What are the requirements for a planet to be habitable?

Question

What will be an ideal response?

Answer 1

If a planet is to be a suitable home for living things, it must be in a stable orbit around its sun. That is simple in a planetary system like our own, but most planet orbits in binary star systems are unstable unless the component stars are very close together or very far apart. Astronomers can calculate that in binary systems with stars separated by distances of a few astronomical units (AU), the planets should eventually be swallowed up by one of the stars or ejected from the system. Half the stars in the galaxy are members of binary systems, and many of them are unlikely to support life on planets.Moreover, just because a star is single does not necessarily make it a good candidate for sustaining life. Earth required between 0.5 and 1 billion years to produce the first cells and 4.6 billion years for intelligence to emerge. Massive stars that live only a few million years do not meet this criterion. If the history of life on Earth is at all representative, then stars more massive and luminous than spectral type F5 are too short-lived for complex life to develop. Main-sequence stars of types G and K, and possibly some of the M stars, are the best candidates.The temperature of a planet is also important, and that depends on the type of star it orbits and its distance from the star. Astronomers have defined a habitable zone around a star as a region within which planets have temperatures that permit the existence of liquid water. The Sun's habitable zone extends from around the orbit of Venus to the orbit of Mars, with Earth right in the middle. A low-luminosity star has a small habitable zone, and a high-luminosity star has a large one.