Is Earth the only habitable planet in our galaxy? Many astronomers believe that it is not and that there are many other planets in a habitable zone. Although, just because it is in this habitable zone, doesn’t mean that it is actually habitable, there are many other factors that go into determining if it is actually a planet that humans can live on. Another question that is brought up is, are we alone in the universe? This can have a variety of answers, but the answer to how many habitable planets there are will lead to an answer for if we are alone.
Planets are all over the Milky Way galaxy, but how many of them are actually habitable? First, the planets have to be discovered before it can be determined if they are habitable or not. According to NASA, there are 5 different ways to discover planets. The first being, watching for a wobble with stars. As a planet orbits around a star, it cause the star to have a slight wobble in it, the gravity from the planet has an effect on the star even though it is a whole lot less than what the star has on the planet. As the planet orbits around the star, it causes light waves to stretch out and then compress, which changes the color of light we see. This was the first successful way that planets a planet was actually discovered. This method is considered radial velocity.
The next method according to NASA is the transit method. This method is when a planet passes in front of a star causing the light of the star to dim quite a bit. The size of the planet and the distance away from the observer can be determined from a transit. The further away the planet is, the longer it takes to pass in front of the star, causing the transit to be longer. The bigger the planet is, the more light that gets blocked. As a result of this, the light curve is deeper and more profound. What makes the transit method a challenge is when there are multiple planets passing at the same time. If this happens, the light curves combine and it just takes the astronomers more time to try and figure out which part of the curve is which planet. It gives the same information as if one planet was passing, but the two or more planets make one big curve in the graph. Depending on what color light passes through the planets atmosphere that passes by the start, astronomers can figure out what that planet’s atmosphere is made out of or the temperature of the planet. The transit method is what is responsible for the findings in the Kepler missions and finding thousands of new planets.
A method that is still being worked on, is direct imaging of exoplanets. These planets that are being found are significantly dimmer than the stars that surround them, so finding them can be difficult. NASA compares it to finding a flea in a lightbulb. Astronomers had to build a light blocker for the camera to drown out the light from the stars. This method is still being worked on, but it is expected to be a key tool to figuring out the characteristics of exoplanets as well as finding new planets.
Gravitational Microlensing is the next method NASA talks about. This method comes from Albert Einstein’s thinking about gravity, big objects, like planets or stars, warp the fabric in space. This method happens when the gravity of a planet or star concentrates the light of another star, more distant, in a way that makes it seem brighter. The gravity of planets focuses the light rays of a distant star on the observer. This lensing process cannot be predicted, so astronomers have to watch the sky for a single blip of light, but when this happens they can gather information about the size.
The final process that NASA uses to discover new planets is similar to the radial velocity method. The wobble that is found with radial velocity can also be found as the stars position in space changes. The movement is so small that it is hard to detect the wobble in stars. The movement is captured by taking a series of pictures of the star with other stars that are near it to use as reference and then they compare the distance between the star and the reference stars for each picture. Once it is determined if the star has moved, astronomers can analyze for signs of exoplanets.
Most of these processes to find a new planet, utilizes all of the stars in the galaxy. One problem is that there are a whole lot of stars out there, but how many? These stars are also so far away, so by the time the light reaches us, they may have died out by now. There is no way to know the exact number of stars in the Milky Way, but we can take an estimate. This is done by finding the mass of the galaxy and then the percentage of the mass that is stars, so there is not an exact number of stars that are known. The problem with this form of estimating is that we do not know the mass of each star individually because they are not all the same size. The numbers that are thrown around are anywhere between 100 billion stars and 400 billion stars. The stars that are in the Milky Way are classified as O, B, A, F, G, K, and M. These are determined based upon the color of the star, the luminosity, and surface temperature. The stars that should be investigated for habitable planets are the ones that are most solar like. These types of stars would be F, G, and K. These stars are most like the sun, so there will be enough heat for the planets and if they were too small, the planets would be too cold and wouldn’t be habitable.
The habitable zone that has been brought up can be defined as the area around the star where water can exist in liquid state. In other words, it is a region where the surface temperature of a planet would allow for water to remain in liquid state. This gives us a better understanding of the conditions of which basic life is able the form and be able to survive there. The ability for water to stay in a liquid state relies of the distance the planet is from the star and the brightness of a star because as the planet and the star get closer, the hotter the planet will be, as well as how bright the star is affects the temperature. Based on what type of star it is, the habitable zone will change. The warmer and most sun like star will mean a larger habitable zone, meaning the inner and outer edges will be situated further from the star than that of a cooler star. The variance in atmospheric composition plays a role when determining the surface temperature of a planet, with carbon dioxide and water playing a key role.
The boundaries of the habitable zone are determined based upon the star that it is around. The stellar properties of the star and the stellar flux are the two main things that help determine the habitable zone. Stellar flux can be defined as radiation that is emitted by the star passing by the star every second through a unit area. The bigger that star, means that there is more solar radiation emitted and are a lot hotter, so the habitable zone boundaries will be further out. The opposite would be true for smaller planets, they would be cooler and would emit less radiation. There are still some unanswered questions about the habitable zone because if a planet gets hit with a solar flare from the star, then it may not be a pleasing place to live but it would still be in the habitable zone, so is it habitable or not? If it is in the habitable zone, that means that there is water still in the liquid state and that would block the radiation so there could still be life on the planet in the form of sea life, so technically speaking it is still habitable.
To answer the question, how many habitable planets are in the Milky Way, there really isn’t one number that astronomers are sure on. There are a lot of estimates that need to be taken, like how many new stars there are each year and how many of those stars have planets around them. Now a day, astronomers use the drake equation to try and calculate this number. The drake equation is . Where R* = The rate of formation of stars suitable for the development of intelligent life, fp = The fraction of those stars with planetary systems, ne = The number of planets, per solar system, with an environment suitable for life, fl = The fraction of suitable planets on which life actually appears, fi = The fraction of life bearing planets on which intelligent life emerges, fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space, and L = The length of time such civilizations release detectable signals into space. When astronomers use this equation with their estimates, they come around a number of 100 billion habitable planets.
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