The Habitable Zone

Is there any other evidence besides the atmosphere that may indicate that the earth can support life? We can impose additional requirements on the stars carrying the planets:

The planet exists in the area at the proper distance from the star (or its satellite) of the planet and keeps the water in a liquid state (ie it is not too cold or too hot).

For these two statements, most of the stars looking for a planet of life are F, G, K or M stars. O, B, and most A stars are very short lived, so we expect that their planets will fail to develop complex shapes of life. For low-quality stars with long lifetimes, astronomers can define the area (or HZ) that they can reside as the area around the star and the water can maintain its liquid state. In the figure below, the blue band shows the location where you can live. As expected, in the case of low mass cooled stars, this area is close to stars, in the case of high quality hot stars, this area is away from the stars. In this particular case, the earth seems to be in the middle of the area where the sun can live.

Another visualization of the habitable area is shown below. The red part is too hot, the blue part is too cold, and the green part is just for liquid water. Because it can be described like this, it is sometimes called "blond girls area."

The size of the habitable area is clearly determined by the brightness of the star which determines the equilibrium temperature of the planet. However, modern models of the extent of residential areas take into account more subtle effects such as regulation of carbon dioxide in the planetary atmosphere by the carbonate-silicate cycle. Scientists at Pennsylvania State University, including Professor James Caster, showed that in this particular process the residential areas are farther than originally assumed. In the solar system, the earth is inside the modified HZ near its inner edge and Mars is just outside the outer edge. Our colleague Ravi Kopparapu holds the latest visualization of habitable areas, including all known extrasolar planets in parents' HZ

When we are studying star evolution, you see the trajectory of the evolution of stars in the HR diagram: stars do not maintain the same color and brightness throughout their lifecycle. When a star begins a stable hydrogen fusion on the main sequence it is placed in a specific position in the HR plot. This is called zero era master series or ZAMS. However, as stars become older, it gradually disappears and becomes brighter. When the brightness changes, the location of the place where you can live will also change. You can define Continuous Habitability Area (or CHZ) as the area where liquid water may be present while the whole main series of stars are alive.

Last point on CHZ In our solar system, remember that the satellites Europa and Titans are considered to be places where life may exist. But these two satellites are far beyond the CHZ around the sun. So CHZ is an interesting place to investigate planets around other stars that may support life, but it is not the only place in the planetary system that can support life.

When astronomers Peter Ward and Donald Brownlee introduced the concept of "Galaxy residential area" in 2000, the residential area theory was updated, and later developed with Guillermo Gonzalez later. Area Galaxy Habitability Area is defined as the most likely area of ​​life in the Milky Way, including those closer to the center of the Milky Way, where stars are richer, heavier elements, less frequent than galaxies, planetary orbit and life . It will be destroyed at the center of the Milky Way by the common point of violent radiation and huge gravity.

A residential area, an orbit area around a star, a planet on the earth can have liquid water on its surface and can support life. The definition of the zone that can live is based on the assumption that extraterrestrial life shares this requirement, since liquid water is essential to all life on the planet. This is a very conservative (but observable) definition. Because the surface temperature of the planet depends not only on its proximity to the star but also on its atmospheric greenhouse gas, its reflectivity, and the atmosphere or ocean circulation. In addition, internal energy sources such as radioactive decay and tidal heating can warm the surface of the earth to the melting point of water. Because these energy sources also maintain groundwater reservoirs, planets can contain life even though they are not in the area where their stars can live.