The media often brings news about newly-discovered planets in the “habitable zone” of some star beyond our solar system. They usually mean that scientists think the planet may be the right distance from the right kind and size of star for water to exist in liquid form. Calling such a planet “habitable” may be misleading because there are many factors required to support life.
One of those factors is that the star also has to be extremely stable, as our Sun is. Consider the fact that a change in the Sun’s luminosity of only two percent would make Earth uninhabitable. That seems hard to believe, but scientists have calculated and modeled it.
If the Sun were only two percent dimmer and everything else the same, there would be more snowfall. Because snow reflects more of the Sun’s heat than land or water does, Earth’s surface would become cooler. The cooling would cause more snowfall, resulting in more reflection of the Sun’s rays and, therefore, more cooling. The result would be a runaway freezing of the surface water, and the entire Earth would become covered with ice and snow.
On the other hand, a two percent increase in the Sun’s brightness would cause greater evaporation of Earth’s surface water. The resulting water vapor would act as a greenhouse gas, trapping more of the Sun’s heat in the atmosphere. The increased heat would cause more evaporation resulting in more water vapor and an increased greenhouse effect. The result would be global warming on a massive scale.
Either way, life on Earth would not be possible. We don’t know if there are any other planets in the universe with all the factors required to support life. Our finely-tuned Sun is only one of many features that allow life on this planet. Some people would suggest that our just-right Sun is merely an accident, but we think this is another case of design by a wise Creator.
The more scientists study Earth and other objects that surround us in space, the more variables we realize must be carefully controlled for life to exist. Many times before, in our posts, our videos, our books, and our printed quarterly, we have discussed the growing list of parameters that must be carefully chosen. NASA posted a graphic of different kinds of stars in the cosmos and whether they could support life. This picture of stars and habitable zones adds to our understanding of the unique qualities of our Sun.
Water is essential for life. Science defines life as having properties such as moving, breathing, eating, reproducing, and responding to outside stimuli. We don’t discuss “rock people” or “gas people” because they don’t fit that definition. For that reason, scientists are interested in stars and habitable zones–the just-right “Goldilocks zone” surrounding a star where water can exist as a liquid.
In their daily posting on apod.nasa.gov for January 31, 2020, NASA gives the distribution of Goldilocks zones for G spectral stars like our Sun, which are yellow, K dwarf stars, which are orange, and M stars, which are red. The other spectral groupings, such as blue stars, are not considered because of their high radiation levels and activity, which would make life impossible.
The most common type of star in our galaxy, making up 73% of all stars in the Milky Way, are M stars. These red stars have very active magnetic fields and massive radiation. Their Goldilocks zone would be minimal and very close to the star. Orange K stars make up 13% of the stars in the Milky Way. They have a modest Goldilocks zone but are fairly active with some radiation levels. Yellow G type stars like our Sun, make up only 6% of the stars in the Milky Way. These stars have very large Goldilocks zones, and they are very quiet compared to K stars.
As we consider stars and habitable zones, we must realize that the type of star is just the beginning of the variables necessary for a star system to support life. Other critical factors include the size of the star, the location of the planet relative to the star, and the shielding a planet has for protection from the radiation of the star. Also, the stability of the star’s location in the Milky Way is another factor that goes into a life-supporting planetary system.
Our existence is not a product of chance. The more we learn about the Earth, the Sun, and the stars and habitable zones within the Milky Way, the more we understand that the statement, “In the beginning, God created the heaven and the earth” is a massive understatement of what God did to make a place for us to exist.
Does it matter how far away the Sun is? Absolutely yes. The picture shows the order of the planets in our solar system, but not their distance from the Sun. So how far away is the Sun from Earth?
Any star that has planets orbiting it may potentially create a “habitable zone” where the light and heat are just right for the possibility of life to exist. Earth resides in the middle of the Sun’s habitable zone with Venus and Mars near the edge of the zone. Of course, there are many other factors required to support any kind of life, and it appears that Earth is the only planet in our solar system that has all of those factors. Earth has everything needed to support not just primitive life, but advanced life.
So what is the range of the habitable zone? That depends on the star. The size and brightness of the star are critical. Another essential factor is the type of radiation emitted by the star. Our Sun has the just-right radiation. Other stars may emit x-rays, gamma rays, or other deadly radiation in amounts that would destroy all life and prevent a habitable zone from existing.
Back in the eighteenth century, scientists determined the distance to the Sun by watching a transit of Venus across the Sun. Venus passes between the Earth and the Sun twice every hundred years or so. By measuring the time of the transit of Venus from two locations on Earth, scientists were able to use triangulation and simple math to calculate the distance to the Sun.
But the question was, how far away is the Sun? The Sun is about 93,000,000 miles (150,000,000 km) away from us. Since the speed of light is 186,000 miles (300,000 km) per second, it takes about eight and one-third minutes for the light from the Sun to reach the surface of the Earth. The energy the Sun delivers to our planet is just right to make life possible.
On a clear, moonless night, you can look up and see the Milky Way. Actually, we are in the Milky Way, a spiral galaxy of 200 billion stars one of which is our Sun. We are located in a spiral arm of that galaxy 26,000 light-years from its center. Our location seems to indicate many galactic coincidences.
At the center of the Milky Way (and perhaps all galaxies), there’s a black hole sending out lethal radiation to a distance of 20,000 light-years. Farther out than 26,000 light-years from the center, heavy elements that are vital to our existence and survival are scarce. We are in what astronomers call the “galactic habitable zone.”
Spiral galaxies rotate, and we are near the co-rotation spot where our solar system moves at almost the same rate as the spiral arm we are in. If we were in precisely the co-rotation spot, we would experience gravitational “kicks” which could send us out of the habitable zone. If we were far away from the co-rotation spot, we would fall out of the arm and be subjected to deadly radiation.
In the vast majority of spiral galaxies, the habitable zone and co-rotation spot do not overlap. Most other spiral galaxies are not as stable as ours. Most galaxies are not spiral galaxies and would not have a stable location for advanced life.
Furthermore, galaxies exist in clusters, and our cluster called the “Local Group” has fewer, smaller, and more spread-out galaxies than nearly all other clusters. Most galaxies are in dense clusters with giant or supergiant galaxies which create deadly radiation and gravitational distortion making advanced life impossible.
Yesterday we mentioned an article by John Gribbin in Scientific American (September 2018, page 96 or online HERE.) The title of the article was “Are Humans Alone in the Milky Way?” Although Gribbin suggests that some form of life exists elsewhere in the galaxy, he insists there could be no sentient beings like ourselves. The reasons for concluding that we are alone in the Milky Way galaxy are these “amazing” and “implausible” “coincidences.”
SPECIAL TIMING. The elements that make up a terrestrial planet like Earth are produced from hydrogen and helium by thermonuclear fusion. We see supernova explosions producing the heavy metals that make up a terrestrial planet and life itself, but it takes time for this process to create the necessary elements. Most of the exoplanets we see have minimal amounts of the heavy elements because they are early in their stellar evolution. Even the sun itself is 71% hydrogen and 27% helium with only 2% metals. The timing of putting the materials together to make a terrestrial planet is critical.
LOCATION IN THE GALAXY. The location of a solar system in the galaxy makes a difference. The galactic habitable zone is the area where there is a freedom from the concentration of supernovae. Systems near the center of the galaxy have high levels of radiation in the form of X-rays and cosmic rays. There is a massive black hole in the center of our galaxy called Sagittarius A which produces massive amounts of radiation. Gamma-ray bursts occur in certain places in the galaxy. In our area of the galaxy, sterilizing radiation bursts do not happen.
Recent studies of the galactic habitable zone tell us that it extends from 23,000 to 30,000 light-years from the center or only about 7% of the galactic radius. This zone contains only about 5% of the stars, because stars tend to concentrate toward the core of the galaxy. Our Sun is close to the center of the galactic habitable zone providing rare long-term stability.
TYPE OF PLANET. So far astronomers have discovered about 50 “earth-like planets.” What that means is that they have found rocky planets in the habitable zone that are about the same size as Earth. Venus would qualify as an “Earth-like planet,” but it is an excellent example of how misleading that statement is. Venus has a thick crust with no sign of plate tectonics, no magnetic field, no way to recycle materials, and no stabilizing moon. Our Moon keeps the tilt of Earth’s axis at 23 ½ degrees providing a stable climate.
Realize that all of these factors are just to have a ball of rock in the right place at the right time with the right materials with which to make life. Now we would need to calculate the odds of getting the right chemicals together at the right time in the right place with the right catalyst to make the first living thing. Books have been written about how improbable those steps are. The writers are not religious fanatics, but scientists who are doing the research.
The Scientific American article, concludes that we are alone in the Milky Way:
“As we put everything together, what can we say? Is life likely to exist elsewhere in the galaxy? Almost certainly yes, given the speed with which it appeared on Earth. Is another technological civilization likely to exist today? Almost certainly no, given the chain of circumstances that led to our existence. These considerations suggest we are unique not just on our planet but in the whole Milky Way. And if our planet is so special, it becomes all the more important to preserve this unique world for ourselves, our descendants and the many creatures that call Earth home.”
One of the interesting scientific discoveries of the past decade has been that there are planets orbiting other stars (called exoplanets) and that many of these planets may have temperatures that would allow liquid water to exist on their surfaces. There has been a special interest in M dwarf exoplanets.
In theory, all stars could have a possible planet in a zone where the temperatures would be between zero and 100 degrees Celcius. However, that zone could be very small, and there are many factors required to make life possible, and many that would make life impossible. In an article in Science News dated June 24, 2017, (page 18) some of those factors were mentioned. They include stellar flares, gravitational locking, and especially the life expectancy of the star.
Stars age and the period during which their habitable zone could exist in a stable form is very short. M dwarf stars are held up as having long enough lifetimes for water to exist and biological processes to take place. Since they are the most common type of star in the Milky Way (70% of all the stars in our galaxy) scientists are studying them closely. We have reported before on one of them called TRAPPIST-1.
As more data comes in, it is becoming apparent that although M dwarf exoplanets remain as they are for very long times, they are still not stable enough to sustain life. Scientists hoping to find another “earth” orbiting another star are learning that M dwarfs are not good candidates even though they have some of the conditions necessary for life.
An exoplanet is a planet orbiting a star other than our Sun. At the time that I am writing there are 3,565 known exoplanets, but by the time you read this, there could be over 4,000. In spite of what the media says, there have been no “Earth-twin,” “habitable,” or “Earth-like” planets found. These are cliches the media throws around which have very little scientific validity. When a scientist identifies a planet as being in a habitable zone, it simply means that water could exist on the planet in a liquid form. Scientists consider water, carbon, and oxygen essential for life. But there are many other variables that must be carefully chosen before an “Earth-like” planet could actually have life on it.
The exoplanets that have been discovered so far are an incredibly varied group. Most are too big, too hot, too gassy (like our gas giant Jupiter), or they have orbits that are too eccentric (ovals, not circles) to support life. Some planets are so hot that they rain glass. Others are so cold that no biological organism could exist on them. The stars around which these planets orbit are also an incredibly varied group with enormous ranges in size, activity, temperature, and radiation levels. In most cases, those stars are orbiting other stars making life nearly impossible.