There are many planets in the entire universe, but as far as we know, there is only a handful which could possibly support life. Space scientists and astronomers continuously scour the vastness of space for finding signs of life elsewhere in the solar system and even outside of it. Now, taking one celestial body at a time and devoting all your time and resources to ascertain whether it supports life would be too imprudent and wasteful. That’s why, astronomers look for certain pointers in a celestial body, and then determine if it has the potential to support life, or in other words, whether it’s habitable.
So, what are the requirements for a planet to support life?
Factors that make a planet habitable:
- It has to be a comfortable distance away from a star (Habitable Zone)
- The stars around it have to be ‘stable’.
- It should not have a very low mass.
- It must rotate on its axis and revolve.
- It should have a molten core.
- It should hold an atmosphere.
In the recent years, there has been a great deal of talk about finding liquid water on Mars and the possibility of life existing on the Red Planet. All this coverage has resulted in a lot of questions being raised for people always staring out into space, including a few interesting ones. For example, is the existence of water the only condition that needs to be present on a planet in order to host life? To better answer that question, let’s look at the various factors that make a planet habitable.
Taking Cues from Earth
As of now, we do not have any concrete proof of the existence of life anywhere else in the universe. Therefore, in order to establish the criteria of habitability of a planet (or a natural satellite), the conditions that support life on Earth also need to be extrapolated for other celestial objects. In other words, since we haven’t found proof of life anywhere else, the basic conditions that support life on Earth can be taken as a benchmark for the sustenance of life on any planet.
There are a number of conditions that a celestial body must fulfill in order to support life. These conditions involve certain geochemical, astrophysical, astrological and geophysical criteria. According to NASA, for a celestial body to sustain life, there should be “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism.”
In gauging the habitability of a planet, a number of factors must be considered, including the planet’s bulk composition, orbital properties, atmosphere, and potential chemical interactions. Some of the requisite conditions are as follow.
For a planet to sustain life, it has to be a comfortable distance away from a star, such as the sun of our solar system. Around a star, there is a shell-shaped region of space, called the Habitable Zone (HZ), where a planet can maintain liquid water on the surface. If a planet lies in this region, then there are good chances for the habitability of life on the surface. However, if life could exist without water in a certain part of the universe, then the definition of an HZ will change drastically. Also, since a star becomes more luminous as it ages, a planet must be farther away from it in the HZ to sustain life.
Stability of Stars
For a planet, it’s important that the stars closest to it are stable in terms of their luminosity. Although every star’s luminosity increases with time, it should not be too severe, or else it could simply burn up everything on the closest planet.
Since Earth is a terrestrial planet and is habitable, it is assumed that a planet must be made up of rocks, and not gases. Therefore, we don’t expect to find life on gas giants like Jupiter, Saturn, and Uranus. However, there could be life on the cloud tops of these planets, but it is highly unlikely, as there is no surface and the gravity of these planets is very high.
A planet with low mass is not suitable for habitation because low mass means low gravity. Low gravity further means that the planet won’t be able to retain an atmosphere, as constituent gases will easily reach escape velocity and be lost in open space. However, there are some exceptions to this condition: one of Jupiter’s moons, Io, is a small celestial body, yet it is volcanically dynamic and has distant chances of harboring life.
Rotation and Revolution
A planet must also rotate on its axis and revolve around its parent star (like the Earth going around the sun) to be habitable. Furthermore, if life on the planet is to be given a chance to evolve, certain other conditions have to be met in its rotational motion. For example, there should be some axial tilt perpendicular to its orbit, which will result in seasons on the planet or celestial object.
A Molten Core
To sustain any type of life, a planet requires a rapidly rotating magnetic field to protect it from flares from nearby stars. This is what we call the core of the planet. A planetary core is a terrific source of geothermal energy, allows the cycling of raw materials, and spawns a magnetic field around the planet to protect it from harmful radiation. It should be noted that Mars was known to have a liquid core at one time, but its heat dissipated quickly because Mars is a smaller planet.
Holding an Atmosphere
Earth’s atmosphere not only fulfills our most basic needs of providing oxygen but also keeps the planet warm by trapping carbon dioxide and other gases. It also protects life on the planet by blocking the vast majority of harmful radiation. Therefore, any habitable planet must have all the necessary conditions in place to have an atmosphere or at least a protective layer of essential gases.
As stated earlier, since we do not know of the existence of any life outside of Earth, these assumptions are deduced from the particular conditions that support life on Earth. If, however, there are organisms who can survive in altogether different conditions than what we have experienced here on Earth, then who really knows…maybe we already have company!
Are there more habitable planets?
Our satellites have been picking up on earth-like habitable planets for some time now. We have found many planets which seem to fit the bill partially, but them being far away has stopped us from getting any proper evidence. We shall look at a list consisting of such planets. Now, these planets have been deemed liveable (or we think that they are liveable) because they lie in the habitable zone of their respective stars. This means that they could have liquid water and thus sustain life. They also fulfill some factors that have been mentioned above. So now let’s see the list and how far these planets are from us.
- Proxima b – This planet revolves around the star Proxima Centauri. The star is 4.2 Light years away from the sun. It weighs around 1.3 times more than earth. It could have liquid water, but that depends on its atmospheric conditions. But as nothing is known about its atmosphere, we cannot be sure about it having water.
- Wolf 1061c – It is located in the constellation Ophiuchus and revolves around the red dwarf Wolf 1061. The planet is approximately 13.8 light years from Earth It’s around 4.3 times larger than earth. It has a short revolution time of about 17.9 days.
- Gliese 832c – It revolves around the red dwarf Gliese 832 and is present in its habitable zone. Its size is around 5 times that of Earth. Because of this, it’s often called a Super-earth. The planet is 16 light years away from our planet.
- TRAPPIST-1d – This planet is around 40 light years away from us and is in the constellation of Aquarius. It revolves around an ultracool red dwarf known as TRAPPIST-1. You can read about TRAPPIST-1d in detail in this article.
- Lunar and Planetary Institute
- New York Center for Astrobiology
- Genetic Science Learning Center
- The Astrophysical Journal