Isn't the Goldilocks Zone somewhere in the vicinity of Ursa Major, Ursa Minor and Ursa Media?
– An h2g2 Researcher

The habitable zone is the area of space around a star where conditions are favourable for life, eg the temperature wouldn't be too extreme if a planet orbited there. Some astronomers call it the 'Goldilocks Zone', referring to the porridge analogy in the fable Goldilocks and the Three Bears. In a nutshell, the Earth orbits in the (just right) habitable zone of our Solar System: Mercury and Venus are too hot (too close to the Sun), and Mars and the outer gas giants are too cold (too far away from the Sun).

Astronomers call the distance between Earth and the Sun an astronomical unit, it measures 150m km (93m miles) and this is the yardstick used for other planets, eg. Saturn is noted to be 9.5 AU distant from the Sun (that's neater than writing out 1,426,725,400 km). Not all stars are the same as our Sun, which is a main sequence  yellow dwarf, therefore the habitable zone varies from star to star. Astrophysicists and exobiologists have compiled a list of criteria whereby life could thrive on a planet within that system's habitable zone:

• The parent star should be stable.

• The planet's orbit would need to be circular, or nearly so.

• The planet (or moon) should be a rocky world with enough mass to create a magnetic field (to protect any inhabitants from being irradiated by the star), and have sufficient gravity to retain an atmosphere.

• The planet (or moon) should have a stable axial tilt in order to maintain a survivable diurnal¹ period.

• The presence of water (H₂O) in liquid form is considered essential for living creatures by most biologists.

• The planet would have to be young enough to be still active; carbon dioxide gas (CO₂) is released through plate tectonics (movement of the planet's crust), making photosynthesis possible.

The first 100 extrasolar planets discovered were all gas giants, like our own Jupiter. Such planets are incapable of supporting life (as we know it), but that doesn't exclude any attendant rocky moons (Jupiter has over 60 moons). Should a gas giant planet in a habitable zone have a rocky moon with enough mass to retain an atmosphere, possess suitable gravity, and have a magnetic field to shield it from the parent star's radiation, that could make life a possibility.

Planets in the Habitable Zone

• The Earth.

• HD 142415 is a yellow dwarf star the same class and age as our own Sun. It is a member of the southern sky's Norma constellation. Orbiting the sun-like star is a gas giant planet dubbed HD 142415 b. Its year lasts 386 days which places it at 1 AU (the same as Sun-Earth distance).

• Delta² Phoenicis b is a gas giant planet member of a binary star system in the mythical firebird constellation Phoenix. Delta² Phoenicis b takes 353 days to complete a year, so any moons in attendance will probably have a comfortable climate if they aren't too close to the gravitational forces of the planet. Speculation about any potential inhabitants has already been mentioned in the light-hearted Entry 'Broadcasting to our Galactic Neighbours'.

• 55 Cancri A is a system which is home to five planets. Unfortunately they are all gas giants, but one of them does orbit in the habitable zone. A large-enough rocky moon attending 55 Cancri Af may be suitable for life.

• Mu Arae is a yellow dwarf similar to our own Sun and planets have been detected within that system. Two of them reside in the habitable zone, planets mu Arae b and mu Arae d. Unfortunately both worlds are gas giants like Neptune (except they're warmer). This doesn't exclude them though, as they could have rocky moons in orbit around them which might be a suitable home.

• Nu Octantis, the luminary of Octans, is a spectroscopic binary star system some 69 light years² distant. A gas giant planet orbits at 1.2 AU; that's a 418-day year around a star about one and a half times the size of our Sun. We know that the two stars orbit each other, and the planet orbits the larger star at 0.45 that distance. Nu Octantis b is an intriguing enigma as it shouldn't exist, but astronomers' best guess is that its orbit would have to be retrograde, that is, in the opposite direction to the orbiting star. Quite how the dynamics of this three-body system works is unknown, but even though the planet is orbiting backwards, at least it's in the right place.

• Within the borders of Sextans are three extrasolar planetary systems - two of which have planets in their respective habitable zones. HD 92788 b is almost four times the mass of Jupiter orbiting its star at a distance of 0.97 AU; that's an acceptable 325-day-long year. BD-082823 is an orange dwarf star somewhat cooler and less massive than our Sun. Two planets were discovered in this system in 2009, and they couldn't be more different. BD-082823 b is a scorched, barren, tidally-locked rocky world, but its sibling BD-082823 c, a third the mass of Jupiter but still classed a gas giant, orbits at a more comfortable distance of 0.6 AU.

• The Gliese 581 (HO Librae) system had the most planets detected up to September 2010. Two of them, Gliese 581 c and Gliese 581 g, were super-Earths thought to be orbiting within the habitable zone until this data was discounted in 2014. Super-Earths are rocky worlds which have a greater mass than the Earth, although the thought of a clone of Earth where the trains run on time and English Heritage membership is free does appeal.

The Search Continues

According to SETI³ founder Frank Drake's estimation there could be 30 billion rocky worlds like the Earth within the Milky Way galaxy. What planet hunters are really interested in detecting are rocky (terrestrial) worlds like the Earth, orbiting at just the right distance from their stars, giving them a chance for life to thrive on them. A rocky planet-finder project is underway and astronomers have a list of likely candidate stars within 50 light years which they are checking out.

Search at Home

Anyone with a home computer can join in the search for suitable planets which might harbour life outside our own Solar System. An amateur extrasolar planet search project called Systemic has been set up for participants to analyse data in their spare time and submit results. Up to the time of writing (September 2010) some discoveries are awaiting confirmation by the scientific community. You don't have to be a rocket scientist to join in the fun, and you could eventually be the discoverer of Earth Mark Two!