The universe is infinite, composed of many worlds and animated by common life and common cause.
– Giordano Bruno, author of On the Infinite Universe and Worlds (1583)

The Holy Grail of many astronomers is to find the right-sized rocky planet orbiting its parent star at the correct distance for life¹ to be viable and sustainable - a planet similar to Earth. Imperatively, its orbit would need to be circular, or nearly so, and the parent star should be stable. Another of the main criteria would be the presence of water (H₂O) in liquid form, considered essential for living creatures by most botanists.

This 'just right' set of circumstances, called the habitable zone, has earned the nickname 'Goldilocks zone' after the children's story Goldilocks and the Three Bears in which the porridge is neither too hot nor too cold, but just right. As of July 2009, no such planets (or large moons) have been found, but there are some likely candidates being studied.

55 Cancri A

There has been one extrasolar planetary system found in the constellation Cancer the Crab up to 2009, belonging to the star 55 Cancri A (this is the star's Flamsteed designation, after the system used by the first Astronomer Royal, John Flamsteed, 1646 – 1719). This star also has the designations Rho¹ Cancri² and HD 75732³, and it is almost 42 light years⁴ distant from us. At 6th magnitude, yellow dwarf 55 Cancri A is approximately the same mass as our Sun, and it rotates in 42 days. It is visible through binoculars, but its binary companion, 55 Cancri B, a red dwarf of 13th magnitude, is only visible in a telescope. The two stars are gravitationally bound even though the distance between them is over 1,000 AUs (1AU – astronomical unit – is the average distance between the Earth and its Sun).

The 55 Cancri A system is the first known quintuple-planet system discovered, and it remains a record-breaker at the time of writing (July 2009). It features in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission. Also, 55 Cancri A was one of the five⁵ targets of the Valentine's Day 2003 Cosmic Call by Team Encounter participants. The Cosmic Call included a duplicate broadcast of the 1974 Arecibo message⁶, names of Team Encounter and personal messages from them. Also included were letters from schoolchildren around the world, female astronaut Dr Sally Ride and musician Greg Lake of Emerson, Lake and Palmer. Due to the distance involved, the Cosmic Call won't be received in 55 Cancri's realm of the galaxy until around 2046, and even if there's anyone to decipher it and respond, their reply would not reach us before 2088.

The Planets

This system is interesting because there's a giant planet [Ad] at 6AU and four smaller planets inward of 0.8AU, with a huge remaining gap in between, right where we would expect to find an Earth-sized planet.
– Geoff Marcy, astronomy professor at UC Berkeley

The nomenclature that has been decided upon for planets is to use a lower-case letter after the parent star catalogue number (or name) eg '55 Cancri Ab'. These letters start at b (rather than a) so there is no confusion between binary star classification. The sequence of these letters denotes the order in which they were discovered rather than the position of the planets around the star. Therefore the first-discovered planet of 55 Cancri A is 55 Cancri Ab even though 55 Cancri Ae has a closer orbit to the shared sun 55 Cancri A.

The planet 55 Cancri Af is in the system's 'Goldilocks zone' (260 day/year equates to 0.78AU) but as a gas giant it's not a candidate for the search for extra-terrestrial life. However, should the planet have a large, rocky moon⁷ with an atmosphere and liquid water, that would make it a distinct possibility.

The idea of life existing in a binary star system on a moon next to a gas giant is quite extraordinary.
– An h2g2 Researcher

If there were life on one of the hypothetical ocean moons of planet Af, there'd be six major things in the sky: its sun 55 Cancri A, its own planet (Af), three (known) inner planets and the binary red dwarf 55 Cancri B, which would provide interesting observational opportunities. An artistic concept of such a view is found at NASA's JPL website.

Planets Ab and Ac would regularly transit their shared sun and there would be exciting occultations to witness. The outermost planet Ad would also be viewable at times, though the innermost (Ae) would likely be elusive due to its extreme proximity to the sun. On planet Ae there is no respite from the excessive heat and constant solar radiation would make conditions unbearable. These types of hot gas giants have the fastest atmospheric winds known to science. The atmosphere is superheated to such an extent that it is energised and the excited wind rushes around the planet at supersonic speed, heating up the cooler air on the far side, creating a worldwide scorching temperature.

Figures given in the table below include the length of the planets' orbital periods around the parent star, which we know of as a year, in order of their distance from the star. The mass of the extrasolar planet is usually compared to that of Jupiter, our Solar System's largest planet, known by astronomers as the 'Jovian scale', because most of the planets discovered so far have been gas giants. So many extrasolar planets have now been detected that astronomers are more effectively describing sub-jovians by comparing them with other gas giants Neptune and Saturn.

Future Observations

The powerful planetfinder James Webb Space Telescope is due to be launched in 2014. The 55 Cancri A system will be monitored for signs of planet Ag, which may be in the 'gap' already noted by Professor Marcy. Interestingly, Ag is the chemical symbol for silver, although if astronomers discover an Earth-type planet, they'll reckon they have struck gold!

What the world calls great we will admire less and all the nullities most of the people set their heart on we despise noble, because we will know that myriads of settled and equally good fitted worlds like ours exist.
– Christiaan Huygens from The Discovery of Celestial Worlds c1690

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¹ As we know it.
² The Greek letter combined with the genitive of the constellation name is known as the 'Bayer designation', after the man who devised the system.
³ HD stands for the Henry Draper catalogue.
⁴ A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.
⁵ The other four selected candidates were: 47 Ursae Majoris, HD 245409 (in Orion), HD 10307 (in Andromeda) and HIP 4872 (in Cassiopeia).
⁶ The 1974 Arecibo message was put together by Frank Drake and Dr Carl Sagan and transmitted towards M13 in Hercules. It was made up of 1,679 binary digits (almost 205 bytes). It lasted almost three minutes, was transmitted at a frequency of 2,380 MHz and sent only once. At the time it was the strongest man-made signal ever broadcast. The main drawback was that the journey to its destination would take 25,000 years. This project obviously wasn't about establishing communication, but more a 'we were here' message.
⁷ At least the size of Mars.