplanets are brightest."
The idea is similar to noise-cancelling headphones that "null" background interference to enable music to be heard clearly.
Surface water is unlikely to exist in liquid form on most of the bizarre worlds found so far. But, scientists agree, it's a matter of time before more Earth-sized exoplanets, that might be favourable to life, show up.
The nearest habitable orb, in fact, may be little more than 6.5 light years away, the latest research suggests.
"This will soon be tested by a newly approved NASA satellite, called TESS, which stands for Transiting Exoplanet Survey Satellite," says Melbourne University astrophysicist Alan Duffy. "It's expected to launch in 2017 and will find habitable Earth-like exoplanets near us across the entire sky."
Other projects slated to start within the next few years include the Gemini Planet Imager and the Spectro-Polarimetric High-contrast Exoplanet Research, or SPHERE for short.
Two decades ago, Dr Duffy points out, scientists knew of only one star with planets, our sun; today more than 800 such systems have been catalogued. They range from giant planets larger than Jupiter but hotter than Mercury to frozen worlds orbiting moribund stars.
The latest revolutionary discoveries, Dr Duffy explains, beg the question: how many more exoplanets are out there, waiting to be found? "And, most excitingly, will any of them resemble Earth?"
Above all, how likely is it that any of these celestial bodies host alien life forms? Scientists are sharply divided over this question, their views ranging from near certainty that alien life exists to being almost as certain that it does not.
Countless books, radio and TV programs and films have been produced on the subject, yet science thus far been lacking in answers.
"The probability of life emerging on an Earth-like planet was considered to be virtually zero in 1970 and one, by some, today," says the eminent physicist and astrobiologist Paul Davies, director of the Beyond Centre for Fundamental Concepts in Science and co-director of the Cosmology Initiative at Arizona State University.
The scientific facts, he says, have not changed. "It can still be anywhere from zero to one, and nothing we currently know gives a clue," explains Professor Davies. "We have no way to estimate the probability that life will emerge even on an Earth-like planet, let alone non-Earth-like ones. The known facts are consistent with ours being the only planet in the universe with life."
Swinburne University astronomer Christopher Fluke agrees. "During the last 50 years, astronomers have searched without success for evidence that we are not alone," he notes, referring to the global Search for Extraterrestrial Intelligence (SETI), which so far has drawn a blank – despite having monitored multiple radio frequencies for signs of anything odd. "Should we be trying to send messages of our own?" Associate Professor Fluke wonders.
Scientists ought to rid their minds of preconceptions as to what an extraterrestrial intelligence might be like or what sorts of technologies aliens may have acquired, Professor Davies suggests. This, he says, is imperative if we are likely to find ET.
His Beyond Centre is trailblazing an ambitious program to search for signs of alien microbes among some of the more extraordinary organisms on Earth that can withstand hostile conditions such as extreme cold, heat, acidity, radioactivity or saltiness.
"There are major unanswered questions in the search for recognisable, or even intelligent, life," acknowledges Melbourne University theoretical physicist Katherine Mack.
"What are the chances other life forms would be able to communicate across interstellar space?" asks Dr Mack. "And if an alien civilisation exists, how likely is it we'd find it before it had time to destroy itself?"
Not all exoplanets are that far flung. One strikingly similar to Earth – at least in terms of its mass – has been found just over four light years away in the triple-star system of Alpha Centauri, our closest stellar neighbours.
The exoplanet is "the lightest orbiting a solar-type star and the closest to the solar system found to date", its discoverers, a Geneva-based team using the European Southern Observatory in Chile, write in the British journal Nature.
Most scientists dismiss the chance of life existing on the chunk of rock that orbits its parent star, Alpha Centauri B, at one-tenth of the distance Mercury orbits our sun. It's probably an utterly desolate place with a scorching surface of about 2000 degrees, they conclude.
Nonetheless, the finding has raised hopes of finding an Earth twin hiding somewhere in Alpha Centauri – perhaps in a habitable zone.
"The Swiss team should be congratulated on this discovery," says NASA astrophysicist Mario Perez in Washington. "The search for a planet around the closest star to our own is finally over. It demonstrates that the presence of such planets around stars may be fairly common."
For all we know, extraterrestrial life, if it exists, may lie within our own neck of the cosmic woods. NASA's Curiosity rover, for example, is hard at work scouring the desiccated deserts of Mars for life.
"Mars might still contain life in some protected and hospitable environments, such as those deep underground," says Mark Sims, a professor of astrobiology and space instrumentation at Britain's University of Leicester and former mission manager of the European Space Agency's Beagle 2 mission.
Curiosity is now tackling this question obliquely by trying to establish whether Mars has or once had potentially habitable environments. To do this, the rover is sampling soil and rocks on the way to a nearby mountain with interesting geologies, including clays and sulphates that might once have been associated with water.
"The mountain enables one to have a cross-section of Martian history over several billion years," Professor Sims says.
NASA, and its European counterpart, ESA, meanwhile, are planning robotic missions to the solar system's icy outer suburbs. Prospects include the icy depths of subsurface oceans on Saturn's moons, Enceladus and Titan, and on Jupiter's moon Europa.
Learn more about the planned TESS telescope at: www.nasa.gov/home/hqnews/2013/apr/HQ_13088_Astro_Explorer_Mission_.html
Visit NASA's comprehensive Exoplanet Archive at: http://exoplanetarchive.ipac.caltech.edu/
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