Should this description differ a lot from  actual scientific or popular books and descriptions in the web what concerns habitable zones and conditions for life: this is intentional. Especially the paragraph which treats the role of the moon will be very different and you may compare by yourself when other descriptions take over these arguments. Also some formulae exist to this subject which only reflect the much too simplistic view of its developper (and of the people who repeat this formula in books, newspapers and television).

Conditions for life ( only the most essential are listed here)

The first distinction we should make is between low forms of life and higher forms of life. Low forms of life is as we know today possible in many different environments. Maybe if certain aspects of their environment would have been more friendly to these life forms they could even have evolved to higher forms of life. But this is at the moment only speculation, so we should constrain ourself to life as we know it from earth and to higher forms of life.

The first precondition for higher forms of life as we know it from earth is quite naturally the right distance of the planet from its star. This is in the first place a question of the temperature on the planet which shouldn't be too high or too low. This is intentionally expressed so vague.

A moon of the size of our moon is essential for development of higher forms of life on a planet. It stabilizes the rotation of the planet, it is essential for the magnetic poles and thereby the magnetosphere, it is essential for surface water on a planet and thereby the atmosphere. Even the volcanic activity of the earth - which is due to the moon, as you can read in other parts of this site -  was essential for development of life on our planet. And last but not least by producing the tides a moon is essential for the evolution of higher life forms in general. There are even theories that make the tides a prerequisite for the spreading of life from the aquatic environment to the land environment. But the moon serves many other purposes to all sorts of life forms on earth: it serves as a timer for many different animals and plants, for the corals for instance to release sperm and larva at the same time. There are many more implications of a moon of the size of our moon: for instance a  moon serves as shelter against impacts of meteorites and asteroides.

Big outer planets are essential and give an inner terrestrial planet a certain shelter against havy impacts. Instable ortbits around stars with high debris endanger evolution of complex life. A high number of cometary impacts create global extinction events too often.

The orbits of all planets around a star should have low excentricities and high gaps between planet orbits. These conditions are essential for the long term stability of a  system.

There should be no source of radiation in the proximity of such a planet. Negative effects of radiation would not only damage a protective ozone layer of the planet but also endanger life on such a planet in many other ways. For instance in the case of pole reversal and therefor lack of protective magnetic shield extinction of all higher forms of life could occur. High-energy particles  produce secondary particle cascades in the planet's atmosphere. Increased radiation levels at the surface of the planet would be the consequence.

The rotational rate of the planetary system around the Galactic Center should be synchronized with the rotational rate of the spiral arms. A low eccentricity of the planetary systems orbit should be given. The system should not be located too close to the Galatic Center because this means gravitational disturbances.

Finally: Water was always thought to be a scarse resource in the universe. More recent investigations seem to show that this is not the case.

Detection of earth like planets

I) An exoplanet’s atmosphere can be tested as it transits or passes in front of its host star. During a transit, the molecules in a planet’s atmosphere absorb some of the starlight that passes through the atmosphere. Thus it is possible to identify those molecules by studying the changes in the spectrum of colors emitted in the light that radiates from the star. Because it is since long known what kinds of colors (patterns) are absorbed at certain wavelengths by certain molecules(Fraunhofer), scientists study spectra patterns to pinpoint the types of molecules that may exist in a planet’s atmosphere. They can also learn how much heat an exoplanet radiates and absorbs by observing it as it passes behind its host star (known as occultation).

II) If the conditions are right the surface of a planet emits energy in the infrared portion of the electromagnetic spectrum. The gases in a planet's atmosphere absorb some of these wavelengths. Which wavelengths get absorbed depends on which atoms and molecules are present in the planet's atmosphere.

Spectroscopic measurements can be used to detect  ozone, which serves as an indicator of oxygen (see pictures below). If there are water pattern in the spectrogram a further hint for similar conditions as on earth is found which means that next methane is searched for in the spectrogram.  Methane is a strong indicator for biogenic activity as we know it from earth. Carbon dioxide is an indicator for an atmosphere. Other gases such as nitrous oxide can also be indicators of life. But since we know today that life has found many ways to exist and evolve also for sulfuric or even arsenic  signs of life we should test new found planets. As our knowledge increases in this point even other environments should be tested for.  Today earth's atmosphere contains molecules of nitrogen (N2), oxygen (O2), and trace gases that include argon (Ar), methane (CH4) and carbon dioxide (CO2). Water vapor (H₂O) occurs in varying amounts. Earth is shielded from ultraviolet radiation by a layer of ozone (O3), which only forms from oxygen atoms.

Earth-like planets in other stadiums than our earth will be more difficult to detect. An early Earth-like planet that has not yet developed an oxygen-rich atmosphere will be difficult to analyze, because scientists do not know much about what early life on Earth was like, or what sort of gases were released by early life forms. Oxygen was probably a toxic gas to early life, this alone shows that contradictous measurements can lead to similar results.

Oxygen (O2) and Ozone (O3)

Carbon Dioxide (CO2)

Methane (CH4)

Water Vapor (H2O)