As our search for extraterrestrial life gathers pace with the advance of technology, we seem to be identifying more and more planets that have the conditions where life could conceivably develop. When we do eventually discover one¹ where it has, it would be great to hop aboard a spaceship, Star Trek style, and pop over there to say 'Hi!' to the neighbours. But there's one small problem: distance. As Douglas Adams famously once wrote:

You may think it's a long way down the road to the chemist, but that's just peanuts to space.

These distances are frankly so huge it even takes light itself many years to get there. A spaceship travelling at a speed of 10km per second would take well over 100,000 years to reach our nearest star², Proxima Centauri. Any habitable planets would be even further than that. We'd need Scotty the engineer to develop something akin to warp drive for us to have a chance of getting there before the bunch of flowers wilted.

But that doesn't stop us from communicating with them. Admittedly, they haven't made much contact with us, if you discount the Wow! signal, but we've been zapping all sorts of broadcasts into the ether for decades now, and these travel at the speed of light. The signals will be pretty weak by the time they arrive, but some of them may well have been detected. Assuming their rooftop aerials are sufficiently sensitive and correctly adjusted, our interstellar neighbours may have been enjoying them for years.

The trouble is: they won't quite be up to date with the latest transmissions. Even the nearest extrasolar planets are around 10.4 light years away³, so they'll be over ten years behind us. Even so, they'd be able to recognise that there's intelligent life on Earth. Having said that, at the time of writing this Entry, the nearest of them will soon be receiving the first transmissions of Big Brother, so we should possibly reserve judgment.

Meet the Neighbours

Our Sun is only one star among perhaps 400 billion in our galaxy, the Milky Way, so there'll undoubtedly be a large number of planetary systems to investigate. However, it's difficult to contemplate just how far away some of these are. Light from the Earth will take 27,000 years to reach the centre of the galaxy, and maybe three times as long to reach the furthest stars on the other side of it.

There are countless more stars in other galaxies, but some of the ones we observe are themselves billions of light years away. Even if we'd started transmitting signals when the Earth was first formed, they won't have reached some of these distant worlds before our Sun's increasing luminosity makes Earth itself uninhabitable again in a few billion years or so. Extrasolar communication is an exercise that only has a realistic chance of success with those worlds much closer to home.

And so, in the spirit of galactic understanding, we've listed a few of our potential neighbours and which Earth broadcasts they'd be enjoying⁴, as of July, 2008:

Gliese 581 c (20.5 light years)

This planet, unofficially nicknamed Ymir, is one of three which orbit the star known as Gliese 581, in the constellation Libra. The star is a lot cooler than our Sun - it's what astronomers call a 'red dwarf'. It makes you wonder what they'd think of the BBC sci-fi comedy series Red Dwarf, which appeared on UK screens between 1988 and 1999. At the time of writing, the Ymirians are viewing Earth as it was in winter 1987, so they'll shortly be able to enjoy the first series.

If they're watching the news, they'll be seeing Mikhail Gorbachev and Ronald Reagan signing the INF Treaty to halt the nuclear arms race. If they're cricket fans⁵, they'll be watching England captain Mike Gatting embroiled in a public row with Pakistani umpire Shakoor Rana. Being Librans, they might well be interested in the scales of justice operating on champion jockey Lester Piggott, who'd just been sentenced to three years imprisonment for tax evasion.

55 Cancri Af (42 light years)

It's 1966 on 55 Cancri Af, one of five planets circling the star 55 Cancri A in the constellation of Cancer. If they're England football fans, then at the time of writing they're cheering the team on to victory in the World Cup final. It's sad to know they won't experience this feeling again for at least another 44 years (and counting).

Some people are on the pitch. They think it's all over... It is now.

The planet itself is believed to be very large - half the mass of Saturn. If it had moons, these could be capable of supporting life. The planet orbits its sun in around 260 Earth days.

Upsilon Andromedae c (43.9 light years)

Any inhabitants of the three planets encircling the star upsilon Andromedae are basking in a sun very much like our own. Astronomers call it a 'main sequence' star, one which generates heat and light by converting hydrogen to helium in its core. They were particularly excited in 1999 to discovery this planetary system around it. The middle planet of the three, upsilon Andromedae c, is considered to be the most habitable, orbiting its sun once every 242 Earth days.

Earth watchers in the area are in 1964, enjoying the Swinging Sixties and Beatlemania. They may have been particularly interested in the BBC's dark science-fiction series A for Andromeda and its sequel The Andromeda Breakthrough, co-written by astronomer Fred Hoyle, which aired a couple of years previously. These TV series are among many from that period thought to have been lost from the BBC archives. So, if we ever do make contact with the Andromedans, we could always ask if they managed to record them.

Mu Arae d (49.8 light years)

One of four planets orbiting the star mu Arae in the Southern Hemisphere constellation Ara 'the Altar', mu Arae d is rocky and about 10 times larger than the Earth. It bombs around its star like nobody's business, completing its yearly orbit in less than 10 Earth days. This is hardly enough time for a summer holiday; the season would be here and gone in the space of a weekend.

If the Muons (for want of a better name) aren't too dizzy to watch Earth TV, currently showing early 1959, they might be particularly interested in the BBC's new astronomy show The Sky at Night, now in its third year, presented by a dapper young Patrick Moore.

News-wise, they'll shortly be watching the first living Earth creatures to survive a space flight: monkeys Able and Baker, who reached an altitude of 300 miles in the nose cone of a Jupiter missile fired by the the USA from Cape Canaveral. They fared better than Laika, the dog sent into orbit by the Russians two years before. Sadly, Laika didn't return alive.

Iota Horologii b (56.2 light years)

Another planet encircling a main sequence star, and favoured target for ET-watchers, is iota Horologii b, in the constellation Horologium 'the Pendulum Clock'. The planet is a gas giant, many times the size of Jupiter. It orbits its sun at a similar distance to the Earth.

Iotans (let's not call them 'swingers') who detect our 1952 news broadcasts are seeing a troubled year for the British monarchy - the death of King George VI and the accession of Queen Elizabeth II. They may well be developing colour TV sets in order to watch next year's coronation ceremony.

70 Virginis b (59 light years)

It's summer 1949 on this planet orbiting the star 70 Virginis⁶, in the constellation Virgo. One of the first extrasolar planets to be discovered, scientists believe it's about 7.5 times the size of Jupiter and has enormously high gravity. So, assuming the inhabitants aren't too flattened to watch US TV, they're enjoying such gems as Candid Camera and Hopalong Cassidy, and looking forward to the first episode of The Lone Ranger. UK offerings include Muffin the Mule, with Come Dancing due to start in September.

They may also be watching the start of the Jet Age, as the de Havilland Comet makes its inaugural flight. They're also witnessing the beginning of the Apartheid regime in South Africa.

Delta² Phoenicis b (67 light years)

An inconspicuous star in the constellation Phoenix, delta² Phoenicis is a yellow sub-giant that's orbited by a gas giant planet, delta² Phoenicis b. Living on gas isn't too amenable to life, so again our neighbours would instead be found on any rocky moons. As it's a binary system - that is, there are two suns - they may not get much in the way of night, so it's difficult to see them as evening armchair viewers of soaps.

They're 67 light years away, so at the time of writing they're experiencing our world in 1941, gripped by the World War II. The BBC was off the air during this time, only resuming in June, 1946. But in the USA big things were happening. The year 1941 saw the launch of the first commercial TV stations, as well as the first game show, Truth or Consequences. The US public couldn't completely escape the war; a report on the Pearl Harbour attack was broadcast in December, unfortunately interrupting the transmission of a hockey game.

HD 82943 b (89 light years)

Planet HD 82943 b sadly lacks a common name, but it's in the constellation Hydra. Discovered in 2000, it's a gas giant that's about one and a half times the size of Jupiter, but may have moons which are habitable. It circles its star, a yellow dwarf, once every 440 or so days.

Inhabitants of these moons, if they exist, are so distant from us that at the time of writing they're only now starting to receive some of the early radio signals from Earth, as broadcast in 1919. Known as wireless telegraphy, the technology, developed by Marconi, hadn't become established in homes yet. The UK government was setting up its own infrastructure at the time - indeed, in 1919, Marconi was in the High Court suing the postmaster general for patent royalties. In 2011, our neighbours on HD 82943 b will start to receive the first scheduled radio transmissions from around the world.

The Digital Switch-off

In fact, the days of these unintentional broadcasts, started all those years ago by Marconi, may be numbered. New technology, in the form of low-power digital broadcasting and directional satellite transmissions, may result in our neighbours suddenly seeing the signals stop. Were this to happen, it may seem to them that some sort of disaster had befallen the Earth. They may even be driven to investigate why.

This scenario is examined in an episode of the animated sitcom Futurama. The Earth is invaded by aliens from Omicron Persei 8, who demand to see the final episode of Single Female Lawyer (a spoof on Ally McBeal), first transmitted but interrupted in 1999. A similar situation can be found in the plot of the comedy film Galaxy Quest, also from 1999. In it, aliens come to Earth and take back the crew of a spaceship they've seen on their TV screens, in order to save their planet.

Bespoke Messages

Not content with beaming nearly 100 years of broadcast material into space, scientists have tried to send specific messages to individual extrasolar planets - those they believe would have the best chance of receiving them. 55 Cancri f (described above) was one of five targets for the message Cosmic Call 2, sent by the METI⁷ project in 2003. Earlier messages were sent to other stars in 1974 (the Arecibo Message), in 1991 (Cosmic Call 1) and in 2001 (the Teen Age Message). The messages themselves don't make a lot of sense unless decoded properly, but advanced civilisations may recognise our digital representations of numbers, the structure of DNA and graphics of a human being and our solar system.

And if the cosmic calls don't stir them, then a good blast of pop music might. In February, 2008, NASA beamed into the void the Beatles song 'Across the Universe', on the 40th anniversary of it being recorded.

I see this as the beginning of the new age in which we will communicate with billions of planets across the Universe.
- Yoko Ono.

But, finally, spare a thought for the beleaguered inhabitants of a planet near the star 47 Ursa Majoris. Not only has it been sent two METI signals, but it was also the target of the first interstellar commercial on 12 June, 2008, when an advertisement for Doritos tortilla chips was zapped at it by a radar array at the EISCAT European space station.