We pick up a signal from ET. What do we do next?

The International Academy of Astronautics (IAA) SETI Permanent Committee is the world’s only international forum for the discussion of SETI. It meets every year at the International Astronautics Congress.

In 1989, it adopted a set of nine principles concerning what happens after the detection of an extraterrestrial signal.

Here's what we'll do if we receive an alien signal:

1. Anyone who thinks they have detected an extraterrestrial signal should not rush to make a public announcement. Instead they should first seek to verify their conclusion.
2. Once the discoverer has completed their due diligence, other organisations signed up to the guidelines should be informed (and the discoverer’s government) so that they too can seek to confirm the discovery.
3. If these partners find that the signal is credible evidence of extraterrestrials, the discoverer should then inform international bodies, including the Central Bureau for Astronomical Telegrams and the UN.
4. The detection should now be communicated promptly, openly and widely. The discoverer should have the privilege of making the first public announcement.
5. Data concerning the detection should be made available to the international scientific community.
6. The source of the signal should be monitored and any additional detections should be distributed for further analysis.
7. The International Telecommunication Union should be notified so that the frequencies on which the signal was detected can be protected.
8. No response to the signal should be sent unless international agreement to do so is reached.
9. An international committee of scientists and other experts should be established as a touchstone for analysis of the signal.

The full wording of the Declaration Of Principles Concerning Activities Following The Detection Of Extraterrestrial Intelligence can be found at iaaseti.org.

How would we recognise an alien signal?

Recognising a possible extraterrestrial signal is a process of elimination. The first thing to do is weed out natural signals. These tend to be smeared over a large number of radio frequencies. Artificial signals, however, are confined to narrow bands around a single, central frequency. Most of these narrow-band transmissions will be interference from Earth, though. The way to weed those out is to look for traces of movement in the signal. Most terrestrial interference will be from transmitters on Earth, which are not moving relative to the telescope, while celestial signals will appear to move at the rate of the Earth’s rotation.

Movement shows up in radio signals through something called the Doppler Effect. This is the same phenomenon that makes the sound of an emergency vehicle’s siren seem to change speed as it approaches and passes you. As the Earth rotates, it will impart a specific Doppler Effect on a celestial source that astronomers can isolate and distinguish from other moving sources such as aircraft and satellites.

Read more about aliens:

• Dyson spheres: How humans (and aliens) could capture a star's energy
• Project Galileo: The search for alien tech hiding in our Solar System