The “habitable zone” is the range of planets’ orbits in which they receive the exact amount of heat from their stars that is necessary for Earth-like life to exist. It is generally thought to be stable for hundreds of millions of years and is determined primarily by the luminosity of the local sun. However, a recent study shows that this is not the case.
What is a “habitable zone”
It is believed that the key factor that influences whether an exoplanet will be habitable or not is whether its orbit is inside the so-called “habitable zone”. Simply put, whether it’s too hot or too cold. However, it’s really not so simple, and researchers from Visva-Bharati University in India recently published a paper on this.
Now exoplanets located in the habitable zone of their stars are known not so few – about 150. However, the question arises here, what actually should be considered such a zone, because a simple and clear formula at first glance, “where a star heats the celestial body for the existence of liquid water” at a closer look, can not 100% guarantee something, because there still need to take into account the features of the celestial body itself, on which the search for life.
That is why in practice, instead of one habitable zone, scientists use two in their calculations: optimistic and conservative. The first one is defined as a distance greater than where the world received as much energy as Venus 1 billion years ago, but less than where Mars’ level heat supply was 3.8 billion years ago.
The second zone of life is conservative, that is, narrower than the first. It is defined between the distance at which the greenhouse effect on a planet becomes uncontrollable and the distance at which it becomes ineffective to compensate for insufficient heat from a star.
External factors
However, it’s actually more complicated, and what other factors could affect the planet’s suitability for life has been focused on by the authors of a new study. They suggested that the interstellar environment of the system strongly influenced all of this.
The first external hazard that could affect the planet’s habitability is sources of high-energy rays. They are mostly supernovae that erupt nearby, but radiation can also be produced by neutron stars and black holes in binary systems.
The second dangerous factor is close passages of stars. They can strongly affect the orbits of the system’s planets and even throw it away from the system. There are other hazards as well. In order to somehow take them all into consideration, the researchers introduced two coefficients: the Solar Similarity Index (SSI) and the Neighborhood Similarity Index (NSI).
The first characterizes how much the star’s luminosity and other physical characteristics differ from the Sun’s. The second is the difference in the environment of the stellar system: the density of stars, the presence of luminaries that can explode as supernovae, black holes, and neutron stars.
What the research showed
Scientists have explored the vicinity of systems with planets in the habitable zone, about which they know for sure. To their surprise, the fluctuations in the NSI index turned out to be quite small. Only for two systems, TOI-1227 and HD 48265, were massive stars found nearby that could potentially explode as supernovae.
As for the risk of a really close flyby that would exactly hit the planets’ orbits, such a situation has only been detected once at all. So the NSI index doesn’t have so much of an impact on a planet’s habitability.
But that’s not the case with SSI. The luminosities of stars with planets differ too much. And they change excessively over the course of their existence. In other words, it turns out that traditional factors are the ones that influence the earth-like nature of the planet the most.
Provided by phys.org
