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What is an exoplanet anyway?
EXOPLANETS, OR EXTRASOLAR planets, are planets in orbit around stars other than the Sun (which is the nearest star to Earth). More than 350 have been discovered so far… more information.

Why Gliese 581d?
With at least two potentially habitable planets and the most Earth-like planet discovered so far, the Gliese 581 system is one of the best candidates for life outside our Solar System… more information.

How will the messages be sent to Gliese 581d?
After the final message was collected on Monday 24 August 2009, messages were exported as a text file and sent to NASA’s Jet Propulsion Laboratory in California, where they were encoded into binary, packaged and tested before transmission. The resulting collection beeps and pauses (on and off radio signals) was sent back to Australia to the NASA/CSIRO Canberra Deep Space Communication Complex at Tidbinbilla. At midday on Friday 28 August 2009, the 70-metre main antenna, known as DSS43, transmitted the signal to Gliese 581d at a frequency of 7.145 gigahertz and a power of 18 kilowatts. The resulting signal, repeated twice over two hours, was equivalent to using the combined power of over 300 billion mobile phones at the same time. The DSS43 has supported many missions exploring our Solar System and beyond, including keeping in touch with the Apollo astronauts, providing two-way radio contact with the Mars Exploration Rovers, and deep-space missions such as the twin Voyager spacecraft and NASA’s New Horizons probe to Pluto and the Kuiper Belt.

When will the messages arrive?
The signal will reach the solar system of Gliese 581 (the parent star) around December 2029 give or take a few months. Despite travelling at the speed of light, the radio signal will need to cross 20.3 light-years (or 192 trillion km) of interstellar space before reaching the planet. It sounds like a long way, but Gliese is one of the 100 closest stars to Earth, making it our best target for sending and hopefully receiving a message within our lifetimes. Any response will need to travel the same way back, so unless Gliesans have improved communication technologies, the soonest we could hope to receive an answer would be in 42 years around 2051.

How far will the signal travel?
After it reaches Gliese 581, the signal will continue almost indefinitely into space, diminishing as it recedes. The signal will be strong enough for the message to be read by any intelligent life with the capacity to do so up to 100 light-years from Earth; its artificial nature should be detectable up to 10,000 light-years distant. Since Gliese is in the direction of the constellation Libra, near the galactic centre, the signal has a reasonable chance of reaching the solar systems of other stars before it fades completely.

What should I write?
If you were face-to-face with an alien what would you say? How can we create messages that can be understood by beings on other worlds? These kinds of questions are part of the scientific prerogative of SETI (the Search for Extraterrestrial Intelligence) Institute’s program in interstellar message composition. As yet, there’ve been no incoming messages to answer. But if science fiction is anything to go by, it’s probably a good idea not to include insults or your home address. Sending out these cosmic calls encourages us to think about “the big questions of existence,” says Paul Davies, an astrophysicist and the director of BEYOND: the Centre for Fundamental Concepts in Science at Arizona State University in the USA. These include: “what is life?”, “what is intelligence?” and “what is mankind’s place and destiny in the universe?”, he says. Check our Top Messages to see the best ideas so far.

What’s the chance that anyone’s listening?
Sending one small signal into the vastness of space is a bit like yelling for help from the ocean floor. But that doesn’t mean that such cosmic calls are doomed to failure. In 1961, astronomer Frank Drake formulated an equation that predicted the likelihood of detecting other intelligent civilisations in our galaxy. In 2007, Drake told NASA’s Astrobiology Magazine that he estimates we’ll eventually find existing intelligent life in “one in 10 million stars.”

What would Gliesians need to receive the message?
The message is encoded in binary and sent at a radio frequency at a particular bandwidth. To receive the message, Gliesians would need a radio receiver, and to understand it, the ability to interpret its binary nature (binary language, the language used by computers, consists of a series of zeroes and ones or on-and-off settings in a pattern).

What happens if someone or something responds?
Anyone can send a message to aliens – if they’re listening, and are from a star very close by, aliens have already enjoyed TV and radio signals that form part of the general background radio noise of our planet. But if any aliens respond, Earth protocol steps in. Response to any non-human communication received from another planet needs to be coordinated by the SETI Post-Detection Taskgroup of the International Academy of Astronautics, chaired by Paul Davies. If anyone or anything calls back, the protocol asks for the discoverer to inform all relevant scientific bodies, such as the taskgroup, as well as the government of the country concerned, says Davies. “The protocol says nobody on Earth should attempt to reply until international consultations have taken place. To safeguard this, the sky coordinates of the transmitting planet should be kept secret.”

Have we sent messages to aliens before?
Periodically, humanity is driven to cast what astronomer Carl Sagan called “bottles into the cosmic ocean”. The first messages sent into space were the simple plaques on NASA’s Pioneer probes (10 and 11, launched in 1972 and 73), and golden records on the Voyager probes (1 and 2, launched in 1977). Other cosmic message bottles include the beaming of the Beatles song Across the Universe, sent by NASA in 2008 from Madrid towards the star Polaris, and the Cosmic Call project, which in 1999 beamed a three-hour long message to four Sun-like stars within 70 light-years. This was the first time such a call had been made since astronomer Frank Drake sent a three-minute message on 16 November 1974. That binary message included the atomic number of DNA bases and a sketch of our Solar System and was sent from the Arecibo observatory in Puerto Rico toward the globular cluster Messier 13. There have been many SETI initiatives to scan of the skies for alien signals over the past 30 years, but despite a few false alarms (such as the ‘WOW! signal’ received in 1977), no-one has yet detected irrefutable evidence of an extraterrestrial signal sent to Earth. NASA is currently collecting names to go to Mars with the Mars Science Laboratory rover in 2011.