Planetary Habitability Factors and Stellar Life Cycles

Essential Factors for Planetary Habitability

For life to evolve and thrive, several critical conditions must be met:

• Distance from the Star (Habitable Zone): If a planet is too close or too distant, the prevailing temperature prevents the existence of liquid water.
• Sufficient Planetary Gravity: If gravity is too low (as on Mars), the planet cannot retain its atmosphere. The resulting absence of atmospheric pressure causes the hydrosphere to vaporize.
• Molten Metal Core: The rotation of the core generates a magnetic field that shields the planet from harmful X-ray and gamma radiation emitted by the star.
• Presence of a Large Satellite: Without a large gravitational anchor (like the Moon), the inclination of the planet's axis of rotation might vary considerably over time, causing major climate changes.
• Stellar Lifetime: Massive stars live much shorter lives. Since life requires billions of years to develop, only solar-type stars and stars less massive than the Sun have stable activity long enough for life to evolve.
• Existence of Nearby Giant Planets: Thanks to their strong gravitational attraction, giant planets can divert asteroids, protecting inner planets from potential impacts.
• Location within the Milky Way: Planets must be far from the galactic center, where supernova explosions, which emit large amounts of radiation harmful to living things, are much more frequent.

Stellar Evolution and Cosmic Phenomena

The Life Cycle of Solar-Type Stars

1. The hydrogen, lithium, and other light metals in the body of the star react. Energy is released, and the initial contraction stops.
2. When lithium and other light materials are consumed, contraction resumes. The star enters the main sequence stage of development, where hydrogen fuses into helium at very high temperatures through the catalytic action of carbon and nitrogen.

This thermonuclear reaction is characteristic of main sequence stars and continues until all the hydrogen is consumed. This process can last 10 billion years. When the release of energy ends, the contraction starts again, and the temperature of the star rises again.

Evolution of Massive Stars and Stellar Remnants

Stars much more massive than the Sun undergo more rapid evolution, lasting only a few million years from birth to the explosion of a supernova. The remains of the star may be a neutron star. However, there is a limit to the size of neutron stars, beyond which these bodies are forced to shrink until they become a black hole, from which no radiation can escape.

Cosmic Background Radiation Discovery

Penzias and Wilson discovered that a very weak radiation came from all points of the universe. This radiation was so weak that it must have originated from something very distant, and it is called the Cosmic Background Radiation.